Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2021 • 191 A hysteresis model for the unfrozen liquid content in freezing porous media
    Saberi, P.S. and Meschke, G.
    Computers and Geotechnics 134 (2021)
    The description of the freezing characteristics of porous media is one of the most conspicuous ingredients in flow and heat transport models that involve freezing and thawing processes. Unfrozen liquid content (ULC) shows strong hysteresis during freezing and thawing cycles in different types of soils and other porous media. We discuss the possible mechanisms of hysteresis in porous media and develop a numerical model for the unfrozen liquid content that is capable of describing the hysteresis phenomenon in freezing and thawing cycles. We present a coupled finite element model as the framework for the numerical simulation of fluid flow and heat transport in partially frozen porous media. The implementation aspects of the ULC model as well as its integration into numerical codes are discussed in detail. We investigate the potential impact of the hysteresis phenomenon on the numerical simulation of transport processes in porous media through benchmark examples and validate the behavior of the model against available laboratory measurement data. © 2021
    view abstractdoi: 10.1016/j.compgeo.2021.104048
  • 2021 • 190 Design and perspective of amorphous metal nanoparticles from laser synthesis and processing
    Liang, S.-X. and Zhang, L.-C. and Reichenberger, S. and Barcikowski, S.
    Physical Chemistry Chemical Physics 23 11121-11154 (2021)
    Amorphous metal nanoparticles (A-NPs) have aroused great interest in their structural disordering nature and combined downsizing strategies (e.g. nanoscaling), both of which are beneficial for highly strengthened properties compared to their crystalline counterparts. Conventional synthesis strategies easily induce product contamination and/or size limitations, which largely narrow their applications. In recent years, laser ablation in liquid (LAL) and laser fragmentation in liquid (LFL) as "green"and scalable colloid synthesis methodologies have attracted extensive enthusiasm in the production of ultrapure crystalline NPs, while they also show promising potential for the production of A-NPs. Yet, the amorphization in such methods still lacks sufficient rules to follow regarding the formation mechanism and criteria. To that end, this article reviews amorphous metal oxide and carbide NPs from LAL and LFL in terms of NP types, liquid selection, target elements, laser parameters, and possible formation mechanism, all of which play a significant role in the competitive relationship between amorphization and crystallization. Furthermore, we provide the prospect of laser-generated metallic glass nanoparticles (MG-NPs) from MG targets. The current and potential applications of A-NPs are also discussed, categorized by the attractive application fields e.g. in catalysis and magnetism. The present work aims to give possible selection rules and perspective on the design of colloidal A-NPs as well as the synthesis criteria of MG-NPs from laser-based strategies. This journal is © 2021 the Owner Societies.
    view abstractdoi: 10.1039/d1cp00701g
  • 2021 • 189 Examination of the Liquid Volume Inside Metal Tanks Using Noncontact EMATs from Outside
    Rieger, K. and Erni, D. and Rueter, D.
    IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 68 1314-1327 (2021)
    This article discusses ultrasound echo examination of the liquid volume inside a metal enclosure using noncontact electromagnetic acoustic transducers (EMATs) from outside the tank. Because only longitudinal sound waves exist in liquids, a novel and powerful EMAT design (kA currents in small coils) with a specifically enhanced transduction of longitudinal ultrasound is presented. Different wall materials (aluminum, steel, and stainless steel) and working frequencies are considered. A particular challenge, in addition to the already weak EMAT signals across an air gap, is the highly reflective interface between the metal wall and liquid because the acoustic impedances of the two media differ considerably. With thicker metal walls (here, 3mm), only either low frequencies < 100 kHz or frequencies close to the corresponding thickness resonance of the wall (here, about 1 MHz) are more suitable. The higher frequencies are preferred, as they show advantages due to a directed beam profile, shorter wavelengths in the liquid, and an overall better pulse fidelity. The simultaneous operation of several and closely neighboring EMATs demonstrates the feasibility of more demanding detection tasks, ultimately leading toward a 3-D localization inside the liquid using a noncontact EMAT array with eight independent elements. © 1986-2012 IEEE.
    view abstractdoi: 10.1109/TUFFC.2020.3022946
  • 2021 • 188 Ignition and propagation of nanosecond pulsed plasmas in distilled water - Negative vs positive polarity applied to a pin electrode
    Grosse, K. and Falke, M. and Von Keudell, A.
    Journal of Applied Physics 129 (2021)
    Nanosecond plasmas in liquids are being used for water treatment, electrolysis, or biomedical applications. The exact nature of these very dynamic plasmas and, most importantly, their ignition physics are strongly debated. The ignition itself may be explained by two competing hypotheses: ignition in water may occur (i) via field effects at the tip of the electrode followed by tunneling of electrons in between water molecules causing field ionization or (ii) via gaseous processes of electron multiplication in nanovoids that are created from liquid ruptures due to the strong electric field gradients. Both hypotheses are supported by theory, but experimental data are very sparse due to the difficulty in monitoring the very fast processes in space and time. In this paper, we analyze nanosecond plasmas in water that are created by applying a positive and a negative polarity to a sharp tungsten electrode. The main diagnostics are fast camera measurements and fast emission spectroscopy. It is shown that plasma ignition is dominated by field effects at the electrode-liquid interface either as field ionization for positive polarity or as field emission for negative polarity. This leads to a hot tungsten surface at a temperature of 7000 K for positive polarity, whereas the surface temperature is much lower for negative polarity. At ignition, the electron density reaches 4 × 10 25 m - 3 for the positive and 2 × 10 25 m - 3 for the negative polarity. At the same time, the emission of the H α light for the positive polarity is four times higher than that for the negative polarity. During plasma propagation, the electron densities are almost identical of the order of 1- 2 × 10 25 m - 3 followed by a decay after the end of the pulse over 15 ns. It is concluded that plasma propagation is governed by field effects in a low density region that is created either by nanovoids or by density fluctuations in supercritical water surrounding the electrode that is created by the pressure and temperature at the moment of plasma ignition. © 2021 Author(s).
    view abstractdoi: 10.1063/5.0045697
  • 2021 • 187 Limited Elemental Mixing in Nanoparticles Generated by Ultrashort Pulse Laser Ablation of AgCu Bilayer Thin Films in a Liquid Environment: Atomistic Modeling and Experiments
    Shih, C.-Y. and Chen, C. and Rehbock, C. and Tymoczko, A. and Wiedwald, U. and Kamp, M. and Schuermann, U. and Kienle, L. and Barcikowski, S. and Zhigilei, L.V.
    Journal of Physical Chemistry C (2021)
    Pulsed laser ablation in liquids (PLAL) is a promising technique for the generation of colloidal alloy nanoparticles that are of high demand in a broad range of fields, including catalysis, additive manufacturing, and biomedicine. Many of the applications have stringent requirements on the nanoparticle composition and size distributions, which can only be met through innovations in the PLAL technique guided by a clear understanding of the nanoparticle formation mechanisms. In this work, we undertake a combined computational and experimental study of the nanoparticle formation mechanisms in ultrashort PLAL of Ag/Cu and Cu/Ag bilayer thin films. Experimental probing of the composition of individual nanoparticles and predictions from large-scale atomistic simulations provide consistent evidence of limited mixing between the two components from bilayer films by PLAL. The simulated and experimental distributions of nanoparticle compositions exhibit an enhanced abundance of Ag-rich and Cu-rich nanoparticles, as well as a strongly depressed population of well-mixed alloy nanoparticles. The surprising observation that the nanoscale phase separation of the two components in the bilayer films manifests itself in the sharp departure from the complete quantitative mixing in the colloidal nanoparticles is explained by the complex dynamic interaction between the ablation plume and liquid environment revealed in the simulations of the initial stage of the ablation process. The simulations predict that rapid deceleration of the ablation plume by the liquid environment results in the formation of a transient hot and dense metal region at the front of the plume, which hampers the mixing of the two components and, at the same time, contributes to the stratification of the plume in the emerging cavitation bubble. As a result, nanoparticles of different sizes and compositions are produced in different parts of the emerging cavitation bubble during the first nanoseconds of the ablation process. Notably, the diameters of the largest nanoparticles generated in the simulations of the initial stage of the ablation process are more than twice larger than the thickness of the original bilayer films. This observation provides a plausible scenario for the formation of large nanoparticles observed in the experiments. The conclusion on limited elemental mixing in the nanoparticles is validated in simulations of bilayers with different spatial order of Cu and Ag layers, even though the two systems exhibit some notable quantitative differences mainly related to the different strength of electron-phonon coupling in Cu and Ag. Overall, the results of this study provide new insights into the formation mechanism of bimetallic nanoparticles in ultrashort PLAL from thin bilayer targets and suggest that the formation of alloy nanoparticles from immiscible elements may be hampered for targets featuring distinctive elemental segregation. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.0c09970
  • 2021 • 186 Phase-sensitive detection of gas-borne Si nanoparticles via line-of-sight UV/VIS attenuation
    Asif, M. and Menser, J. and Endres, T. and Dreier, T. and Daun, K. and Schulz, C.
    Optics Express 29 21795-21809 (2021)
    The distinct optical properties of solid and liquid silicon nanoparticles are exploited to determine the distribution of gas-borne solid and liquid particles in situ using line-of-sight attenuation measurements carried out across a microwave plasma reactor operated at 100 mbar. The ratio between liquid and solid particles detected downstream of the plasma varied with measurement location, microwave power, and flow rate. Temperatures of the liquid particles were pyrometrically-inferred using a spectroscopic model based on Drude theory. The phase-sensitive measurement supports the understanding of nanoparticle formation and interaction and thus the overall gas-phase synthesis process. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstractdoi: 10.1364/OE.426528
  • 2021 • 185 Thermodynamic Properties of Biogenic Amines and Their Solutions
    Huxoll, F. and Heyng, M. and Andreeva, I.V. and Verevkin, S.P. and Sadowski, G.
    Journal of Chemical and Engineering Data (2021)
    Vapor pressures of the biologically and industrially relevant amines 2-phenylethan-1-amine, 2-amino-1-phenylethanol, α-(methylaminomethyl)benzyl alcohol, 1-phenylmethanamine, and N,N-diethylundecan-1-amine were measured via the transpiration method. Pure-component parameters for the thermodynamic model PC-SAFT were fitted to these vapor pressures and to liquid densities. The pure-component parameters were validated with measured liquid densities of binary mixtures dimethylsulfoxid + 4-(2-aminoethyl)phenol, dimethylsulfoxid + 2-amino-1-phenylethanol, dimethylsulfoxid + α-(methylaminomethyl)benzyl alcohol, and dimethylsulfoxid + 1-phenylmethanamine at 0.102 MPa and temperatures from 298.15 to 343.15 K at different amine mass fractions. Solid-liquid equilibria at 0.1 MPa were measured in binary mixtures of α-(methylaminomethyl)benzyl alcohol + water and 4-(2-aminoethyl)phenol + water at 298.15 and 308.15 K. Finally, the presence of liquid-liquid phase separation for these systems was qualitatively predicted using PC-SAFT based on the solid-liquid equilibria only and validated for the system α-(methylaminomethyl)benzyl alcohol + water by experiments at 293.15 and 323 K at 0.1 MPa. © 2021 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.1c00202
  • 2020 • 184 A compact and powerful EMAT design for contactless detection of inhomogeneities inside the liquid volume of metallic tanks Ein einfaches und leistungsstarkes EMAT-Design für die kontaktlose Detektion von Inhomogenitäten in metallischen Flüssigkeitsbehältern
    Rieger, K. and Erni, D. and Rueter, D.
    Technisches Messen 87 349-359 (2020)
    A simple and powerful design of an electromagnetic acoustic transducer (EMAT) without bulky permanent magnets is presented. The EMAT is operated in a pulse echo modality and generates longitudinal ultrasound at about 1 MHz. Unlike shear waves, these longitudinal ultrasound pulses can propagate in liquids. The generally addressed application scenario is the examination of a liquid volume inside a metallic container or tank, e. g., the detection of inhomogeneities within the liquid. The herein proposed EMAT operates for virtually all metallic containers, i. e., it succeeds for container walls made of aluminum or ferromagnetic steel, and even for non-ferromagnetic (stainless) steel. Moreover, unlike piezo transducers, EMAT techniques allow for a noncontacting ultrasound transduction: the air gap between the EMAT sensor coil and the tank s metallic surface extends up to 2 mm. Even with this relatively large air gap, the biasing magnetic field approaches a flux density of 3.2 T at the surface, more than what is possible to achieve with the permanent magnets of conventional and bulkier EMATs. Strong fields improve the coupling efficiency of the principally low-efficiency EMAT mechanism, which is important for both ultrasound transmission and reception. For that superior field intensity, a unipolar current pulse of up to 3.6 kA is applied through the thin windings (0.5 mm) of the EMAT coil. This paper presents a novel solid-state EMAT circuitry for such strong currents and MHz pulsed voltages >1 kV. As a particularly delicate task, the powerful circuitry must also detect the rather weak echo signals in the V range. A very short recovery time is required after such a strong emission burst. The discussed circuitry consists of three unipolar high-current modules, which can each be independently launched. This allows for received echo signals that can be timed independently, e. g., objects deep inside the liquid tank can be specifically addressed. In general, this work concentrates on the novel circuitry in parallel connection, the general pulse-echo functionality and the magnetic fields. A detailed analysis and shaping of the ultrasonic fields through different EMAT coil geometries would exceed the scope of this contribution and is to be reported separately. © 2020 De Gruyter Oldenbourg. All rights reserved.
    view abstractdoi: 10.1515/teme-2019-0124
  • 2020 • 183 A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy
    Tarnev, T. and Cychy, S. and Andronescu, C. and Muhler, M. and Schuhmann, W. and Chen, Y.-T.
    Angewandte Chemie - International Edition 59 5586-5590 (2020)
    A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, NixB nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201916419
  • 2020 • 182 Chemistry in nanosecond plasmas in water
    Chauvet, L. and Nenbangkaeo, C. and Grosse, K. and von Keudell, A.
    Plasma Processes and Polymers 17 (2020)
    Discharges in liquids are the basis of a range of applications in electrochemistry, wastewater treatment, or plasma medicine. One advantage of discharges in water is their ability to produce radicals and molecules directly inside liquid with a high conversion efficiency. In this study, H2O2 production in a 10 ns pulsed discharge in water is investigated. The dynamic of these discharges is based on plasma ignition directly inside liquid followed by the formation of a bubble that expands in time before it eventually collapses. This sequence can be well described by cavitation theory. H2O2 is produced using different plasma conditions varying the treatment time, the pulse frequency between 1 and 100 Hz, and the applied voltage in a range from 15–30 kV. The resulting H2O2 concentration is measured using absorption spectroscopy ex situ based on a colorimetry method. The results indicate that the main parameter controlling the H2O2 production constitutes the applied voltage. The measured concentrations are compared with a global chemistry model simulating the chemistry involved during a single pulse using pressures and temperatures from the cavitation model. In addition, a global chemical equilibrium model for H2O2 creation is evaluated as well. The models show a good agreement with the data. The energy efficiency for the production of H2O2 reaches values up to 2 g/kWh. © 2020 The Authors. Plasma Processes and Polymers published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstractdoi: 10.1002/ppap.201900192
  • 2020 • 181 Continuous-Flow Flat Jet Setup for Uniform Pulsed Laser Postprocessing of Colloids
    Zerebecki, S. and Reichenberger, S. and Barcikowski, S.
    Journal of Physical Chemistry A 124 11125-11132 (2020)
    Pulsed laser postprocessing (PLPP) of colloidal nanoparticles and related laser fragmentation in liquid (LFL) using a liquid jet setup have become an acknowledged tool to reduce the nanoparticle diameter down to a few nanometers, alter the crystal phase, or increase the defect density under high-purity and continuous-flow conditions. In recent studies on LFL that were conducted with a cylindrical liquid jet, intensity gradients and related incomplete illumination of the volume element passing through the laser beam path were reported to cause a broadening of the product particle size distribution, melting, and phase segregation. In this paper, we present a new flat jet design, which reduces the deviation of the laser intensity up to 10 times compared to the conventional cylindrical liquid jet. The experimental threshold intensity for gold nanoparticle fragmentation found with the cylindrical setup strongly deviates from the theoretical prediction, while they are in very good agreement for the flat jet setup. Additionally, a narrow product size fraction of 3 ± 2 nm was found for the flat jet, while the main product fraction gained from the cylindrical jet was 10 ± 8 nm in size under the same conditions. Consequently, the flat jet setup allows us not only to study laser fragmentation mechanisms with higher precision but also to gain product particles with narrow particle size distribution at single pulse per particle conditions even at elevated mass concentrations (>50 mg L-1). In future studies, these promising results also render the flat jet setup relevant for the other disciplines of PLPP such as laser melting and defect engineering. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpca.0c08787
  • 2020 • 180 Design of a 1000 L pilot-scale airlift bioreactor for nitrification with application of a three-phase hydrodynamic mathematical model and prediction of a low liquid circulation velocity
    Pelivanoski, B. and Detmann, B. and Ooms, K. and Winkler, M. and Vasyukova, E. and Denecke, M.
    Chemical Engineering Research and Design 153 257-262 (2020)
    In this study, a 1000 L pilot scale internal loop airlift bioreactor was operated and compared to a mathematical model to determine the best design for optimal supply of oxygen for nitrification and sufficient air for biomass fluidization. The design model is based on parameters such as geometry, carrier density, and airflow of the 1000 L pilot scale bioreactor. The model predicts a range of superficial air velocities (0.009–0.013 m/s) under which the airlift bioreactor was fluidized. Three superficial air velocities (0.009 m/s, 0.011 m/s and 0.013 m/s) were experimentally tested in the pilot plant and the obtained circulation velocities were compared with the predicted design scenarios. The predicted velocity was in agreement with the measured velocity. The aim of the mathematical model and the calculations of different geometry scenarios was to define the optimal geometry design for the physical model. The results show that the ratio of the cross-sectional area between the riser and the downcomer of 1.33 resulted in the lowest superficial liquid velocity of 0.076 m/s in the riser at a relative low superficial air velocity of 0.011 m/s and a carrier density of 1030 kg/m3. This bioreactor design enabled longest retention time of particles in the oxygenated riser. © 2019 Institution of Chemical Engineers
    view abstractdoi: 10.1016/j.cherd.2019.10.018
  • 2020 • 179 Dynamics of laser-induced cavitation bubbles at a solid-liquid interface in high viscosity and high capillary number regimes
    Hupfeld, T. and Laurens, G. and Merabia, S. and Barcikowski, S. and Gökce, B. and Amans, D.
    Journal of Applied Physics 127 (2020)
    No unified model is available yet to explain the dynamics of laser-induced cavitation bubbles during laser ablation of solid targets in liquids, when an extremely high capillary number is achieved (>100), i.e., when the viscous forces strongly contribute to the friction. By investigating laser-induced bubbles on gold and yttrium-iron-garnet targets as a function of the liquid viscosity, using a nanosecond laser and an ultrafast shadowgraph imaging setup, we give a deeper insight into what determines the bubble dynamics. We find that the competition between the viscous forces and the surface tension (capillary number Ca), on the one hand, and the competition between the viscous forces and inertia (Reynolds number Re), on the other hand, are both key factors. Increasing the viscous forces, and hereby Ca up to 100 has an impact on the bubble shape and results in a very pronounced rim, which separates the bubble in a spherical cap driven by inertia and an interlayer. The temporal evolution of the footprint radius of the interlayer can be addressed in the framework of the inertiocapillary regime. For an intermediate viscosity, the thickness of the interlayer is consistent with a boundary layer equation. Interestingly, our data cannot be interpreted with simplified hydrodynamic (Cox-Voinov) or molecular-kinetic theory models, highlighting the originality of the dynamics reported when extremely high capillary numbers are achieved. Upon bubble collapse, spherical persistent microbubbles are created and partly dispersed in water, whereas the high-viscous polyalphaolefines lead to long-standing oblate persistent bubbles sticking to the target's surface, independent of the ablated target. Overall, liquid's viscosity determines laser ablation-induced cavitation. © 2020 Author(s).
    view abstractdoi: 10.1063/1.5116111
  • 2020 • 178 Enzymatic epoxidation of cyclohexene by peroxidase immobilization on a textile and an adapted reactor design
    Wunschik, D.S. and Ingenbosch, K.N. and Süss, P. and Liebelt, U. and Quint, S. and Dyllick-Brenzinger, M. and Zuhse, R. and Menyes, U. and Hoffmann-Jacobsen, K. and Opwis, K. and Gutmann, J.S.
    Enzyme and Microbial Technology 136 (2020)
    A textile-based reaction system for new peroxidase reactions in non-native media was implemented. The epoxidation of cyclohexene by the commercial peroxidase MaxiBright® was realized with the textile-immobilized enzyme in an adapted liquid-liquid two-phase reactor. A commercially available polyester felt was used as low-price carrier and functionalized with polyvinyl amine. The covalent immobilization with glutardialdehyde lead to an enzyme loading of 0.10 genzyme/gtextile. The textile-based peroxidase shows a high activity retention in the presence of organic media. This catalyst is shown to enable the epoxidation of cyclohexene in various solvents as well as under neat conditions. A model reactor was produced by 3D printing which places the textile catalyst at the interphase between the liquid reaction phase and the product extracting solvent. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.enzmictec.2020.109512
  • 2020 • 177 Evaluation of spray impact on a sphere with a two-fluid nozzle
    Strob, R. and Babaria, T. and Rodeck, M. and Schaldach, G. and Walzel, P. and Thommes, M.
    Journal of Aerosol Science 140 (2020)
    The generation of a secondary aerosol after impact, consisting of smaller droplets at a given velocity and mass flow, is relevant for various applications. Thus far, the investigations and modelling approaches on spray impact are based on extrapolation of the single-droplet impingement or empirical correlations. The validity of the models presented is limited to the given experimental setup and conditions such as initial droplet size, velocity and the impact surface characteristics. The aim of this work was to empirically evaluate the spray impact of a two-fluid nozzle on a sphere. A small-scale nozzle was used, which produced a primary aerosol with a mass median diameter of about 12μm (liquid-to-gas mass flow ratio = 1, gas pressure: ΔpG = 5 bar). After impact on a sphere, a multimodal distribution was observed and a higher mass flowrate of droplets in the small micrometer range (2 and 3μm) was produced for a liquid mass flow rate in the range of 1.2–6 kg/h and an atomizing gas mass flow rate of 1–4 kg/h. For easier observation, a geometrically similar, larger nozzle was used, which produced an aerosol with a mass median diameter of about 80μm (liquid-to-gas mass flow ratio = 4, gas pressure: ΔpG = 1 bar). The measured droplet size after impact is smaller for a lower liquid-to-gas mass flow ratio and increased atomizing gas inlet pressure. Droplet formation mechanisms such as splashing, crown formation and spreading on the sphere surface were observed. A characteristic film with large variations in thickness was generated. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jaerosci.2019.105483
  • 2020 • 176 Investigation of Synergistic Effects between Co and Fe in Co3-xFexO4 Spinel Catalysts for the Liquid-Phase Oxidation of Aromatic Alcohols and Styrene
    Waffel, D. and Budiyanto, E. and Porske, T. and Büker, J. and Falk, T. and Fu, Q. and Schmidt, S. and Tüysüz, H. and Muhler, M. and Peng, B.
    Molecular Catalysis 498 (2020)
    Transition metal oxides are attractive catalyst alternatives in liquid-phase oxidation reactions due to their lower cost and higher abundance compared with conventional noble metal catalysts. We investigated the catalytic properties of a systematic series of Co3-xFexO4 spinel catalysts synthesized by a hard-templating method, which were applied in the liquid-phase oxidation of styrene, benzyl alcohol and cinnamyl alcohol. O2 and tert-butyl hydroperoxide (TBHP) were used as the oxidants in a comparative manner. For alcohol oxidation, TBHP leads to similar or slightly higher selectivity to the corresponding aldehydes compared with O2. For the activation of C=C bonds, TBHP favors the oxidative cleavage pathway, while O2 favors the epoxidation pathway. The comparison of the catalytic performance revealed that the activity of Co3O4 does not benefit from Fe doping using O2 as the oxidant, while the substitution of Fe ≤ 10 % in the spinel structure is beneficial when TBHP is used. This is attributed to the different activation mechanisms of the oxidizing agents, being spin transfer in case of O2 and partial decomposition in case of TBHP. Heterogeneity tests and reusability studies demonstrated the stability of the spinel catalysts. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.mcat.2020.111251
  • 2020 • 175 Mixed Poiseuille-Knudsen flow model for Gas Liquid Displacement porometry data treatment
    Islam, M.A. and Hossain, M.S. and Garcia-Payo, C. and Khayet, M. and Ulbricht, M.
    Journal of Membrane Science 612 (2020)
    A comprehensive methodology has been developed for treating Gas Liquid Displacement (GLD) porometry data with a flow model called Weber model (WM) describing mixed Poiseuille-Knudsen flow regime. The model has been applied in two options: i) considering that the gas viscosity in porometry experiments is the same as that available in reference books for Poiseuille flow regime and ii) equating the expression for Darcy coefficient in gas flow to that obtained in additional liquid permeability experiments and thus leaving the gas viscosity to be an adaptable parameter. In the analysis of GLD porometry data for a range of different microfiltration membranes, it is found that with the WM in both options identical relative pore-number distribution is estimated; and this distribution satisfactorily reproduces both dry and wet flow data from the GLD experiments. The absolute pore-number distributions obtained by the two options are quite similar, but differ in the absolute value of the pore numbers. The pore-number distribution obtained by the second option describes the liquid permeability well, while the first option fails. The WM as a method of GLD porometry data treatment is quite similar to the earlier introduced variable viscosity Poiseuille model (VVPM), and the variable viscosity from the latter model appears to be a combined effect of an uncertainty about actual gas viscosity and the contribution of Knudsen flow. It is concluded that a standard test method for determining pore-size distribution by GLD porometry must include prediction or description of liquid permeability of the membrane. Then, any acceptable gas flow model with adjusted Darcy coefficient obtained from liquid permeability experiment will be suitable for advanced GLD porometry data treatment, beyond the methods typically implemented in gas flow-based porometers, currently used in academia and industry. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2020.118422
  • 2020 • 174 Nanosecond pulsed discharges in distilled water-Part II: Line emission and plasma propagation
    Von Keudell, A. and Grosse, K. and Schulz-Von Der Gathen, V.
    Plasma Sources Science and Technology 29 (2020)
    Nanosecond plasmas in liquids can initiate chemical processes that are exploited in the fields of water treatment, electrolysis or biomedical applications. The understanding of these chemical processes relies on unraveling the dynamics of the variation of pressures, temperatures and species densities during the different stages of plasma ignition and plasma propagation as well as the conversion of the liquid into the plasma state and the gas phase. This is analyzed by monitoring the emission of nanosecond pulsed plasmas that are generated by high voltages of 20 kV and pulse lengths of 10 ns applied to a tungsten tip with 50 μm diameter immersed in water. The spectra are acquired with a temporal resolution of 2 ns and the emission pattern is modelled by a combination of black body radiation from the hot tungsten tip and the pronounced emission lines of the hydrogen Balmer series. The data indicate two contributions of the hydrogen line radiation that differ with respect to the degree of self-absorption. It is postulated that one contribution originates from a recombination region showing strong self absorption and one contribution from an ionization region showing very little self-absorption. The emission lines from the ionization region are evaluated assuming Stark broadening, that yielded electron densities up to 5 × 1025 m-3. The electron density evolution follows the same trend as the temporal evolution of the voltage applied to the tungsten tip. The propagation mechanism of the plasma is similar to that of a positive streamer in the gas phase, although in the liquid phase field effects such as electron transport by tunneling should play an important role. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aba4b9
  • 2020 • 173 Role of pre-ordered liquid in the selection mechanism of crystal polymorphs during nucleation
    Menon, S. and Díaz Leines, G. and Drautz, R. and Rogal, J.
    Journal of Chemical Physics 153 (2020)
    We investigate the atomistic mechanism of homogeneous nucleation during solidification in molybdenum employing transition path sampling. The mechanism is characterized by the formation of a pre-structured region of high bond-orientational order in the supercooled liquid followed by the emergence of the crystalline bulk phase within the center of the growing solid cluster. This precursor plays a crucial role in the process as it provides a diffusive interface between the liquid and crystalline core, which lowers the interfacial free energy and facilitates the formation of the bulk phase. Furthermore, the structural features of the pre-ordered regions are distinct from the liquid and solid phases and preselect the specific polymorph that nucleates. The similarity in the nucleation mechanism of Mo with that of metals that exhibit different crystalline bulk phases indicates that the formation of a precursor is a general feature observed in these materials. The strong influence of the structural characteristics of the precursors on the final crystalline bulk phase demonstrates that for the investigated system, polymorph selection takes place in the very early stages of nucleation. © 2020 Author(s).
    view abstractdoi: 10.1063/5.0017575
  • 2020 • 172 Room-Temperature Laser Synthesis in Liquid of Oxide, Metal-Oxide Core-Shells, and Doped Oxide Nanoparticles
    Amendola, V. and Amans, D. and Ishikawa, Y. and Koshizaki, N. and Scirè, S. and Compagnini, G. and Reichenberger, S. and Barcikowski, S.
    Chemistry - A European Journal 26 9206-9242 (2020)
    Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect-engineering in liquid. Here, established laser-based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non-equilibrium compounds, metal-oxide core–shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser-assisted methodologies, there is still a lot of room to expand the library of nano-crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser-based synthesis and processing of colloids for future studies of oxide nanomaterial-oriented sciences. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/chem.202000686
  • 2020 • 171 Studies on ultra-short pulsed laser shock peening of stainless-steel in different confinement media
    Elango, K. and Hoppius, J.S. and Kukreja, L.M. and Ostendorf, A. and Gurevich, E.L.
    Surface and Coatings Technology 397 (2020)
    We investigate the role of liquid confinement media on ultra-short pulsed Laser Shock Peening (LSP). The LSP of stainless-steel 316 and 316 L was studied using Ti: Sapphire laser pulses of about 2 ps duration, maximum energy of about 1 mJ and pulse repetition rate of 5 kHz in different liquid confinement media of Ethanol, Deionized water and separate aqueous solutions of NaCl and Glycerol. It is found that the laser fluence and/or energy attenuating mechanisms like self-focusing, filamentation, plasma breakdown in the confinement media are less significant with ps laser pulses than those with sub-ps or fs pulse durations. It is shown that the resulting surface hardness of the peened steel as a function of laser fluence depends significantly on the confinement media and the relative increase in the hardness increases monotonically with the acoustic impedance of the liquid of the confinement medium used during LSP. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2020.125988
  • 2020 • 170 Tailored SiNx-based Anode Processing for Li-Ion Batteries
    Bapat, S. and Oezcan, F. and Kilian, S.O. and Wiggers, H. and Segets, D.
    ECS Transactions 97 185-193 (2020)
    In order to successfully implement promising new battery materials at industrial production rates, it is important to provide suitable recipes for information. Typically, active materials are processed as dispersions in liquid-phase together with further additives aiming at the improvement of battery slurry properties. Interactions between particles and dispersing liquid(s) are decisive for electrode manufacturing and performance. Hence, we combined Hansen parameter approach with analytical centrifugation for dispersing SiNx nanoparticles as promising next generation battery material. Suitable probe liquids were chosen for the identification of beneficial dispersion properties. Electron microscopy was employed for a first qualitative plausibility analysis. Diacetone alcohol showed favorable dispersion properties for SiNx particles while toluene was found to be not suitable. Our approach and findings are an excellent starting point for the systematic characterization and evaluation of new battery materials with regard to processability. © 2020 ECS - The Electrochemical Society.
    view abstractdoi: 10.1149/09707.0185ecst
  • 2020 • 169 Thermoregeneration of Plastrons on Superhydrophobic Coatings for Sustained Antifouling Properties
    Simovich, T. and Rosenhahn, A. and Lamb, R.N.
    Advanced Engineering Materials 22 (2020)
    A popular and desirable function of superhydrophobic coatings is their remarkable ability to retain an entrapped layer of air, called a plastron, when submerged underwater. The drawback is that the air layer is short-lived due to solvation into the surrounding liquid. While manipulating the solubility of gases using temperature is a possible approach, it generally requires inefficiently heating large volumes of water. Following the demonstrated ability to maintain air bubbles on superhydrophobic surfaces for drag reduction, this article introduces a novel method of extracting gas from water to replenish and stabilize the plastron on superhydrophobic surfaces for sustained antifouling abilities. This method involves locally heating the liquid surrounding a superhydrophobic coating, reducing gas solubility, and causing the gas to nucleate at the liquid–air interface. The approach requires a relatively low energy input, due to the small volume of locally heated water. With a constant supply of equilibrated water and minimal energy input, the plastron can survive indefinitely without the need for a mechanical delivery of air. The thermoregenerating superhydrophobic samples were shown to exhibit excellent antifouling behavior and inhibited diatom attachment over a period of 5 days. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201900806
  • 2019 • 168 Development of a MOF-FF-compatible interaction model for liquid methanol and Cl− in methanol
    Siwaipram, S. and Bopp, P.A. and Soetens, J.-C. and Schmid, R. and Bureekaew, S.
    Journal of Molecular Liquids 285 526-534 (2019)
    If complex systems are to be studied in molecular simulation, one usually attempts to combine existing interaction models in order to describe the new system. This is, however, not always feasible. We thus propose here a new pairwise-additive interaction model for liquid methanol and solvated Cl− to be used to study the immersion of Metal-Organic Frameworks (MOFs) in methanol. Practically, it entails that all interactions must be written to be compatible with the family of MOF-FF models, which have been specifically developed and then widely employed in molecular simulations of such MOFs, in particular flexible ones. The new model for liquid methanol has been mostly tailored to provide densities and dielectric constants as close to experiment as possible in a large temperature domain. This is important since the flexible MOFs modify their shapes according to their loading with guest molecules of various types, and also according to the thermodynamic conditions. The model yields excellent agreement for the density-temperature, dielectric constant-temperature, and self-diffusion-temperature relationships, properties. Other properties such as e.g. the compressibilities or thermal expansion coefficients are of the correct order of magnitude. Since some MOF frameworks are electrically charged, counterions will be present in these cases. The interactions of Cl− with the liquid are thus also considered here. The solvation of this ion is also found to be satisfactory when compared to other MD studies. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.molliq.2019.04.068
  • 2019 • 167 Discrimination of effects leading to gas formation during pulsed laser ablation in liquids
    Kalus, M.-R. and Reimer, V. and Barcikowski, S. and Gökce, B.
    Applied Surface Science 465 1096-1102 (2019)
    Pulsed laser ablation of a bulk target in liquid induces the formation of cavitation bubbles and persistent gas bubbles, which both shield subsequent laser pulses leading to a decrease in nanoparticle productivity. A further shielding entity and a source for gas formation when post-irradiated are the synthesized nanoparticles. In this study, an experimental setup is developed, which allows quantitative measurement of the gas volume produced by these shielding entities. It can be shown that 1 cm3 gas is produced in 10 min ablation time when 8 W picosecond-laser power is applied. By a combined experimental and mathematical approach, the gas volumes induced by silver bulk ablation and post-irradiation effects of the produced colloids are discriminated. It is shown that a characteristic nanoparticle mass concentration threshold exists, where post-irradiation effects mostly dominate gas formation. In a synergistic process, the effective laser fluence available for bulk ablation decreases with increasing nanoparticle mass concentration and up to 80% of the laser power is coupled into the nanoparticles. At the same time, the interparticle distance between the nanoparticles decreases favoring the laser-induced breakdown of the liquid. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.09.224
  • 2019 • 166 Evalution of Drude parameters for liquid Germanium nanoparticles through aerosol-based line-of-sight attenuation measurements
    Daun, K.J. and Menser, J. and Asif, M. and Musikhin, S. and Dreier, T. and Schulz, C.
    Journal of Quantitative Spectroscopy and Radiative Transfer 226 146-156 (2019)
    The objective of this study is to infer Drude model parameters for liquid germanium nanoparticles from extinction measurements made across an aerosol within a microwave plasma reactor using a halogen lamp (410–700 nm) and a laser-driven light source (205–585 nm). The plasma frequency and relaxation time are inferred using Rayleigh theory, Mie theory, and a fourth-order Mie approximation. These parameters are compared with those found using the ellipsometry-derived complex dielectric function as well as the bulk density and electrical resistivity of liquid germanium. The analysis is carried out in a probabilistic context using Bayesian inference, which accounts for both the measurement noise and model error. While all the candidate models can reproduce the shape of the experimentally-derived extinction spectra, the Bayesian inference showed that extinction-derived parameters differed from those obtained from the density and electrical resistivity in a statistically-significant way. This highlights the limitations of the free-electron model that underpins Drude theory, and suggests potential opportunities for model refinement. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.jqsrt.2019.01.021
  • 2019 • 165 Impact of Macromolecular Crowding and Compression on Protein-Protein Interactions and Liquid-Liquid Phase Separation Phenomena
    Julius, K. and Weine, J. and Gao, M. and Latarius, J. and Elbers, M. and Paulus, M. and Tolan, M. and Winter, R.
    Macromolecules 52 1772-1784 (2019)
    We determined the intermolecular interaction potential, V(r), of dense lysozyme solutions, which governs the spatial distribution of the protein molecules and the location of its liquid-liquid phase separation (LLPS) region, in various crowding environments applying small-angle X-ray scattering in combination with liquid-state theory. We explored the effect of polyethylene glycol (PEG) on V(r) and the protein's phase behavior over a wide range of temperatures and pressures, crossing from the dilute to the semidilute polymer regime, thereby mimicking all crowding scenarios encountered in the heterogeneous biological cell. V(r) and hence the protein-protein distances and the phase boundary of the LLPS region strongly depend on the polymer-to-protein size ratio and the polymer concentration. The strongest effect is observed for small-sized PEG molecules, leading to a marked decrease of the mean intermolecular spacing of the protein molecules with increasing crowder concentration. The effect levels off at intermolecular distances where the proteins' second hydration shells start to penetrate each other. Strong repulsive forces like hydration-shell repulsion and/or soft enthalpic protein-PEG interactions must be operative at short distances which stabilize the protein against depletion-induced aggregation, also at pressures as high as encountered in the deep sea, where pressures up to the kbar-level are encountered. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.macromol.8b02476
  • 2019 • 164 Liquid-Liquid Equilibria for Separation of Alcohols from Esters Using Deep Eutectic Solvents Based on Choline Chloride: Experimental Study and Thermodynamic Modeling
    Samarov, A. and Prikhodko, I. and Shner, N. and Sadowski, G. and Held, C. and Toikka, A.
    Journal of Chemical and Engineering Data 64 6049-6059 (2019)
    Deep eutectic solvent (DES) formed by choline chloride and glutaric acid was tested for the separation of azeotropic mixtures of ethanol-ethyl acetate, n-propanol-n-propyl acetate, n-butanol-n-butyl acetate, ethanol-ethyl propionate, n-propanol-n-propyl propionate, and n-butanol-n-butyl propionate. For this aim, the experimental data of liquid-liquid equilibria (LLE) were obtained at a temperature of 313.15 K and atmospheric pressure. Liquid-liquid tie-lines were determined and analyzed. The extraction performance of DES was characterized with distribution coefficients and values of selectivity with respect to alcohol. The NRTL model for LLE data correlation was used. Perturbed-chain statistical associating fluid theory had also been applied for modeling LLE using a "pseudo-component" approach for the DES. Both models were shown to give reasonable estimates for the selectivity values. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.9b00884
  • 2019 • 163 Microfiltration membrane characterization by gas-liquid displacement porometry: Matching experimental pore number distribution with liquid permeability and bulk porosity
    Islam, M.A. and Ulbricht, M.
    Journal of Membrane Science 569 104-116 (2019)
    The variable viscosity Poiseuille model (VVPM), developed recently to treat gas-liquid displacement (GLD) porometry data for track-etched polyethylene terephthalate (PET) membranes, is applied to polyethersulfone (PES) and polypropylene (PP) microfiltration membranes prepared by phase inversion. It is found that in general aspects the pore size distribution as estimated by the model, with the intrinsic assumption of isolated capillary pores, perfectly reproduces wet and dry fluxes during porometry measurements, but the estimated porosity appeared to be much higher than unity. To eliminate the discrepancy, two additional parameters, namely non-uniformity and tortuosity coefficient, have been introduced in the flux and porosity estimating equations. The apparent viscosity of the gas in porometry analysis is found to be several fold higher than that available in literature, and also the viscosity range appeared to be membrane type dependent. The pore size distribution curve shifts along the diameter axis depending on the gas flow direction applied in porometry data acquisition. However, because the membranes had narrow pore size disribution, the estimated parameters had been averaged within acceptable confidence range. With the inclusion of non-uniformity and tortuosity of the capillaries, the revised version of the VVPM made an advancement in porometry data analysis of membranes with both isolated and interconnected pore structure; it characterizes the assumed model capillaries in terms of Young-Laplace pore diameter, but also non-uniformity and tortuosity of pores. The procedure for data treatment has been illustrated in detail and the method could easily be adapted to gas-liquid displacement porometer software system for regular data treatment. Based on a set of readily available experimental data, including the typical output from standard GLD porometry, the here established extended version of the VVPM allows the step-by-step determination of all the parameters which characterize the pore structure of the membranes. Also the absolute pore number density distributions can be obtained, including estimates of additional pore characteristics such as non-uniformity and tortuosity. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2018.09.030
  • 2019 • 162 Nanoporous carbon: Liquid-free synthesis and geometry-dependent catalytic performance
    Xu, R. and Kang, L. and Knossalla, J. and Mielby, J. and Wang, Q. and Wang, B. and Feng, J. and He, G. and Qin, Y. and Xie, J. and Swertz, A.-C. and He, Q. and Kegnæs, Sø. and Brett, D.J.L. and Schüth, F. and Wang, F.R.
    ACS Nano 13 2463-2472 (2019)
    Nanostructured carbons with different pore geometries are prepared with a liquid-free nanocasting method. The method uses gases instead of liquid to disperse carbon precursors, leach templates, and remove impurities, minimizing synthetic procedures and the use of chemicals. The method is universal and demonstrated by the synthesis of 12 different porous carbons with various template sources. The effects of pore geometries in catalysis can be isolated and investigated. Two of the resulted materials with different pore geometries are studied as supports for Ru clusters in the hydrogenolysis of 5-hydroxymethylfurfural (HMF) and electrochemical hydrogen evolution (HER). The porous carbon-supported Ru catalysts outperform commercial ones in both reactions. It was found that Ru on bottleneck pore carbon shows a highest yield in hydrogenolysis of HMF to 2,5-dimethylfuran (DMF) due to a better confinement effect. A wide temperature operation window from 110 to 140 °C, with over 75% yield and 98% selectivity of DMF, has been achieved. Tubular pores enable fast charge transfer in electrochemical HER, requiring only 16 mV overpotential to reach current density of 10 mA·cm-2. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.8b09399
  • 2019 • 161 Optimization of femtosecond laser processing in liquids
    Hoppius, J.S. and Maragkaki, S. and Kanitz, A. and Gregorčič, P. and Gurevich, E.L.
    Applied Surface Science 467-468 255-260 (2019)
    In this paper we analyze femtosecond laser processing of metals in liquids searching for optimal conditions for predictable ablation. Incident laser pulses are stretched or compressed, self-focused and scattered on bubbles and on surface waves in the liquid environment. Influence of these effects on the laser intensity distribution on the target surface is discussed and optimal processing parameters are suggested. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.10.121
  • 2019 • 160 Pump-probe microscopy of femtosecond laser ablation in air and liquids
    Kanitz, A. and Förster, D.J. and Hoppius, J.S. and Weber, R. and Ostendorf, A. and Gurevich, E.L.
    Applied Surface Science 475 204-210 (2019)
    The ablation process of femtosecond laser pulses of iron in air and different liquids was investigated for fluences of 0.5 J/cm2 and 2 J/cm2 by means of femtosecond pump-probe microscopy. Measurements of the relative change in reflectivity suggest that the surrounding liquid has a significant impact on the ablation process. During the heating phase of the metal within the first picoseconds after laser beam impact, the change in reflectivity in air and liquids is similar. Afterwards, the vapor and melt expulsion in air leads to a strong decrease in reflectivity, while the change in reflectivity in the liquids shows a more complex fluence and time-dependent behavior. This behavior is suggested to be triggered by the expansion of the molten surface and chemical reactions on the picosecond timescale. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2018.12.184
  • 2019 • 159 Relationship between bubble characteristics and hydrodynamic parameters for single bubbles in presence of surface active agents
    Lewandowski, B. and Fertig, M. and Ulbricht, M. and Krekel, G.
    Chemical Engineering Science 199 179-198 (2019)
    Rising single air bubbles were investigated in aqueous solutions of hexadecylamine (HDA) and methyl isobutyl carbinol (MIBC) as surface active agents at varying concentrations at a constant gas flow rate. Shadowgraphy was applied to determine main bubble characteristics, such as equivalent diameter, morphology, and rising velocity. From these characteristics, critical parameters like the concentration at the minimum bubble velocity were derived. Simultaneous application of Particle Image Velocimetry (PIV) provided information about hydrodynamic parameters, e.g. the induced liquid velocities and vortex shedding. From surface tension measurements, the concentration of adsorbed species on the interface and packing densities of HDA and MIBC on the bubble surface could be calculated. HDA exhibited a better adsorption and a higher packing density on the bubble surface compared to MIBC due to the ionic character and the straight hydrocarbon chain. The bubble characteristics were therefore more strongly affected by HDA than by MIBC. Combining the Shadowgraphy and PIV results it was found that the mean liquid velocity as well as the amount of induced turbulent kinetic energy increased with increasing concentration of surfactants in the solutions, while the investigated bubble characteristics such as equivalent diameter and rising velocity decreased. The increase in mean liquid velocity and induced turbulent kinetic energy could be correlated with the oscillating frequency of the bubble trajectory, which also increased with increasing surfactant concentration. The vortex shedding process could be visualised using Proper Orthogonal Decomposition (POD) revealing the micro-process of energy cascading. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.ces.2019.01.026
  • 2019 • 158 Review on experimental and theoretical investigations of the early stage, femtoseconds to microseconds processes during laser ablation in liquid-phase for the synthesis of colloidal nanoparticles
    Kanitz, A. and Kalus, M.-R. and Gurevich, E.L. and Ostendorf, A. and Barcikowski, S. and Amans, D.
    Plasma Sources Science and Technology 28 (2019)
    Laser ablation in liquid-phase (LAL) has been developed since the 1990s, but the interest in laser synthesis of colloids has emerged in the last decade due to a significant improvement in the production rate, proven comparative advantages in biomedical and catalysis applications, and recent commercialization. However, the method relies on highly transient phenomena, so that the fundamental understanding lacks behind the LAL synthesis refinement research. The complexity of the physics and chemistry involved has led to experimental and theoretical investigations that attempt to provide a basic description of the underlying processes but face the challenge of temporal and spatial resolution as well as non-equilibrium conditions. It appears that the processes occurring at the early time scales, ranging from femtoseconds to several microseconds are critical in the definition of the final product. The review is mainly dedicated to the comprehensive description of the processes occurring at early time scales, which include the description of laser-matter interaction for ultrashort and short laser pulses, plasma formation processes as well as comparison of the measured plasma parameters at these time scales, and subsequent description of the cavitation bubble dynamics. Furthermore, the plasma and cavitation bubble chemistry are addressed, and their impact on the nanoparticle formation is emphasized. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab3dbe
  • 2019 • 157 The Key Role of Water Activity for the Operating Behavior and Dynamics of Oxygen Depolarized Cathodes
    Röhe, M. and Botz, A. and Franzen, D. and Kubannek, F. and Ellendorff, B. and Öhl, D. and Schuhmann, W. and Turek, T. and Krewer, U.
    ChemElectroChem 6 5671-5681 (2019)
    Advanced chlor-alkali electrolysis with oxygen depolarized cathodes (ODC) requires 30 % less electrical energy than conventional hydrogen-evolution-based technology. Herein, we confirm that the activities of hydroxide and water govern the ODC performance and its dynamics. Experimental characterization of ODC under varying mass transfer conditions on the liquid side reveals large differences in the polarization curves as well as in potential step responses of the electrodes. Under convective transport in the liquid electrolyte, the ODC is not limited by mass transfer in its current density at j&gt;3.9 kA m−2, whereas transport limitations are already reached at j≈1.3 kA m−2 with a stagnant electrolyte. Since gas phase conditions do not differ significantly between the measurements, these results are in contrast the common assumption that oxygen supply determines ODC performance. A dynamic model reveals the strong influence of the electrolyte mass transfer conditions on oxygen availability and thus performance. Dynamic responses of the current density to step-wise potential changes are dominated by the mass transport of water and hydroxide ions, which is by orders of magnitude faster with convective electrolyte flow. Without convective liquid electrolyte transport, a high accumulation of hydroxide ions significantly lowers the oxygen solubility. Thus, a fast mass transport of water and hydroxide is essential for high ODC performance and needs to be ensured for technical applications. The predicted accumulation of ions is furthermore validated experimentally by means of scanning electrochemical microscopy. We also show how the outlined processes can explain the distinctively different potential step responses with and without electrolyte convection. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/celc.201901224
  • 2019 • 156 Thermodynamic Properties of Systems Comprising Esters: Experimental Data and Modeling with PC-SAFT and SAFT-γMie
    Haarmann, N. and Siewert, R. and Samarov, A.A. and Verevkin, S.P. and Held, C. and Sadowski, G.
    Industrial and Engineering Chemistry Research 58 6841-6849 (2019)
    In this work, new experimental vapor-pressure data of 14 esters were obtained using the transpiration method. Besides dimethyl fumarate, dimethyl maleate, diethyl maleate, benzyl ethanoate, benzyl propanoate, and benzyl butanoate, eight representatives of the homologous series of ethyl alkanoates were investigated. The pure-component vapor pressures and liquid densities were modeled by means of Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) and SAFT-γMie. Satisfying modeling results could be achieved with both equations of state. Furthermore, the molar excess enthalpies of 12 binary mixtures benzyl ethanoate + n-alkane were modeled. Only one binary interaction parameter was fitted for PC-SAFT to quantitatively predict the molar excess enthalpies of all binary mixtures under study, while SAFT-γMie predicts these properties in qualitative agreement with the experimental data. Finally, the liquid-liquid equilibria of three binary mixtures ester (benzyl ethanoate, dimethyl maleate, and diethyl maleate) + water were investigated. These systems show a very low and almost temperature-independent solubility of the ester in the aqueous phase, whereas the moderate solubility of water in the organic phase is temperature-dependent. Promisingly, both PC-SAFT and SAFT-γMie predicted broad and unsymmetrical miscibility gaps for these mixtures, which is in qualitative agreement with the experimental data. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.9b00714
  • 2019 • 155 Time and Mechanism of Nanoparticle Functionalization by Macromolecular Ligands during Pulsed Laser Ablation in Liquids
    Letzel, A. and Reich, S. and Dos Santos Rolo, T. and Kanitz, A. and Hoppius, J. and Rack, A. and Olbinado, M.P. and Ostendorf, A. and Gökce, B. and Plech, A. and Barcikowski, S.
    Langmuir 35 3038-3047 (2019)
    Laser ablation of gold in liquids with nanosecond laser pulses in aqueous solutions of inorganic electrolytes and macromolecular ligands for gold nanoparticle size quenching is probed inside the laser-induced cavitation bubble by in situ X-ray multicontrast imaging with a Hartmann mask (XHI). It is found that (i) the in situ size quenching power of sodium chloride (NaCl) in comparison to the ablation in pure water can be observed by the scattering contrast from XHI already inside the cavitation bubble, while (ii) for polyvinylpyrrolidone (PVP) as a macromolecular model ligand an in situ size quenching cannot be observed. Complementary ex situ characterization confirms the overall size quenching ability of both additive types NaCl and PVP. The macromolecular ligand as well as its monomer N-vinylpyrrolidone (NVP) are mainly effective for growth quenching of larger nanoparticles on later time scales, leading to the conclusion of an alternative interaction mechanism with ablated nanoparticles compared to the electrolyte NaCl, probably outside of the cavitation bubble, in the surrounding liquid phase. While monomer and polymer have similar effects on the particle properties, with the polymer being slightly more efficient, only the polymer is effective against hydrodynamic aggregation. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.8b01585
  • 2018 • 154 Coexistence of two structural relaxation processes in monohydroxy alcohol-alkyl halogen mixtures: Dielectric and rheological studies
    Bierwirth, S.P. and Gainaru, C. and Böhmer, R.
    Journal of Chemical Physics 149 (2018)
    Evidence for the existence of two glass transitions is found in binary mixtures of monohydroxy alcohols with an aprotic alkyl halide by means of dielectric spectroscopy and, markedly, also shear rheology. In the mechanical data, an enormous separation of two components becomes obvious for suitable compositions. The observation of bimodal motional heterogeneity is possible despite the fact that the glass transition temperatures of these substances differ by only 40 K. Obviously, the hydrogen-bond driven formation of supramolecular structures in one of the mixture components facilitates the emergence of dynamic contrast which for other binary liquids was so far only observed in the presence of much larger glass transition temperature differences. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5037037
  • 2018 • 153 Determination of Hansen parameters for particles: A standardized routine based on analytical centrifugation
    Süß, S. and Sobisch, T. and Peukert, W. and Lerche, D. and Segets, D.
    Advanced Powder Technology 29 1550-1561 (2018)
    The Hansen Solubility Parameters (HSP) are powerful descriptors to evaluate interactions of (polymer) molecules and their solubility in different liquids. Although approaches do exist to transfer the HSP-concept to the question of dispersibility of particles, HSP determination of slowly sedimenting (nano)particles (NPs) is time consuming and depends on the subjective evaluation of the experimenter. Herein, we introduce a new method for HSP determination for colloidal systems using analytical centrifugation (AC) which was applied to standardize and accelerate the experimental procedure. However, as not dissolution but dispersion is in focus, we propose to use the term Hansen Dispersibility Parameters (HDP) instead of HSP whenever dispersibility and stability of particles against agglomeration/flocculation are to be discussed. First, we implemented a standard dispersion routine for the well-known, industrially highly relevant pigment carbon black (CB). Then, a standardized method for the evaluation of measured AC profiles and appropriate ranking of NPs dispersibility in different media was developed. We demonstrate outstanding reproducibility of our results by comparing HDP derived for the same CB material from independent experiments performed at two different affiliations. Finally, we show the predictive power of HDP and the accuracy of our approach by evaluating the dispersibility of CB in additional liquids and mixtures of so-called “good” and “poor” liquid media crossing the border from stable to unstable medium conditions. Our study evidences the enormous potential of AC to determine the HDP of colloidal systems using a standardized and non-subjective method to access particle interactions and colloidal stability. © 2018 The Society of Powder Technology Japan
    view abstractdoi: 10.1016/j.apt.2018.03.018
  • 2018 • 152 Development of a low-cost FPGA-based measurement system for real-time processing of acoustic emission data: Proof of concept using control of pulsed laser ablation in liquids
    Wirtz, S.F. and Cunha, A.P.A. and Labusch, M. and Marzun, G. and Barcikowski, S. and Söffker, D.
    Sensors (Switzerland) 18 (2018)
    Today, the demand for continuous monitoring of valuable or safety critical equipment is increasing in many industrial applications due to safety and economical requirements. Therefore, reliable in-situ measurement techniques are required for instance in Structural Health Monitoring (SHM) as well as process monitoring and control. Here, current challenges are related to the processing of sensor data with a high data rate and low latency. In particular, measurement and analyses of Acoustic Emission (AE) are widely used for passive, in-situ inspection. Advantages of AE are related to its sensitivity to different micro-mechanical mechanisms on the material level. However, online processing of AE waveforms is computationally demanding. The related equipment is typically bulky, expensive, and not well suited for permanent installation. The contribution of this paper is the development of a Field Programmable Gate Array (FPGA)-based measurement system using ZedBoard devlopment kit with Zynq-7000 system on chip for embedded implementation of suitable online processing algorithms. This platform comprises a dual-core Advanced Reduced Instruction Set Computer Machine (ARM) architecture running a Linux operating system and FPGA fabric. A FPGA-based hardware implementation of the discrete wavelet transform is realized to accelerate processing the AE measurements. Key features of the system are low cost, small form factor, and low energy consumption, which makes it suitable to serve as field-deployed measurement and control device. For verification of the functionality, a novel automatically realized adjustment of the working distance during pulsed laser ablation in liquids is established as an example. A sample rate of 5 MHz is achieved at 16 bit resolution. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/s18061775
  • 2018 • 151 Human Apolipoprotein A1 at Solid/Liquid and Liquid/Gas Interfaces
    Dogan, S. and Paulus, M. and Forov, Y. and Weis, C. and Kampmann, M. and Cewe, C. and Kiesel, I. and Degen, P. and Salmen, P. and Rehage, H. and Tolan, M.
    Journal of Physical Chemistry B 122 3953-3960 (2018)
    An X-ray reflectivity study on the adsorption behavior of human apolipoprotein A1 (apoA1) at hydrophilic and hydrophobic interfaces is presented. It is shown that the protein interacts via electrostatic and hydrophobic interactions with the interfaces, resulting in the absorption of the protein. pH dependent measurements at the solid/liquid interface between silicon dioxide and aqueous protein solution show that in a small pH range between pH 4 and 6, adsorption is increased due to electrostatic attraction. Here, the native shape of the protein seems to be conserved. In contrast, the adsorption at the liquid/gas interface is mainly driven by hydrophobic effects, presumably by extending the hydrophobic regions of the amphipathic helices, and results in a conformational change of the protein during adsorption. However, the addition of differently charged membrane-forming lipids at the liquid/gas interface illustrates the ability of apoA1 to include lipids, resulting in a depletion of the lipids from the interface. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b12481
  • 2018 • 150 In Aqua Electrochemistry Probed by XPEEM: Experimental Setup, Examples, and Challenges
    Nemšák, S. and Strelcov, E. and Guo, H. and Hoskins, B.D. and Duchoň, T. and Mueller, D.N. and Yulaev, A. and Vlassiouk, I. and Tselev, A. and Schneider, C.M. and Kolmakov, A.
    Topics in Catalysis 61 2195-2206 (2018)
    Recent developments in environmental and liquid cells equipped with electron transparent graphene windows have enabled traditional surface science spectromicroscopy tools, such as scanning X-ray photoelectron microscopy, X-ray photoemission electron microscopy (XPEEM), and scanning electron microscopy to be applied for studying solid–liquid and liquid–gas interfaces. Here, we focus on the experimental implementation of XPEEM to probe electrified graphene–liquid interfaces using electrolyte-filled microchannel arrays as a new sample platform. We demonstrate the important methodological advantage of these multi-sample arrays: they combine the wide field of view hyperspectral imaging capabilities from XPEEM with the use of powerful data mining algorithms to reveal spectroscopic and temporal behaviors at the level of the individual microsample or the entire array ensemble. © 2018, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.
    view abstractdoi: 10.1007/s11244-018-1065-4
  • 2018 • 149 Light-scattering data of protein and polymer solutions: A new approach for model validation and parameter estimation
    Voges, M. and Herhut, M. and Held, C. and Brandenbusch, C.
    Fluid Phase Equilibria 465 65-72 (2018)
    The development of separation processes for polymers or proteins from aqueous solutions requires a high amount of experimental effort, including phase-equilibrium data such as solid-liquid, liquid-liquid or vapor-liquid equilibria. This effort can be reduced by means of thermodynamic models. This work presents a new method for parameter estimation and validation of thermodynamic models by means of static-light-scattering (SLS) measurements. In this work the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) was used to predict directly the SLS data (Rayleigh ratio) of macromolecular solutions. In a first step, SLS data were measured for binary water/polyethylene glycol (PEG molecular weight ranging from 2000 to 12000 g/mol) mixtures and for binary water/lysozyme mixtures. Applying pure-component PC-SAFT parameters from literature, the SLS data of these binary mixtures were successfully predicted with PC-SAFT. In a second step, one binary interaction parameter between lysozyme and PEG was adjusted to new experimental SLS data of buffered PEG/lysozyme/water solutions with PEG6000 concentration of 20 g/L. Finally, SLS data for buffered ternary PEG/lysozyme/water solutions with PEG of different molecular weights (2000–12000 g/mol) and different concentrations (1–50 g/L) were accurately predicted with PC-SAFT. Thus, (1) the proposed approach allows predicting SLS data, and (2) the method provides an access to the estimation of model parameters by means of experimental SLS data, which are accessible with much less effort than experimental phase-equilibrium data. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2018.02.022
  • 2018 • 148 Maximum Likelihood Analysis of Reaction Coordinates during Solidification in Ni
    Díaz Leines, G. and Rogal, J.
    Journal of Physical Chemistry B 122 10934-10942 (2018)
    Understanding the underlying mechanism of crystal nucleation is a fundamental aspect in the prediction and control of materials properties. Classical nucleation theory (CNT) assumes that homogeneous nucleation occurs via random fluctuations within the supercooled liquid, that the structure of the growing clusters resembles the most stable bulk phase, and that the nucleus size is the sole reaction coordinate (RC) of the process. Many materials are, however, known to exhibit multiple steps during crystallization, forming different polymorphs. As a consequence, more complex RCs are often required to capture all relevant information about the process. Here, we employ transition path sampling together with a maximum likelihood analysis of candidate order parameters to identify suitable RCs for the nucleation mechanism during solidification in Ni. In contrast to CNT, the analysis of the reweighted path ensemble shows that a prestructured liquid region that surrounds the crystal cluster is a relevant order parameter that enhances the RC and therefore plays a key role in the description of the nucleus and the interfacial free energy. We demonstrate that prestructured liquid clusters that emerge within the liquid act as precursors of the crystallization in a nonclassical two-step mechanism, which predetermines the coordination of the selected polymorphs. Copyright © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.8b08718
  • 2018 • 147 Oil desulfurization using deep eutectic solvents as sustainable and economical extractants via liquid-liquid extraction: Experimental and PC-SAFT predictions
    Warrag, S.E.E. and Pototzki, C. and Rodriguez, N.R. and van Sint Annaland, M. and Kroon, M.C. and Held, C. and Sadowski, G. and Peters, C.J.
    Fluid Phase Equilibria 467 33-44 (2018)
    The reduction of the sulfur content in crude oil is of utmost importance in order to meet the stringent environmental regulations. Thiophene and its derivatives are considered key substances to be separated from the crude oil. In previous works, six deep eutectic solvents (DESs) based on tetraethylammonium chloride, tetrahexylammonium bromide and methyltriphenylphosphonium bromide as hydrogen bond acceptors (HBAs) and polyols (ethylene glycol and glycerol) as hydrogen bond donors (HBDs) were successfully applied for the extraction of thiophene from {n-alkane + thiophene} mixtures via liquid-liquid extraction. One of the objectives of this work was to study the effect of the aliphatic hydrocarbon type/length (e.g. n-hexane vs n-octane) on the extraction performance of the same DESs. Extraction performance was evaluated by the selectivity and the thiophene distribution coefficient. Based on new experimental data, higher selectivities and lower thiophene distribution coefficients were obtained when thiophene was extracted from n-octane instead of n-hexane. Another objective was to predict the phase behavior of the ternary systems {n-alkane + thiophene + DES} using Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT). The PC-SAFT “pseudo-pure component” approach was applied, in which a DES was considered as a pseudo-pure compound (not a mixture). The pure-component parameters of the DESs were obtained by fitting to liquid density data, which were measured at temperatures between 298.2 K and 323.2 K. Binary interaction parameters were fitted to experimental binary LLE data for the systems {n-alkane + DES} and {thiophene + DES} at 298.2 K and atmospheric pressure, while the LLE data of the ternary systems {n-alkane + thiophene + DES} were fully predicted. It was found that the distribution coefficients and selectivity of the ternary systems containing DESs could be qualitatively well predicted using this model. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2018.03.018
  • 2018 • 146 Primary particle diameter differentiation and bimodality identification by five analytical methods using gold nanoparticle size distributions synthesized by pulsed laser ablation in liquids
    Letzel, A. and Gökce, B. and Menzel, A. and Plech, A. and Barcikowski, S.
    Applied Surface Science 435 743-751 (2018)
    For a known material, the size distribution of a nanoparticle colloid is a crucial parameter that defines its properties. However, measured size distributions are not easy to interpret as one has to consider weighting (e.g. by light absorption, scattering intensity, volume, surface, number) and the way size information was gained. The radius of a suspended nanoparticle can be given as e.g. sphere equivalent, hydrodynamic, Feret or radius of gyration. In this study, gold nanoparticles in water are synthesized by pulsed-laser ablation (LAL) and fragmentation (LFL) in liquids and characterized by various techniques (scanning transmission electron microscopy (STEM), small-angle X-ray scattering (SAXS), analytical disc centrifugation (ADC), dynamic light scattering (DLS) and UV–vis spectroscopy with Mie-Gans Theory) to study the comparability of different analytical techniques and determine the method that is preferable for a given task related to laser-generated nanoparticles. In particular, laser-generated colloids are known to be bimodal and/or polydisperse, but bimodality is sometimes not analytically resolved in literature. In addition, frequently reported small size shifts of the primary particle mode around 10 nm needs evaluation of its statistical significance related to the analytical method. Closely related to earlier studies on SAXS, different colloids in defined proportions are mixed and their size as a function of the nominal mixing ratio is analyzed. It is found that the derived particle size is independent of the nominal mixing ratio if the colloid size fractions do not overlap considerably. Conversely, the obtained size for colloids with overlapping size fractions strongly depends on the nominal mixing ratio since most methods cannot distinguish between such fractions. Overall, SAXS and ADC are very accurate methods for particle size analysis. Further, the ability of different methods to determine the nominal mixing ratio of sizes fractions is studied experimentally. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.11.130
  • 2018 • 145 Suspension- and powder-based derivation of Hansen dispersibility parameters for zinc oxide quantum dots
    Süß, S. and Lin, W. and Getmanenko, O. and Pflug, L. and Sobisch, T. and Peukert, W. and Lerche, D. and Segets, D.
    Particuology (2018)
    For most particle-based applications, formulation in the liquid phase is a decisive step, and thus, particle interactions and stability in liquid media are of major importance. The concept of Hansen solubility parameters (HSP) was initially invented to describe the interactions of (polymer) molecules and their solubility in different liquids and is increasingly being used in particle technology to describe dispersibility. Because dispersions are not thermodynamically stable, the term Hansen dispersibility parameters (HDP) is used instead of HSP (Süß Sobisch, Peukert, Lerche, & Segets, 2018). Herein, we extend a previously developed standardized and non-subjective method for determination of Hansen parameters based on analytical centrifugation to the important class of quantum materials. As a technically relevant model system, zinc oxide quantum dots (QDs) were used to transfer our methodology to nanoparticles (NPs) with sizes below 10 nm. The results obtained using the standard procedure starting from a dried powder were compared with those obtained through redispersion from the wet sediment produced during the typical washing procedure of QDs, and drying was observed to play an important role. In conclusion, our study reveals the high potential of HDP for quantifying the interfacial properties of NPs as well as their link to dispersibility. © 2018 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences
    view abstractdoi: 10.1016/j.partic.2018.05.010
  • 2018 • 144 The Role of Polyfunctionality in the Formation of [Ch]Cl-Carboxylic Acid-Based Deep Eutectic Solvents
    Crespo, E.A. and Silva, L.P. and Martins, M.A.R. and Bülow, M. and Ferreira, O. and Sadowski, G. and Held, C. and Pinho, S.P. and Coutinho, J.A.P.
    Industrial and Engineering Chemistry Research 57 11195-11209 (2018)
    Aiming at providing an extensive characterization of the solid-liquid equilibria (SLE) of deep eutectic solvents (DESs), the phase diagrams of nine eutectic mixtures composed of choline chloride ([Ch]Cl) and (poly)carboxylic acids, commonly reported in the literature as DESs, were measured experimentally. Contrarily to the behavior reported for eutectic mixtures composed of [Ch]Cl (hydrogen-bond acceptor, HBA) and monofunctional hydrogen-bond donors (HBD) such as fatty acids and fatty alcohols, which have recently been shown to be almost ideal mixtures, a significant decrease of the melting temperature, at the eutectic point, was observed for most of the systems studied. This melting temperature depression was attributed to a pronounced nonideality of the liquid phase induced by the strong hydrogen-bond interactions between the two mixture components. Perturbed-chain statistical associating fluid theory (PC-SAFT) was used to describe these interactions physically. PC-SAFT allowed accurately modeling the experimental phase diagrams over the entire concentration and temperature ranges. Depending on the kind of mixture, up to two temperature-independent binary interaction parameters between HBA and HBD were applied. The PC-SAFT approach was used to provide trustworthy information on the nonideality of the liquid phase (expressed as the activity coefficients of HBA and HBD) as well as to estimate the eutectic points coordinates. The experimental data along with the modeling results allowed us to infer about the importance of the HBD's chemical structure on the formation of [Ch]Cl-based DESs. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.8b01249
  • 2018 • 143 Two mechanisms of nanoparticle generation in picosecond laser ablation in liquids: The origin of the bimodal size distribution
    Shih, C.-Y. and Streubel, R. and Heberle, J. and Letzel, A. and Shugaev, M.V. and Wu, C. and Schmidt, M. and Gökce, B. and Barcikowski, S. and Zhigilei, L.V.
    Nanoscale 10 6900-6910 (2018)
    The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting laboratory and industrial applications. The fundamental understanding of processes leading to the nanoparticle generation, however, still remains elusive. In particular, the origin of bimodal nanoparticle size distributions in femto- and picosecond laser ablation in liquids, where small nanoparticles (several nanometers) with narrow size distribution are commonly observed to coexist with larger (tens to hundreds of nanometers) ones, has not been explained so far. In this paper, joint computational and experimental efforts are applied to understand the mechanisms of nanoparticle formation in picosecond laser ablation in liquids and to explain the bimodal nanoparticle size distributions. The results of a large-scale atomistic simulation reveal the critical role of the dynamic interaction between the ablation plume and the liquid environment, leading to the generation of large nanoparticles through a sequence of hydrodynamic instabilities at the plume-liquid interface and a concurrent nucleation and growth of small nanoparticles in an expanding metal-liquid mixing region. The computational predictions are supported by a series of stroboscopic videography experiments showing the emergence of small satellite bubbles surrounding the main cavitation bubble generated in single pulse experiments. Carefully timed double pulse irradiation triggers expansion of secondary cavitation bubbles indicating, in accord with the simulation results, the presence of localized sites of laser energy deposition (possibly large nanoparticles) injected into the liquid at the early stage of the bubble formation. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7nr08614h
  • 2017 • 142 Advanced SERS Sensor Based on Capillarity-Assisted Preconcentration through Gold Nanoparticle-Decorated Porous Nanorods
    Xue, L. and Xie, W. and Driessen, L. and Domke, K.F. and Wang, Y. and Schlücker, S. and Gorb, S.N. and Steinhart, M.
    Small (2017)
    A preconcentrating surface-enhanced Raman scattering (SERS) sensor for the analysis of liquid-soaked tissue, tiny liquid droplets and thin liquid films without the necessity to collect the analyte is reported. The SERS sensor is based on a block-copolymer membrane containing a spongy-continuous pore system. The sensor's upper side is an array of porous nanorods having tips functionalized with Au nanoparticles. Capillarity in combination with directional evaporation drives the analyte solution in contact with the flat yet nanoporous underside of the SERS sensor through the continuous nanopore system toward the nanorod tips where non-volatile components of the analyte solution precipitate at the Au nanoparticles. The nanorod architecture increases the sensor surface in the detection volume and facilitates analyte preconcentration driven by directional solvent evaporation. The model analyte 5,5'-dithiobis(2-nitrobenzoic acid) can be detected in a 1 × 10-3m solution ≈300 ms after the sensor is brought into contact with the solution. Moreover, a sensitivity of 0.1 ppm for the detection of the dissolved model analyte is achieved. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/smll.201603947
  • 2017 • 141 Atomistic insight into the non-classical nucleation mechanism during solidification in Ni
    Díaz Leines, G. and Drautz, R. and Rogal, J.
    Journal of Chemical Physics 146 (2017)
    Nucleation is a key step during crystallization, but a complete understanding of the fundamental atomistic processes remains elusive. We investigate the mechanism of nucleation during solidification in nickel for various undercoolings using transition path sampling simulations. The temperature dependence of the free energy barriers and rate constants that we obtain is consistent with the predictions of classical nucleation theory and experiments. However, our analysis of the transition path ensemble reveals a mechanism that deviates from the classical picture of nucleation: the growing solid clusters have predominantly non-spherical shapes and consist of face-centered-cubic and random hexagonal-close-packed coordinated atoms surrounded by a cloud of prestructured liquid. The nucleation initiates in regions of supercooled liquid that are characterized by a high orientational order with structural features that predetermine the polymorph selection. These results provide atomistic insight not only into the nucleation mechanism of nickel but also into the role of the preordered liquid regions as precursors for crystallization. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4980082
  • 2017 • 140 Characterization and Modeling of the Liquid Phase of Deep Eutectic Solvents Based on Fatty Acids/Alcohols and Choline Chloride
    Crespo, E.A. and Silva, L.P. and Martins, M.A.R. and Fernandez, L. and Ortega, J. and Ferreira, O. and Sadowski, G. and Held, C. and Pinho, S.P. and Coutinho, J.A.P.
    Industrial and Engineering Chemistry Research 56 12192-12202 (2017)
    The solid-liquid equilibria phase diagrams of eight eutectic systems formed by choline chloride and fatty acids, or fatty alcohols, were measured to characterize the nonideality of the liquid phase of these systems, commonly reported in the literature as examples of type III deep eutectic solvents (DESs), and to evaluate the best modeling approaches to their description. Most of these systems are shown to present only slight deviations from ideal behavior, resulting from a fine balance of the hydrogen bonding between the hydroxyl/carboxylic groups with the chloride anion and the interactions present in the pure compounds. The phase diagrams measured were modeled with an associative equation of state (EoS) and a gE model. As an EoS, the perturbed-chain statistical associating fluid theory (PC-SAFT) was used, and this model was able to accurately describe the experimental data and to provide reliable estimates of the eutectic points using just a single binary temperature-dependent interaction parameter that often correlates with the acid/alcohol chain length. The performance of PC-SAFT was further compared with the gE model, a non-random two-liquid model (NRTL), and was found to provide a better description of the experimental data, especially for the more nonideal systems. Ultimately, the data gathered, and the molecular modeling, allowed the discussion of the behavior of fatty acids or fatty alcohols as hydrogen bond donors in choline chloride-based DESs. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b02382
  • 2017 • 139 Diffusion across the glass transition in silicate melts: Systematic correlations, new experimental data for Sr and Ba in calcium-aluminosilicate glasses and general mechanisms of ionic transport
    Fanara, S. and Sengupta, P. and Becker, H.-W. and Rogalla, D. and Chakraborty, S.
    Journal of Non-Crystalline Solids 455 6-16 (2017)
    Viscosity and diffusivity of silicate melts and glasses are related to each other through relaxation timescales. The systematic is explored based on published data. Diffusion coefficients for Sr and Ba were measured in calcium aluminosilicate glasses at conditions near the glass/supercooled liquid boundary in temperature – time space making use of thin film technology and Rutherford Backscattering Spectroscopy (RBS) to measure concentration profiles on nanoscales. These data extend the range of published diffusion coefficients and combined with the systematic noted above allow the nature of change of diffusion coefficients across the glass transition region to be studied. Activation energies for diffusion in the glassy state (~ 360 kJ/mol) are higher than in the molten liquid (~ 213 kJ/mol). A defect based model of glass transition derived by Ojovan and coworkers, where attainment of a percolation threshold of configuron-type defects accounts for the glass – liquid transition, can explain the observed diffusion behaviour. Data treatment using this model yields a defect formation enthalpy of ~ 146 kJ/mol and a migration enthalpy of ~ 213 kJ/mol. The results of this study provide generalized expressions for the prediction of diffusion coefficients of cations in silicate melts for any composition at any temperature. © 2016
    view abstractdoi: 10.1016/j.jnoncrysol.2016.10.013
  • 2017 • 138 Gradual modification of ITO particle's crystal structure and optical properties by pulsed UV laser irradiation in a free liquid jet
    Lau, M. and Straube, T. and Aggarwal, A.V. and Hagemann, U. and De Oliveira Viestel, B. and Hartmann, N. and Textor, T. and Lutz, H. and Gutmann, J.S. and Barcikowski, S.
    Dalton Transactions 46 6039-6048 (2017)
    Indium tin oxide (ITO) particle coatings are known for high transparency in the visible, good conductive properties and near-infrared absorption. These properties depend on ITO particle's stoichiometric composition, defects and size. Here we present a method to gradually change ITO particle's optical properties by a simple and controlled laser irradiation process. The defined irradiation process and controlled energy dose input allows one to engineer the absorption and transmission of coatings made from these particles. We investigate the role of the surrounding solvent, influence of laser fluence and the specific energy dose targeting modification of the ITO particle's morphology and chemistry by stepwise laser irradiation in a free liquid jet. TEM, SEM, EDX, XPS, XRD and Raman are used to elucidate the structural, morphological and chemical changes of the laser-induced ITO particles. On the basis of these results the observed modification of the optical properties is tentatively attributed to chemical changes, e.g. laser-induced defects or partial reduction. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7dt00010c
  • 2017 • 137 High productive and continuous nanoparticle fabrication by laser ablation of a wire-target in a liquid jet
    Kohsakowski, S. and Santagata, A. and Dell'Aglio, M. and de Giacomo, A. and Barcikowski, S. and Wagener, P. and Gökce, B.
    Applied Surface Science 403 487-499 (2017)
    To scale-up pulsed laser ablation in liquids for nanoparticle synthesis, we combine two promising approaches, a wire-shaped target and a small liquid layer, in one setup. Using thin liquid layers a significant increase in nanoparticle productivity (up to 5 times) is obtained. This increase is attributed to the dynamics, shape of the cavitation bubble and the spring-board like behavior of the wires in the small liquid filament. It is found that despite the increase in productivity, the particle size is independent of the productivity-related ablation parameters such as repetition rate, liquid layer thickness and wire diameter. In addition to the cavitation bubble, further shielding effects have been related to both, the laser ablated material and the presence of generated small vapor bubbles. The obtained results show that this setup can provide a good strategy to realize a continuous and process-stable (particle size and quality) ablation process without the need of target replacement. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.apsusc.2017.01.077
  • 2017 • 136 Impact of liquid environment on femtosecond laser ablation
    Kanitz, A. and Hoppius, J.S. and Fiebrandt, M. and Awakowicz, P. and Esen, C. and Ostendorf, A. and Gurevich, E.L.
    Applied Physics A: Materials Science and Processing 123 (2017)
    The ablation rate by femtosecond laser processing of iron in different liquids is investigated for fluences up to 5 J/cm2. The resulting fluence dependency is modeled by an approach derived from the two-temperature model. In our experiments, the liquid environment strongly affects the effective penetration depth, e.g, the ablation rate in water is almost ten times higher than in toluene. This effect is discussed and introduced phenomenologically into the model. Additional reflectivity measurements and plasma imaging provide improved insight into the ablation process. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00339-017-1280-z
  • 2017 • 135 Influence of water and oil clearance flow on the operational behavior of screw expanders
    Gräßer, M. and Brümmer, A.
    Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 23 38-46 (2017)
    Currently, the simulation of multi-phase rotary displacement machines in reasonable accordance with the experimental results is not possible. Clearance sealing, additional frictional losses, heat transfer and lubrication are among the various effects caused by the presence of a liquid, which would all have to be modelled. Moreover, complex processes, such as condensation and evaporation affecting the thermodynamic equilibrium, as well as expansion of the multi-phase gas-liquid-mixture would have to be included. With the purpose of achieving a better understanding of liquid-flooded screw expanders, this paper describes a theoretical evaluation of clearance sealing by means of a liquid, and the resulting frictional losses. The influence of different auxiliary liquids, namely water and oil, is examined. Thus, after introducing the expander geometry and the auxiliary liquids, the results of a thermodynamic analysis are presented. The multi-chamber model-based simulation tool KaSim, that has been developed at the Chair of Fluidics, is applied to analyze the maximum potential of clearance sealing. Subsequently, dry running and liquid-flooded screw expanders are compared, taking clearance sealing as well as frictional losses into account. On the one hand, the study demonstrates that the influence of liquid water on temperature is negligible. On the other hand, the results show that reasonable modelling of oil requires a consideration of temperature-dependent dynamic viscosity for this auxiliary liquid. Finally, an extension of the presented simulation approach is introduced. © IMechE 2016.
    view abstractdoi: 10.1177/0954408916667411
  • 2017 • 134 Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy
    Nemšák, S. and Strelcov, E. and Duchoň, T. and Guo, H. and Hackl, J. and Yulaev, A. and Vlassiouk, I. and Mueller, D.N. and Schneider, C.M. and Kolmakov, A.
    Journal of the American Chemical Society 139 18138-18141 (2017)
    Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/jacs.7b07365
  • 2017 • 133 Ionic Liquid-Based Route for the Preparation of Catalytically Active Cellulose-TiO2 Porous Films and Spheres
    Wittmar, A. and Ulbricht, M.
    Industrial & Engineering Chemistry Research 56 2967--2975 (2017)
    The present work evaluates the possibilities of processing cellulose with ionic liquids and functional nanoparticles like TiO2 toward a new generation of porous nanocomposites, shaped as films or spheres, which may find direct application in water purification, catalysis, and selfcleaning materials. The focus was set on the factors controlling the formation of the porous film structure during the nonsolvent induced phase separation process from polymer solutions in ionic liquids via immersion in water and during the porous film drying step. Temperature and cosolvent addition facilitate cellulose solubilization and help control the phase separation by improving the mass transfer. The complex relation between the catalytic activity of the porous TiO2-cellulose nanocomposite materials obtained under different processing conditions and their structure has been studied during the photodegradation of model organic dyes like rhodamine B and methylene blue. After drying, the catalytic activity of the nanocomposites decreases as a consequence of the reformation of the intra- and intermolecular hydrogen bonds in cellulose which diminish the flexibility and the mobility of the fine cellulose fibrils network.
    view abstractdoi: 10.1021/acs.iecr.6b04720
  • 2017 • 132 Micro-macro modelling of steel solidification: A continuum mechanical, bi-phasic, two-scale model including thermal driven phase transition
    Moj, L. and Foppe, M. and Deike, R. and Ricken, T.
    GAMM Mitteilungen 40 125-137 (2017)
    This paper addresses a continuum-mechanical, bi-phasic, two-scale numerical model for casting and processing of metallic alloys. The solid and liquid physical states, which represents the solid and molten alloy, are formulated in the framework of the theory of porous media (TPM) including thermal coupling, finite plasticity superimposed by a secondary power creep law and visco-elasticity associated by Darcy's permeability for the solid and the liquid phase, respectively. In view of phase transition during solidification, a two-scale approach considering the phase-field on the micro-scale is proposed, where a double-well potential with two local minima for completely solid and liquid physical states is utilized. The finite element method based on the standard Gallerkin element formulation and the finite difference method was employed for the macro-scale and the micro-scale, respectively. Finally, the performance of the discussed model is demonstrated by the recalculation and validation of a solidification experiment. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/gamm.201720004
  • 2017 • 131 Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel
    Suhuddin, U.F.H. and Fischer, V. and Kostka, A. and dos Santos, J.F.
    Science and Technology of Welding and Joining 1-8 (2017)
    In the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min−1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe2Al5 and Fe4Al13 intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute
    view abstractdoi: 10.1080/13621718.2017.1300744
  • 2017 • 130 Modeling binary mixtures of n-alkanes and water using PC-SAFT
    Haarmann, N. and Enders, S. and Sadowski, G.
    Fluid Phase Equilibria (2017)
    Modeling and measuring the mutual solubility in binary n-alkane + water mixtures is very challenging due to their low order of magnitude. Consequently, experimental data regarding mutual solubilities of these systems scatter remarkably. In this work, the PC-SAFT equation of state has been applied to model liquid-liquid and vapor-liquid-liquid equilibria of binary n-alkane + water mixtures. For this purpose, temperature-dependent binary interaction parameters have been fitted to the n-alkane solubility in the aqueous phase for n-alkanes ranging from n-pentane to n-undecane. Furthermore, these binary interaction parameters have been correlated with the carbon number of the n-alkane in order to predict phase equilibria of binary n-alkane + water mixtures for n-alkanes ranging from n-propane to n-pentadecane. Excellent agreement between modeling results and available experimental data has been observed for the liquid-liquid equilibria including the description of the minimum of n-alkane solubility in water as a function of temperature. Even the prediction of the vapor-liquid-liquid equilibria of the respective mixtures showed remarkably good results compared to experimental data. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2017.11.015
  • 2017 • 129 Orientation of ripples induced by ultrafast laser pulses on copper in different liquids
    Maragkaki, S. and Elkalash, A. and Gurevich, E.L.
    Applied Physics A: Materials Science and Processing 123 (2017)
    Formation of laser-induced periodic surface structures (LIPSS or ripples) was studied on a metallic surface of polished copper using irradiation with multiple femtosecond laser pulses in different environmental conditions (air, water, ethanol and methanol). Uniform LIPSS have been achieved by controlling the peak fluence and the overlapping rate. Ripples in both orientations, perpendicular and parallel to laser polarization, were observed in all liquids simultaneously. The orientation of these ripples in the center of the ablated line was changing with the incident light intensity. For low intensities the orientation of the ripples is perpendicular to the laser polarization, whereas for high intensities it turns parallel to it without considerable changes in the period. Multi-directional LIPSS formation was also observed for moderate peak fluence in liquid environments. © 2017, Springer-Verlag GmbH Germany.
    view abstractdoi: 10.1007/s00339-017-1336-0
  • 2017 • 128 Phase Behavior of Binary Mixtures Containing Succinic Acid or Its Esters
    Altuntepe, E. and Reinhardt, A. and Brinkmann, J. and Briesemann, T. and Sadowski, G. and Held, C.
    Journal of Chemical and Engineering Data 62 1983-1993 (2017)
    This work provides experimental data and thermodynamic modeling on phase equilibria of binary mixtures that are relevant for esterification reactions. The components under investigation include water, succinic acid (SA), ethanol (EtOH), 1-butanol (1-BuOH), and the diesters of SA, namely, diethyl succinate (DES) and dibutyl succinate (DBS), respectively, as well as the organic solvents acetonitrile (ACN) and tetrahydrofuran (THF). Liquid-liquid equilibria (LLE) of water/DBS were measured at ambient pressure for temperatures between 313 and 353 K. Isobaric vapor-liquid equilibria (VLE) were measured for the binary systems ACN/DES, ACN/DBS, 1-BuOH/DBS, and THF/DBS at pressures of 10 or 20 or 30 kPa. Temperature ranges for the isobaric VLE varied between 300 and 500 K. The measured data and phase equilibria reported in literature were accurately modeled using perturbed-chain statistical associating fluid theory (PC-SAFT). For this purpose, pure-component PC-SAFT parameters, which were not already reported in the literature, were adjusted to experimental literature pure-component data. Applying binary interaction parameters allowed precise phase-equilibrium modeling results of the binary systems under investigation. Two different association schemes for water were used ("2B" and "4C"). Both schemes appeared to be suitable to describe phase equilibria of aqueous mixtures; however, a binary parameter for the Wolbach-Sandler mixing rule was required for aqueous mixtures modeled with the 4C scheme. For LLE modeling the 2B scheme was found to give better modeling results. In general, the 4C association scheme for water yields better results for mixtures with two self-associating components while the 2B association scheme for water should be preferred if mixtures are considered with water and a non-self-associating component. Further, the modeling concept of "induced association" has been investigated and discussed. Especially for mixtures with esters, which are of main importance for esterification mixtures, the induced-association approach turned out to be a more accurate modeling strategy compared to the nonassociative approach. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.7b00005
  • 2017 • 127 Photocatalytic and Magnetic Porous Cellulose-Based Nanocomposite Films Prepared by a Green Method
    Wittmar, A. and Fu, Q. and Ulbricht, M.
    ACS Sustainable Chemistry and Engineering 5 9858-9868 (2017)
    The present work expands our previous studies related to cellulose processing with room-temperature ionic liquids and simultaneous integration of functional nanoparticles toward photocatalytically active and easily recyclable nanocomposite porous films based on a renewable matrix material. Porosity can be tuned by the selection of phase separation conditions for the films obtained from the casting solutions of cellulose in ionic liquids or their mixture with an organic co-solvent. TiO2 nanoparticles confer to the nanocomposite photocatalytic activity, while Fe3O4 nanoparticles make it magnetically active. The photocatalytic activity of the cellulose film containing 10 mg of TiO2 was 1 order of magnitude lower than that of the same amount of pure TiO2 nanopowder, due to the reduction of the active catalytic surface which can be reached by UV irradiation after embedment in the polymer matrix. However, this fixation in a solid polymer support allows facile recovery of the catalyst after use. The rate constant when using the cellulose nanocomposite doped with TiO2 and Fe3O4 (k ≈ 0.0019 min-1) is very close to that for the corresponding composite containing only TiO2 (k ≈ 0.0017 min-1), suggesting that co-doping with Fe3O4 nanoparticles did not diminish the photocatalytic activity of the final composite, which can be easily separated from solution with a magnet. Additionally, by Fe3O4 doping, the composite material's temperature can be homogeneously increased by ∼12 K via exposure to a high-frequency alternating magnetic field (AMF) for 5 min. For an optimal thermal response to AMF, the magnetite nanoparticles have to be homogeneously dispersed within the polymer matrix. The preparation method for the casting solution has been found to play an essential role for the one-step fabrication of multifunctional cellulose-based nanocomposite materials. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acssuschemeng.7b01830
  • 2017 • 126 Predicting the Solubility of CO2 in Toluene + Ionic Liquid Mixtures with PC-SAFT
    Canales, R.I. and Held, C. and Lubben, M.J. and Brennecke, J.F. and Sadowski, G.
    Industrial and Engineering Chemistry Research 56 9885-9894 (2017)
    Perturbed-chain statistical associating fluid theory (PC-SAFT) was applied for modeling the vapor-liquid equilibrium of CO2 + toluene + ionic liquid (IL) mixtures and the molar volume of their liquid phases at temperatures between 298.15 K and 333.15 K and at pressures up to 80 bar. ILs used for this study contain the bis(trifluoromethylsulfonylimide) anion ([Tf2N]-) and imidazolium, pyridinium, thiolanium, and phosphonium cations. The pure-IL PC-SAFT parameters were fit to pure-IL liquid density data. Temperature-dependent binary interaction parameters were fit to binary liquid-liquid equilibrium data (i.e., toluene + IL) obtained from the literature and some points measured for this work. Temperature independent binary interaction parameters were fit to vapor-liquid equilibrium data (CO2 + IL, CO2 + toluene) from the literature. The availability of the pure-IL parameters and binary interaction parameters allowed prediction of CO2 solubility in toluene + IL mixtures with an absolute average relative deviation (AARD) of 6.8%, as well as molar volumes of CO2 + toluene + IL mixtures with an AARD of 5.0%, for the four ternary systems under investigation. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.7b01497
  • 2017 • 125 Pulsed laser ablation in liquids: Impact of the bubble dynamics on particle formation
    Reich, S. and Schönfeld, P. and Wagener, P. and Letzel, A. and Ibrahimkutty, S. and Gökce, B. and Barcikowski, S. and Menzel, A. and dos Santos Rolo, T. and Plech, A.
    Journal of Colloid and Interface Science 489 106-113 (2017)
    Pulsed laser ablation in liquids (PLAL) is a multiscale process, involving multiple mutually interacting phenomena. In order to synthesize nanoparticles with well-defined properties it is important to understand the dynamics of the underlying structure evolution. We use visible-light stroboscopic imaging and X-ray radiography to investigate the dynamics occurring during PLAL of silver and gold on a macroscopic scale, whilst X-ray small angle scattering is utilized to deepen the understanding on particle genesis. By comparing our results with earlier reports we can elucidate the role of the cavitation bubble. We find that symmetry breaking at the liquid-solid interface is a critical factor for bubble motion and that the bubble motion acts on the particle distribution as confinement and retraction force to create secondary agglomerates. © 2016 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2016.08.030
  • 2017 • 124 Pulsed laser ablation of wire-shaped target in a thin water jet: Effects of plasma features and bubble dynamics on the PLAL process
    Dell'Aglio, M. and De Giacomo, A. and Kohsakowski, S. and Barcikowski, S. and Wagener, P. and Santagata, A.
    Journal of Physics D: Applied Physics 50 (2017)
    In this paper, emission spectroscopy and fast imaging surveys during pulsed laser ablation in liquid (PLAL) for nanoparticles (NPs) production have been used, in order to provide further details about the process involved and the potentialities offered by a wire-shaped sample ablated in a flowing water jet. This kind of set-up has been explored because the laser ablation efficiency in water increases when a thin water layer and a wire-shaped target are used. In order to understand the physical processes causing the increasing ablation efficiency, both the laser-induced plasma and bubble dynamics generated in a flowing liquid jet have been analysed. The plasma parameters and the bubble behaviour in such a system have been compared with those observed in conventional PLAL experiments, where either a bulk or a wire-shaped target is immersed in bulk water. From the data presented here it is evidenced that the plasma and shockwave induced during the breakdown process can play a direct role in the ablation efficiency variation observed. With regard to the cavitation bubbles evolving near a free surface (the interface between water and air) it should be noted that these have to be treated with caution as a consequence of the strong influence played in these circumstances by the boundary of the water jet during its expansion dynamics. The effects due to the size of the liquid layer, the presence of the water/air interface, the liquid characteristics, the target shape, the plasma evolution and the bubble dynamics together with their outcomes on the NPs' production, are presented and discussed. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa652a
  • 2017 • 123 Size Quenching during Laser Synthesis of Colloids Happens Already in the Vapor Phase of the Cavitation Bubble
    Letzel, A. and Gökce, B. and Wagener, P. and Ibrahimkutty, S. and Menzel, A. and Plech, A. and Barcikowski, S.
    Journal of Physical Chemistry C 121 5356-5365 (2017)
    Although nanoparticle synthesis by pulsed laser ablation in liquids (PLAL) is gaining wide applicability, the mechanism of particle formation, in particular size-quenching effects by dissolved anions, is not fully understood yet. It is well-known that the size of small primary particles (d ≤ 10 nm), secondary particles (spherical particles d > 10 nm), and agglomerates observed ex situ is effectively reduced by the addition of small amounts of monovalent electrolyte to the liquid prior to laser ablation. In this study, we focus on the particle formation and evolution inside the vapor filled cavitation bubble. This vapor phase is enriched with ions from the afore added electrolyte. By probing the cavitation bubbles' interior by means of small-angle X-ray scattering (SAXS), we are able to examine whether the size quenching reaction between nanoparticles and ions starts already during cavitation bubble confinement or if these reactions are subjected to the liquid phase. We find that particle size quenching occurs already within the first bubble oscillation (approximately 100 μs after laser impact), still inside the vapor phase. Thereby we demonstrate that nanoparticle-ion interactions during PLAL are in fact a gas phase phenomenon. These interactions include size reduction of both primary and secondary particles and a decreased abundance of the latter as shown by in situ SAXS and confirmed by ex situ particle analysis (e.g., static SAXS and TEM). (Figure Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b12554
  • 2017 • 122 Structure-property relationships in hydrogen-bonded liquid crystals
    Pfletscher, M. and Hölscher, S. and Wölper, C. and Mezger, M. and Giese, M.
    Chemistry of Materials 29 8462-8471 (2017)
    The structural impact of hydrogen-donating moieties on the liquid crystalline behavior of hydrogen-bonded assemblies (HBAs) is comprehensively investigated. Therefore, a series of phenol derivatives such as phenol, catechol (CA), resorcinol (RE), hydroquinone (HQ), pyrogallol, hydroxyhydroquinone, and phloroglucinol (PHG) were combined with alkoxyazopyridines (Ap-N) yielding 49 new HBAs, which were studied with respect to their mesomorphic properties. The present study revealed significant differences in the liquid crystalline behavior of the structurally diverse assemblies, ranging from the absence of a mesophase to smectic or nematic phases. In contrast to previous studies a comprehensive crystallographic analysis provides insight into the structure-property relationships of the assemblies and proves a correlation between the supramolecular architecture and the macroscopic properties (=liquid crystallinity). More specifically, comparison of the single crystal data with the 2D X-ray diffraction patterns indicates that linear assemblies tend to form crystalline or smectic phases (for the HQ and RE, respectively), while a bent-shaped assembly yields nematic phases (for CA and PHG). Furthermore, our results suggest that segregation of aliphatic and aromatic segments, as observed in the solid state structures, supports the formation of stable mesophases. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.7b03182
  • 2017 • 121 Theoretical investigation of flash vaporisation in a screw expander
    Vasuthevan, H. and Brümmer, A.
    IOP Conference Series: Materials Science and Engineering 232 (2017)
    In the present study flash vaporisation of liquid injection in a twin screw expander for a Trilateral Flash Cycle (TFC) is examined theoretically. The TFC process comprises a pressure increase in the working fluid, followed by heating the liquid close to boiling point. The hot liquid is injected into the working chamber of a screw expander. During this process the pressure of the liquid drops below the saturation pressure, while the temperature of the liquid remains virtually constant. Hence the liquid is superheated and in a metastable state. The liquid jet seeks to achieve a stable state in thermodynamic equilibrium and is therefore partially vaporised. This effect is referred to as flash vaporisation. Accordingly, a two-phase mixture, consisting of vapour and liquid, exists in the working chamber. Thermodynamic simulations were carried out using water as the working fluid for representative screw expander geometry. The simulations presented are performed from two different aspects during the filling process of a screw expander. The first case is the vaporisation of the injected liquid in a state of thermodynamic equilibrium, whereby the two-phase mixture is treated entirely as a compressible and homogeneous gas. The second case considers flashing efficiency. It describes the quantity of flashed vapour and consists of a liquid and vapour domain. Both models are compared and analysed with respect to the operational behaviour of a screw expander. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/232/1/012077
  • 2016 • 120 Analysis of liquid water formation in polymer electrolyte membrane (PEM) fuel cell flow fields with a dry cathode supply
    Gößling, S. and Klages, M. and Haußmann, J. and Beckhaus, P. and Messerschmidt, M. and Arlt, T. and Kardjilov, N. and Manke, I. and Scholta, J. and Heinzel, A.
    Journal of Power Sources 306 658-665 (2016)
    PEM fuel cells can be operated within a wide range of different operating conditions. In this paper, the special case of operating a PEM fuel cell with a dry cathode supply and without external humidification of the cathode, is considered. A deeper understanding of the water management in the cells is essential for choosing the optimal operation strategy for a specific system. In this study a theoretical model is presented which aims to predict the location in the flow field at which liquid water forms at the cathode. It is validated with neutron images of a PEM fuel cell visualizing the locations at which liquid water forms in the fuel cell flow field channels. It is shown that the inclusion of the GDL diffusion resistance in the model is essential to describe the liquid water formation process inside the fuel cell. Good agreement of model predictions and measurement results has been achieved. While the model has been developed and validated especially for the operation with a dry cathode supply, the model is also applicable to fuel cells with a humidified cathode stream. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2015.12.060
  • 2016 • 119 Chemical Surface Characterization of Activated Carbons by Adsorption Excess of Probe Molecules
    Treese, J. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemical Engineering and Technology 39 1144-1150 (2016)
    Activated carbons are one of the most common industrial adsorbents in liquid-phase applications. It is known that the surface groups of the activated carbon can have a significant influence on the adsorption process from the liquid phase. Therefore, it is desirable to measure surface groups on activated carbons. This opens up the possibility to use group-contribution methods to model and predict adsorption isotherms. An idea is presented to characterize the inner surface of activated carbons by three types of surface groups: aromatic, polar, and nonpolar surface groups. The amounts of these surface groups were calculated from excess adsorption isotherms of probe molecules on ten activated carbons. This lays the groundwork for further simulation studies of liquid-phase adsorption using group-contribution methods. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ceat.201500571
  • 2016 • 118 Continuous multigram nanoparticle synthesis by high-power, high-repetition-rate ultrafast laser ablation in liquids
    Streubel, R. and Barcikowski, S. and Gökce, B.
    Optics Letters 41 1486-1489 (2016)
    Utilizing a novel laser system consisting of a 500 W, 10 MHz, 3 ps laser source which is fully synchronized with a polygon scanner reaching scanning speeds up to 500 m/s, we explore the possibilities to increase the productivity of nanoparticle synthesis by laser ablation in liquids. By exploiting the high scanning speed, laser-induced cavitation bubbles are spatially bypassed at high repetition rates and continuous multigram ablation rates up to 4 g/h are demonstrated for platinum, gold, silver, aluminum, copper, and titanium. Furthermore, the applicable, ablation-effective repetition rate is increased by two orders ofmagnitude.The ultrafast ablation mechanisms are investigated for different laser fluences, repetition rates, interpulse distances, and ablation times, while the resulting trends are successfully described by validating a model developed for ultrafast laser ablation in air to hold in liquids as well. © 2016 Optical Society of America.
    view abstractdoi: 10.1364/OL.41.001486
  • 2016 • 117 Debris-free rear-side picosecond laser ablation of thin germanium wafers in water with ethanol
    Zhang, D. and Gökce, B. and Sommer, S. and Streubel, R. and Barcikowski, S.
    Applied Surface Science 367 222-230 (2016)
    In this paper, we perform liquid-assisted picosecond laser cutting of 150 μm thin germanium wafers from the rear side. By investigating the cutting efficiency (the ability to allow an one-line cut-through) and quality (characterized by groove morphologies on both sides), the pros and cons of this technique under different conditions are clarified. Specifically, with laser fluence fixed, repetition rate and scanning speed are varied to show quality and efficiency control by means of laser parameter modulation. It is found that low repetition rate ablation in liquid gives rise to a better cut quality on the front side than high repetition rate ablation since it avoids dispersed nanoparticles redeposition resulting from a bubble collapse, unlike the case of 100 kHz which leads to large nanorings near the grooves resulting from a strong interaction of bubbles and the case of 50 kHz which leads to random cutting due to the interaction of the former pulse induced cavitation bubble and the subsequent laser pulse. Furthermore, ethanol is mixed with pure distilled water to assess the liquid's impact on the cutting efficiency and cutting quality. The results show that increasing the ethanol fraction decreases the ablation efficiency but simultaneously, greatly improves the cutting quality. The improvement of cut quality as ethanol ratio increases may be attributed to less laser beam interference by a lower density of bubbles which adhere near the cut kerf during ablation. A higher density of bubbles generated from ethanol vaporization during laser ablation in liquid will cause stronger bubble shielding effect toward the laser beam propagation and therefore result in less laser energy available for the cut, which is the main reason for the decrease of cut efficiency in water-ethanol mixtures. Our findings give an insight into under which condition the rear-side laser cutting of thin solar cells should be performed: high repetition, pure distilled water and high laser power are favorable for high-speed rough cutting but the cut kerf suffers from strong side effects of ripples, nanoredeposition occurrence, while low laser power at low repetition rate (10 kHz), mixed solution (1 wt% ethanol in water) and moderate scanning speed (100 μm/s) are preferable for ultrafine high-quality debris-free cutting. The feasibility of high-quality cut is a good indication of using rear laser ablation in liquid to cut thinner wafers. More importantly, this technique spares any post cleaning steps to reduce the risk to the contamination or crack of the thin wafers. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2016.01.071
  • 2016 • 116 Designing molecular complexes using free-energy derivatives from liquid-state integral equation theory
    Mrugalla, F. and Kast, S.M.
    Journal of Physics Condensed Matter 28 (2016)
    Complex formation between molecules in solution is the key process by which molecular interactions are translated into functional systems. These processes are governed by the binding or free energy of association which depends on both direct molecular interactions and the solvation contribution. A design goal frequently addressed in pharmaceutical sciences is the optimization of chemical properties of the complex partners in the sense of minimizing their binding free energy with respect to a change in chemical structure. Here, we demonstrate that liquid-state theory in the form of the solute-solute equation of the reference interaction site model provides all necessary information for such a task with high efficiency. In particular, computing derivatives of the potential of mean force (PMF), which defines the free-energy surface of complex formation, with respect to potential parameters can be viewed as a means to define a direction in chemical space toward better binders. We illustrate the methodology in the benchmark case of alkali ion binding to the crown ether 18-crown-6 in aqueous solution. In order to examine the validity of the underlying solute-solute theory, we first compare PMFs computed by different approaches, including explicit free-energy molecular dynamics simulations as a reference. Predictions of an optimally binding ion radius based on free-energy derivatives are then shown to yield consistent results for different ion parameter sets and to compare well with earlier, orders-of-magnitude more costly explicit simulation results. This proof-of-principle study, therefore, demonstrates the potential of liquid-state theory for molecular design problems. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/34/344004
  • 2016 • 115 Diode laser-based standoff absorption measurement of water film thickness in retro-reflection
    Pan, R. and Brocksieper, C. and Jeffries, J.B. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    A dual-wavelength diode laser-based absorption sensor for standoff point measurements of water film thickness on an opaque surface is presented. The sensor consists of a diode laser source, a foil as backscattering target, and off-axis paraboloids for collecting the fraction of the laser radiation transmitted through the liquid layer via retro-reflection. Laser wavelengths in the near infrared at 1412 and 1353 nm are used where the temperature dependence of the liquid water absorption cross section is known. The lasers are fiber coupled and the detection of the retro-reflected light was accomplished through a multimode fiber and a single photodiode using time-division multiplexing. The water film thickness at a given temperature was determined from measured transmittance ratios at the two laser wavelengths. The sensor concept was first validated with measurement using a temperature-controlled calibration cell providing liquid layers of variable and known thickness between 100 and 1000 µm. Subsequently, the sensor was demonstrated successfully during recording the time-varying thickness of evaporating water films at fixed temperatures. The film thickness was recorded as a function of time at three temperatures down to 50 µm. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00340-016-6524-7
  • 2016 • 114 Efficient additive manufacturing production of oxide- and nitride-dispersion-strengthened materials through atmospheric reactions in liquid metal deposition
    Springer, H. and Baron, C. and Szczepaniak, A. and Jägle, E.A. and Wilms, M.B. and Weisheit, A. and Raabe, D.
    Materials and Design 111 60-69 (2016)
    Despite being extremely attractive compounds for strengthening, oxides and nitride particles have found only limited use in metallic materials design, as obtaining appropriate size and dispersion up to now necessitates production by time- and cost-intensive powder metallurgy processes. Here we present an alternative production method, based on the oxide and nitride formation during liquid-metal-deposition procedures in oxygen and/or nitrogen containing atmospheres. Rapid solidification of the small liquid zone suppresses floatation and agglomeration of particles, while subsequent thermo-mechanical treatments densify the material and aids particle dispersion. The in-situ particle formation coupled to the high deposition rates ensures a drastically shortened production chain. The feasibility of the method is exemplarily demonstrated on austenitic stainless steel and commercially available deposition techniques as used in additive manufacturing, performed without shielding gas but instead at air. Even without substantial optimisation of processes and material, &gt; 2 vol.% of hard and stable Cr2N particles with sizes down to 80 nm could be evenly dispersed, resulting in pronounced strengthening at both room temperature and 700 °C without significant loss in ductility. Future possibilities for creating novel generations of cost effective and lean high strength materials, especially for high temperature applications, are outlined and discussed. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.matdes.2016.08.084
  • 2016 • 113 Efficient liquid metallurgy synthesis of Fe-TiB2 high modulus steels via in-situ reduction of titanium oxides
    Baron, C. and Springer, H. and Raabe, D.
    Materials and Design 97 357-363 (2016)
    We studied the in-situ reduction of Ti oxides by Al as an alternative and cost effective route for the liquid metallurgical synthesis of low density, high stiffness steels (high modulus steels) containing about 10 vol.% TiB2. TiO2, TiO1.83 and TiO were inserted via iron tubes into Fe-B melts, with Al either premixed with the oxide powders or liquid in the melt. Depending on Ti oxide type and location of the redox partner Al, greatly differing reaction kinetics, slag formation and corresponding microstructures of the high modulus steels were observed. TiO1.83 and TiO premixed with Al showed the highest TiB2 yield in the cast steel and are thus favourable candidates for the cost effective production of high modulus steels. Based on our findings, a novel synthesis process is proposed, based on filling wire injection into a continuous casting process, allowing the utilisation of the additionally formed oxide particles for the further improvement of the property profile of high modulus steels. © 2016 Elsevier Ltd.
    view abstractdoi: 10.1016/j.matdes.2016.02.076
  • 2016 • 112 Employing ionic liquids to deposit cellulose on PET fibers
    Textor, T. and Derksen, L. and Gutmann, J.S.
    Carbohydrate Polymers 146 139-147 (2016)
    Several ionic liquids are excellent solvents for cellulose. Starting from that finishing of PET fabrics with cellulose dissolved in ionic liquids like 1-ethyl-3-methyl imidazolium acetate, diethylphosphate and chloride, or the chloride of butyl-methyl imidazolium has been investigated. Finishing has been carried out from solutions of different concentrations, using microcrystalline cellulose or cotton and by employing different cross-linkers. Viscosity of solutions has been investigated for different ionic liquids, concentrations, cellulose sources, linkers and temperatures. Since ionic liquids exhibit no vapor pressure, simple pad-dry-cure processes are excluded. Before drying the ionic liquid has to be removed by a rinsing step. Accordingly rinsing with fresh ionic liquid followed by water or the direct rinsing with water have been tested. The amount of cellulose deposited has been investigated by gravimetry, zinc chloride iodine test as well as reactive dyeing. Results concerning wettability, water up-take, surface resistance, wear-resistance or washing stability are presented. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbpol.2016.03.053
  • 2016 • 111 In Situ Hydrocracking of Fischer-Tropsch Hydrocarbons: CO-Prompted Diverging Reaction Pathways for Paraffin and α-Olefin Primary Products
    Duyckaerts, N. and Trotuş, I.-T. and Swertz, A.-C. and Schüth, F. and Prieto, G.
    ACS Catalysis 6 4229-4238 (2016)
    The single-step production of wax-free liquid hydrocarbons from syngas (H2 + CO) via integration of Fischer-Trospch (FT) and hydrocracking catalysts represents an attractive approach toward process intensification in compact gas-to-liquid technologies. Despite current, intensive efforts on the development of hybrid (multifunctional) catalysts to this end, not much is known about the reactivity of different FT primary products on hydrocracking catalysts under syngas. Using model compounds, the individual and collective reactivities of n-paraffin and α-olefin FT primary products were systematically studied on a Pt/nano-H-ZSM-5 hydrocracking catalyst under H2 (standard hydrocracking) and syngas (in situ hydroprocessing) atmospheres. Under H2, both reactants show indistinguishable reactivity as rapid olefin hydrogenation precedes hydrocracking. Under syngas, however, inhibition of (de)hydrogenation functionalities by CO poisoning of metal sites leads to a notable divergence of the reaction pathways for n-paraffins and α-olefins. Under these conditions, α-olefins showed enhanced reactivity, as an initial dehydrogenative activation step is not required, and contributed to moderate secondary cracking, likely via enhanced competitive adsorption on the acid sites. Besides, CO poisoning restored the intrinsic activity of the zeolite for the oligomerization of short-chain (α-)olefins, providing an additional net chain-growth pathway, which contributes to reducing the overall yield to undesired gas (C4-) hydrocarbons. These findings emphasize the key role of not only the chain-length distribution, but also the olefinic content of the FT primary hydrocarbons for the ultimate product distribution, and suggest guidelines for the design of multifunctional catalysts for the single-step synthesis of liquid hydrocarbons from syngas. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00904
  • 2016 • 110 Influence of electrolytes on liquid-liquid equilibria of water/1-butanol and on the partitioning of 5-hydroxymethylfurfural in water/1-butanol
    Mohammad, S. and Grundl, G. and Müller, R. and Kunz, W. and Sadowski, G. and Held, C.
    Fluid Phase Equilibria 428 102-111 (2016)
    The influence of electrolytes on liquid-liquid equilibria (LLE) of water/1-butanol and on the partitioning of 5-hydroxymethylfurfural (HMF) between water-rich and 1-butanol-rich phases was investigated in this study. For that purpose, the LLE of the ternary systems water/1-butanol/HMF, water/1-butanol/salt, and the LLE of the quaternary system water/1-butanol/HMF/salt were measured at 298.15 K under atmospheric pressure. The investigated salts were composed of one of the anions Cl−, CH3COO−, NO3 − and SO4 2− and either Li+ or Na+. By investigating the LLE of the system water/1-butanol/salt it was found that 1-butanol was salted-out from the aqueous phase by all salts, and the strength of the salting-out increased in the following order NO3 − <  CH3COO− ≈ Cl− <  SO4 2−, independently of the cation. Based on the LLE data, the partition coefficient KHMF w of HMF between 1-butanol and aqueous phase was determined. Li2SO4 caused a pronounced salting-out of HMF from the aqueous phase, whereas only a moderate influence was observed for NaCl and CH3COONa. LiCl even caused a salting-in at LiCl molalities above 6 mol/kgH2O. electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT) was successfully used to model the influence of salts on the LLE water/1-butanol. Without fitting parameters to LLE data of the quaternary system water/1-butanol/HMF/salt, ePC-SAFT allowed predicting the salt influence on the partitioning of HMF in these systems in good agreement with the experimental data. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2016.05.001
  • 2016 • 109 Influence of the liquid on femtosecond laser ablation of iron
    Kanitz, A. and Hoppius, J.S. and Gurevich, E.L. and Ostendorf, A.
    Physics Procedia 83 114-122 (2016)
    Ultrashort pulse laser ablation has become a very important industrial method for highly precise material removal ranging from sensitive thin film processing to drilling and cutting of metals. Over the last decade, a new method to produce pure nanoparticles emerged from this technique: Pulsed Laser Ablation in Liquids (PLAL). By this method, the ablation of material by a laser beam is used to generate a metal vapor within the liquid in order to obtain nanoparticles from its recondensation process. It is well known that the liquid significantly alters the ablation properties of the substrate, in our case iron. For example, the ablation rate and crater morphology differ depending on the used liquid. We present our studies on the efficiency and quality of ablated grooves in water, methanol, acetone, ethanol and toluene. The produced grooves are investigated by means of white-light interferometry, EDX and SEM. © 2016 The Authors.
    view abstractdoi: 10.1016/j.phpro.2016.08.022
  • 2016 • 108 Measurements of liquid film thickness, concentration, and temperature of aqueous urea solution by NIR absorption spectroscopy
    Pan, R. and Jeffries, J.B. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 122 (2016)
    A multi-wavelength near-infrared (NIR) diode laser absorption sensor has been developed and demonstrated for real-time monitoring of the thickness, solute concentration, and temperature of thin films of urea–water solutions. The sensor monitors the transmittance of three near-infrared diode lasers through the thin liquid film. Film thickness, urea mass fraction, and liquid temperature were determined from measured transmittance ratios of suitable combinations of lasers. Available laser wavelengths were selected depending on the variation of the NIR absorption spectrum of the solution with temperature and solute concentration. The spectral database was measured by a Fourier transform infrared spectrometer in the range 5500–8000 cm−1 for urea solutions between 5 and 40 wt% and temperatures between 298 and 338 K. A prototype sensor was constructed, and the sensor concept was first validated with measurements using a calibration cell providing liquid layers of variable thickness (200–1500 μm), urea mass fraction (5–40 wt%) and temperature (298–318 K). Temporal variations of film thickness and urea concentration were captured during the constant-temperature evaporation of a liquid film deposited on an optically polished heated quartz flat. © Springer-Verlag Berlin Heidelberg 2016.
    view abstractdoi: 10.1007/s00340-015-6290-y
  • 2016 • 107 Modeling the density of ionic liquids with ePC-SAFT
    Ji, X. and Held, C.
    Fluid Phase Equilibria 410 9-22 (2016)
    ePC-SAFT was used to model the densities of ionic liquids (ILs) up to high pressures and temperatures. The ILs under consideration contained one of the IL-cations [Cnmim]+, [Cnpy]+, [Cnmpy]+, [Cnmpyr]+ or [THTDP]+, and one of the IL-anions [Tf2N]-, [PF6]-, [BF4]-, [tfo]-, [DCA]-, [SCN]-, [C1SO4]-, [C2SO4]-, [eFAP]-, Cl-, [Ac]- or Br-, respectively. Within the ePC-SAFT framework, IL-ion specific parameters were applied that are valid independent of the IL they are part of. Each IL-ion was modeled as a non-spherical species exerting repulsive, dispersive and Coulomb forces. The ePC-SAFT parameters for [Cnmim]+ (n = 2, 4, 6 and 8), [Tf2N]-, [PF6]-, and [BF4]- were taken from our previous work (Fluid Phase Equilibria 2012 (335) 64-73). Based on these parameters, all parameters of the other IL-ions were fitted to experimental density of pure ILs up to high pressures in a broad temperature range. Being provided with ion-specific and linearly molecular-weight-dependent parameters, ePC-SAFT allows reliably representing/predicting pure-IL and mixed-IL density up to high pressures. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2015.11.014
  • 2016 • 106 Modelling interfacial properties of ionic liquids with ePC-SAFT combined with density gradient theory
    Shen, G. and Held, C. and Lu, X. and Ji, X.
    Molecular Physics 114 2492-2499 (2016)
    Highlights: Combination of ePC-SAFT with density gradient theory Calculation of interfacial properties of pure ILs in broad temperature range Quantitative predictions of surface tensions for ILs not used in κ parameter fitting © 2016 Informa UK Limited, trading as Taylor & Francis Group.
    view abstractdoi: 10.1080/00268976.2016.1171408
  • 2016 • 105 Non-monotonic effect of confinement on the glass transition
    Varnik, F. and Franosch, T.
    Journal of Physics Condensed Matter 28 (2016)
    The relaxation dynamics of glass forming liquids and their structure are influenced in the vicinity of confining walls. This effect has mostly been observed to be a monotonic function of the slit width. Recently, a qualitatively new behaviour has been uncovered by Mittal and coworkers, who reported that the single particle dynamics in a hard-sphere fluid confined in a planar slit varies in a non-monotonic way as the slit width is decreased from five to roughly two particle diametres (Mittal et al 2008 Phys. Rev. Lett. 100 145901). In view of the great potential of this effect for applications in those fields of science and industry, where liquids occur under strong confinement (e.g. nano-technology), the number of researchers studying various aspects and consequences of this non-monotonic behaviour has been rapidly growing. This review aims at providing an overview of the research activity in this newly emerging field. We first briefly discuss how competing mechanisms such as packing effects and short-range attraction may lead to a non-monotonic glass transition scenario in the bulk. We then analyse confinement effects on the dynamics of fluids using a thermodynamic route which relates the single particle dynamics to the excess entropy. Moreover, relating the diffusive dynamics to the Widom's insertion probability, the oscillations of the local dynamics with density at moderate densities are fairly well described. At high densities belonging to the supercooled regime, however, this approach breaks down signaling the onset of strongly collective effects. Indeed, confinement introduces a new length scale which in the limit of high densities and small pore sizes competes with the short-range local order of the fluid. This gives rise to a non-monotonic dependence of the packing structure on confinement, with a corresponding effect on the dynamics of structural relaxation. This non-monotonic effect occurs also in the case of a cone-plate type channel, where the degree of confinement varies with distance from the apex. This is a very promising issue for future research with the possibility of uncovering the existence of alternating glassy and liquid-like domains. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/28/13/133001
  • 2016 • 104 Optimization of artificial ground freezing in tunneling in the presence of seepage flow
    Marwan, A. and Zhou, M.-M. and Zaki Abdelrehim, M. and Meschke, G.
    Computers and Geotechnics 75 112-125 (2016)
    Artificial ground freezing is an environmentally friendly technique to provide temporary excavation support and groundwater control during tunnel construction under difficult geological and hydrological ground conditions. Evidently, groundwater flow has a considerable influence on the freezing process. Large seepage flow may lead to large freezing times or even may prevent the formation of a closed frozen soil body. For safe and economic design of freezing operations, this paper presents a coupled thermo-hydraulic finite element model for freezing soils integrated within an optimization algorithm using the Ant Colony Optimization (ACO) technique to optimize ground freezing in tunneling by finding the optimal positions of the freeze pipe, considering seepage flow. The simulation model considers solid particles, liquid water and crystal ice as separate phases, and the mixture temperature and liquid pressure as primary field variables. Through two fundamental physical laws and corresponding state equations, the model captures the most relevant couplings between the phase transition associated with latent heat effect, and the liquid transport within the pores. The numerical model is validated by means of laboratory results considering different scenarios for seepage flow. As demonstrated in numerical simulations of ground freezing in tunneling in the presence of seepage flow connected with the ACO optimization algorithm, the optimized arrangement of the freeze pipes may lead to a substantial reduction of the freezing time and of energy costs. © 2016.
    view abstractdoi: 10.1016/j.compgeo.2016.01.004
  • 2016 • 103 Phase-field simulation of liquid phase migration in the WC-Co system during liquid phase sintering
    Cheng, K. and Zhang, L. and Schwarze, C. and Steinbach, I. and Du, Y.
    International Journal of Materials Research 107 309-314 (2016)
    Liquid phase sintering is a process for forming high performance, multiple-phase components from powders. The process includes very complex interactions between various mass transportation phenomena, among which the liquid phase migration represents an important one in the aspect of forming a gradient structure in cemented carbide. In the present work, phase-field simulation of the liquid phase migration phenomenon during liquid phase sintering is performed in the WC - Co based cemented carbide. The simulation results are analyzed and compared with the experimentally determined key factors of microstructural evolution, such as contiguity and liquid phase migration rate. The diffusion-controlled solution-precipitation mechanism of the liquid phase migration process in the cemented carbide system is confirmed from the current simulation result, which provides deeper understanding of the microstructural evolution during the liquid phase migration process. These simulations can offer guidance in preventing the liquid phase migration process during liquid phase sintering of cellular cemented carbide. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.111353
  • 2016 • 102 Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids
    Streubel, R. and Bendt, G. and Gökce, B.
    Nanotechnology 27 (2016)
    The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h-1 for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s-1. This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/20/205602
  • 2016 • 101 Preformed 2 nm Ag Clusters Deposited into Ionic Liquids: Stabilization by Cation-Cluster Interaction
    Engemann, D.C. and Roese, S. and Hövel, H.
    Journal of Physical Chemistry C 120 6239-6245 (2016)
    Recently, the formation of nanoparticles by sputter deposition of metal atoms onto the surface of room-temperature ionic liquids (RTIL) was reported; however, the growth and stabilization mechanism within the ionic liquid are still in discussion. Here, we present another approach by depositing Ag clusters with a diameter of 2 nm preformed in a supersonic nozzle expansion into an ionic liquid. Thus, the properties and size distribution of the clusters are well-known before deposition. The mixture of the clusters with the ionic liquid is investigated in situ and ex situ with UV/vis measurements and X-ray absorption near-edge structure (XANES) spectroscopy at the Ag L2 edge. The plasmon resonances of the Ag clusters show that up to 10 μg/mL, the clusters stay separated in the RTIL and suggest an interaction process between the cations in the liquid and the surfaces of the clusters, which is confirmed by a shift of the absorption edge in the XANES measurements. For higher cluster concentration and on a longer time scale, the stabilization ability of ionic liquids can be investigated. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b12120
  • 2016 • 100 Salt influence on MIBK/water liquid-liquid equilibrium: Measuring and modeling with ePC-SAFT and COSMO-RS
    Mohammad, S. and Held, C. and Altuntepe, E. and Köse, T. and Gerlach, T. and Smirnova, I. and Sadowski, G.
    Fluid Phase Equilibria 416 83-93 (2016)
    In biotechnological processes, salts might be present during reaction steps and in downstream processes. Salts are known to have a strong impact on phase equilibria of aqueous systems.In this work, the liquid-liquid equilibria (LLE) of ternary salt/MIBK/water mixtures were measured at 298.15 K and 1 bar up to the salt solubility limit. The salts studied in this work were NaCl, LiCl, KCl, NaNO3, LiNO3, Na2SO4, CH3COONa, and CH3COOLi. From these LLE measurements it was found that a high amount of salt is dissolved in the aqueous phase whereas only a very small amount of salt was detected in the MIBK phase. Further, the salting-out behavior of MIBK from the aqueous phase upon addition of different salts was investigated to study ion-specific effects.Two ion-specific models, ePC-SAFT and an extended version of COSMO-RS for electrolytes were used for modeling the binary system MIBK/water and ternary salt/MIBK/water systems. In case of the COSMO-RS based approach, the modeling results were fully predictive. In contrast, ion-specific binary interaction parameters between MIBK and ions were fitted to experimental LLE data of the ternary systems salt/MIBK/water when using ePC-SAFT. The results show that the COSMO-RS based approach allows for predicting the salt influence on LLE with acceptable accuracy, whereas ePC-SAFT allows for almost quantitative correlations of experimental data. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2015.11.018
  • 2016 • 99 Thermodynamic and kinetic solid-liquid interface properties from transition path sampling
    Şopu, D. and Rogal, J. and Drautz, R.
    Journal of Chemical Physics 145 (2016)
    We perform transition path sampling simulations to determine two of the key quantities in solidification, the solid-liquid interface energy and velocity, in a Lennard-Jones system. Our approach is applicable to a wide range of temperature and pressure conditions, at the melting temperature and out-of-equilibrium. We show that small system sizes are sufficient for good values of interface energies and velocities. The transition path sampling method thus offers an attractive and robust alternative for the evaluation of solid-liquid interface properties. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4972583
  • 2015 • 98 A Generalized Finite Element Method for hydro-mechanically coupled analysis of hydraulic fracturing problems using space-time variant enrichment functions
    Meschke, G. and Leonhart, D.
    Computer Methods in Applied Mechanics and Engineering 290 438-465 (2015)
    In computational simulations of hydraulic fracturing problems, consideration of interactions between the propagating fracture zone and the fluid flow through the porous material requires an appropriate up-scaling procedure from the spatial scale of the local crack, which usually is much smaller compared to the scale of typical finite elements in poromechanics problems. This scale transition refers to both the displacement field (discontinuity across cracks) as well as to the fluid flow (accelerated flow within cracks and the interaction with the flow in the bulk material). To resolve the small and the large scale portion of the solution, the Generalized Finite Element Method (GFEM) exploiting the partition of unity property of shape functions is used. Accordingly, the displacements u and the liquid pressure p<inf>l</inf> are locally enriched to better resolve their distribution in the vicinity of cracks by means of an additive decomposition into a large and a small scale part. As far as the representation of cracks is concerned, the Extended Finite Element Method (XFEM) is used by enriching the displacement field by means of a jump function as well as crack tip functions. In the framework of the GFEM physically motivated enrichment functions for the local enrichment of the C1 discontinuity of the liquid pressure field across cracks are proposed in the paper. The space and time variant analytical solutions obtained from the 1D transient response of saturated porous materials subjected to the liquid pressure within the crack are applied as enrichment functions to locally improve the approximation of the liquid pressure field at discontinuities. Applying these space and time variant functions, which are the exact solutions of the pressure field in the vicinity of cracks, as local enrichment functions lead to a significant improvement in the local approximation of the pressure field at discontinuities. The new GFEM model is formulated in a poromechanics framework for fully saturated porous materials. Representative analyses demonstrate the improvement of the solution quality compared to existing FEM and XFEM models. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.cma.2015.03.005
  • 2015 • 97 Continuous electrophoretic deposition and electrophoretic mobility of ligand-free, metal nanoparticles in liquid flow
    Koenen, S. and Streubel, R. and Jakobi, J. and Schwabe, K. and Krauss, J.K. and Barcikowski, S.
    Journal of the Electrochemical Society 162 D174-D179 (2015)
    Direct current electrophoretic deposition (DC-EPD) of ligand-free metal nanoparticles in a flow-through reactor is studied by analyzing the educt colloid and the outflow of the flow through chamber while the concentration of the colloid and the strength of the electric field is varied.Metal nanoparticles synthesized by pulsed laser ablation in liquid (PLAL) are used to ensure that the colloidal nanoparticle surface is free of any ligands and that the colloid's stability and movement in an electric field is solely influenced by electrostatic forces. Electrophoretic mobility and deposition kinetics of these ligand-free nanoparticles on plain surfaces are examined for different electric field strengths. Additionally, a continuous liquid flow DC-EPD process is presented and optimized regarding deposition rate, colloid stability, and liquid flow rate. The reported parameter window for high deposition rates of nanoparticles without a negative impact on the colloid, allows to define an efficient stationary EPD process suitable for high throughput applications. © 2015 The Electrochemical Society.
    view abstractdoi: 10.1149/2.0811504jes
  • 2015 • 96 Different recycling concepts in the homogeneously catalysed synthesis of terpenyl amines
    Färber, T. and Schulz, R. and Riechert, O. and Zeiner, T. and Górak, A. and Sadowski, G. and Behr, A.
    Chemical Engineering and Processing: Process Intensification 98 22-31 (2015)
    The homogeneously catalysed hydroamination reaction of β-myrcene with morpholine to terpenyl amines was investigated. Two different techniques to avoid catalyst losses from the liquid phase were applied: Thermomorphic Multicomponent Solvent (TMS)-systems, in which the temperature-sensitivity of the binodal curve is exploited and Liquid-Liquid Two-Phase (LLTP)-systems, in which the reaction happens at the phase interface. The highest β-myrcene conversion of more than 90% and a product yield of more than 80% was measured in a TMS-system consisting of n-heptane and acetonitrile. The same conversion was reached in a LLTP-system consisting of water and β-myrcene, whereas a product yield of 55% was achieved. Experimental data of the liquid-liquid phase equilibria resulted in thermodynamic fundamentals for the design of chemical reactors for the production of amines. Theoretical prediction of equilibrium compositions using PC-SAFT equation of state agree excellent with measured values. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.cep.2015.09.016
  • 2015 • 95 Elasticity of interfacial rafts of hard particles with soft shells
    Knoche, S. and Kierfeld, J.
    Langmuir 31 5364-5376 (2015)
    We study an elasticity model for compressed protein monolayers or particle rafts at a liquid interface. Based on the microscopic view of hard-core particles with soft shells, a bead-spring model is formulated and analyzed in terms of continuum elasticity theory. The theory can be applied, for example, to hydrophobin-coated air-water interfaces or, more generally, to liquid interfaces coated with an adsorbed monolayer of interacting hard-core particles. We derive constitutive relations for such particle rafts and describe the buckling of compressed planar liquid interfaces as well as their apparent Poisson ratio. We also use the constitutive relations to obtain shape equations for pendant or buoyant capsules attached to a capillary, and to compute deflated shapes of such capsules. A comparison with capsules obeying the usual Hookean elasticity (without hard cores) reveals that the hard cores trigger capsule wrinkling. Furthermore, it is shown that a shape analysis of deflated capsules with hard-core/soft-shell elasticity gives apparent elastic moduli which can be much higher than the original values if Hookean elasticity is assumed. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b00083
  • 2015 • 94 Experimental and numerical analysis of liquid-forming
    Zimmer, J. and Klein, D. and Stommel, M.
    Key Engineering Materials 651-653 842-847 (2015)
    The packaging of liquid products is conventionally realized by using two production stages, which are the stretch blow molding and the filling. In the stretch blow molding process, hot polyethylene terephthalate (PET) preforms are inflated by pressurized air into a cavity to form plastic bottles. In a follow-up process, these packages are filled by a separate machine with the desired liquid product. In contrast to that, liquid-forming combines the blowing and filling stages by directly using the liquid product to form a plastic bottle. Through this substitution, two main challenges arise. Firstly, there are significant inertia effects through the liquid mass, leading to additional reaction forces and a spatially inhomogeneous pressure distribution inside the preform. Secondly, the heat transfer between preform and fluid is drastically increased. Because of this cooling effect, a specific combination of forming speed as well as initial preform and liquid temperatures is necessary to avoid thermally induced preform rupture. This is based on the fact that the formability of PET rapidly declines below its glass transition temperature (Tg). Consequently, a process control requires the knowledge of how the process parameters influence the preform cooling. In this paper, a numerical simulation of the liquid-forming process (LF) is introduced including the preform cooling during forming. In addition, the strain-dependent self-heating effect of PET is implemented. Process experiments under different parameter combinations are conducted using simplified bottle geometry. Through a comparison of the results from experiments and from simulation, the influence of process parameters on the temperature drop and thus on thermally induced failure is determined. In this way, process understanding and control are increased. © (2015) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2015 • 93 Experimental evidence for two distinct deeply supercooled liquid states of water - Response to "comment on 'Water's second glass transition"', by G.P. Johari, Thermochim. Acta (2015)
    Stern, J. and Seidl, M. and Gainaru, C. and Fuentes-Landete, V. and Amann-Winkel, K. and Handle, P.H. and Köster, K.W. and Nelson, H. and Böhmer, R. and Loerting, T.
    Thermochimica Acta 617 200-207 (2015)
    Recently, our earlier data which led us to conclude that deeply supercooled water displays a second glass transition (Amann-Winkel et al., 2013) was reinterpreted (Johari, 2015). In particular, the increase in heat capacity observed for high-density amorphous ice (HDA) samples at 116 K was reinterpreted to indicate sub-T<inf>g</inf> features of low-density amorphous ice's (LDA's) glass transition. We reply to the criticism in detail and report an experiment triggered by the comment on our work. This experiment unequivocally confirms our original interpretation of the observations and reinforces the case for water's second glass transition, its polyamorphism, and the observation of two distinct ultraviscous states of water differing by about 25% in density. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tca.2015.08.030
  • 2015 • 92 Experimental investigation and numerical simulation of liquid supported stretch blow molding
    Zimmer, J. and Chauvin, G. and Stommel, M.
    Polymer Engineering and Science 55 933-944 (2015)
    An innovative production process for PET bottles ad containers is analyzed in this article. Liquid Bi-Orientation (LBO) is a liquid supported stretch blow molding (SBM), which combines the separate blowing and filling phases of conventional SBM. The process modification is mainly characterized by forming the bottle using the desired liquid product instead of pressurized air. Consequently, possible improvements evolve regarding production cycle time, energy consumption and machine footprint. To make use of these capabilities, comprehensive process understanding is required, which can be increased using numerical simulation methods. Therefore, in this article, an LBO process model is set-up and experimentally evaluated. The model explicitly considers the fluid-structure interaction between liquid and PET, which significantly influences the PET forming behavior. The key simulation parameters namely the strong rate and temperature dependency of PET and a realistic process parameter determination are also included. The model is evaluated using two different methods to show the reliability of the process prediction. © 2014 Society of Plastics Engineers.
    view abstractdoi: 10.1002/pen.23961
  • 2015 • 91 Influence of liquid in clearances on the operational behaviour of twin screw expanders
    Gräßer, M. and Brümmer, A.
    IOP Conference Series: Materials Science and Engineering 90 (2015)
    A lot of effort has been expended on understanding the influences of an injected auxiliary liquid on a twin screw expander's performance. Sealed clearances improve performance on the one hand, but involve considerable frictional losses on the other hand. This paper contributes to an evaluation of these opposing effects with regard to the efficiency of screw expanders. First, thermodynamic analyses using the multi-chamber model-based simulation tool KaSim, developed at the Chair of Fluidics, are presented for a test screw expander in order to show the maximum potential of clearance sealing. This analysis involves thermodynamic simulations for sealed and unsealed clearances and leads to an order of priorities for different clearance types. Second, hydraulic losses within front and housing clearances are calculated, applying an analytical model of incompressible one-phase clearance flow. Subsequently dry and wet screw expanders are evaluated while both clearance sealing and frictional losses are considered for the simulation of a liquid-injected machine. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/90/1/012060
  • 2015 • 90 Measurement and modeling of phase equilibria in systems of acetonitrile, n-alkanes, and β-myrcene
    Riechert, O. and Zeiner, T. and Sadowski, G.
    Industrial and Engineering Chemistry Research 54 1153-1160 (2015)
    This work presents a modeling approach using the Perturbed Chain-Statistical Associating Fluid Theory (PC-SAFT) for new liquid-liquid equilibria (LLE) data of ternary systems containing β-myrcene, acetonitrile, and n-alkanes, as well as binary mixtures thereof. The modeling approach is based on parameter estimations from binary systems and allows a general prediction of acetonitrile/n-alkane systems' LLE and their ternary mixtures' LLE with β-myrcene. The binary mixtures' vapor-liquid equilibria (VLE) of β-myrcene with acetonitrile and n-alkanes were measured at 100 mbar. The ternary systems' LLE were measured at ambient pressure and 298.15 K. Experimentally investigated alkanes are n-hexane, n-heptane, and n-octane. The approach was validated by successfully predicting the ternary system containing n-dodecane. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/ie502557g
  • 2015 • 89 Mixed Debye-type liquids studied by dielectric, shear mechanical, nuclear magnetic resonance, and near-infrared spectroscopy
    Bauer, S. and Moch, K. and Münzner, P. and Schildmann, S. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 384-391 (2015)
    Monohydroxy alcohols have been in the focus of scientific research for a long time and their Debye (-like) process was studied predominantly using dielectric spectroscopy. However, a number of other techniques are useful to unravel the dynamics of these supramolecular liquids. For a recent review on neat monohydroxy alcohols, see R. Böhmer, C. Gainaru, R. Richert, Phys. Rep. (accepted). On this background the present article deals mostly with mixtures involving monohydroxy alcohols using various experimental methods. Examples given include dielectric spectroscopy on a mixture which shows a small Debye process well separated from the structural relaxation. For another mixture it is demonstrated that the time scales of the dielectric and the rheological signatures of the Debye process coincide. Isotope labeling is exploited to map out the rotational dynamics of both components in binary mixtures of 1-butanol (BuOH) and 1-bromobutane (BuBr) using spin-lattice relaxation nuclear magnetic resonance (NMR) spectroscopy. While the hydroxyl motion in BuOH becomes faster upon admixture of BuBr, the alkyl bromide dynamics is virtually independent of composition. Two-time and four-time stimulated-echo NMR experiments show that the dynamic exchange in a BuOH-BuBr mixture is similar to that of supercooled liquids devoid of a Debye process and hence it does not provide a rationale to understand the symmetric broadening of the structural dielectric loss peak in this and related mixtures. Finally, a wavelength dependent derivative analysis of near-infrared spectra recorded for pure and mixed monohydroxy alcohols over wide temperature ranges shows that the rearrangement of the hydrogen network differs below and above about 250 K. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.jnoncrysol.2014.07.018
  • 2015 • 88 Modeling of self-healing effects in polymeric composites
    Bluhm, J. and Specht, S. and Schröder, J.
    Archive of Applied Mechanics 85 1469-1481 (2015)
    Polymers and polymer composites are used in many engineering applications, but they can loose a high rate of stiffness and strength due to internal micro cracks/damages during their lifetime cycle. These damages are very hard to detect and nearly impossible to repair. To avoid failure due to such damages, a self-healing system is considered where microencapsulated healing agents and catalysts are embedded in the polymer matrix. For the numerical simulation of such a self-healing material, a thermodynamically consistent multiphase model, based on the Theory of Porous Media, is developed in this contribution. The different phases of the model are the solid matrix material with embedded catalysts, the liquid healing agents, the solid healed material and the gas phase, which represents the volume fraction of the micro cracks in the model. For the description of the healing mechanism, a mass exchange between the liquid healing agents and the solid healed material, in consideration of the change of the aggregate state, is introduced, which depends on the local concentration of catalysts in the polymer matrix. The applicability of the developed model is shown by means of numerical test simulations of a tapered double cantilever beam. © 2014, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00419-014-0946-7
  • 2015 • 87 Modeling thermodynamic derivative properties of ionic liquids with ePC-SAFT
    Shen, G. and Held, C. and Lu, X. and Ji, X.
    Fluid Phase Equilibria 405 73-82 (2015)
    In this work, ePC-SAFT was extended to predict the second order thermodynamic derivative properties of pure ionic liquids (ILs), such as isothermal and isentropic compressibility coefficients, thermal pressure coefficient, heat capacities, speed of sound, thermal expansion coefficient and internal pressure. ePC-SAFT predictions were compared with available experimental data of imidazolium-based ILs. The pure-component ePC-SAFT parameters for the IL-cations [C<inf>2</inf>mim]+, [C<inf>4</inf>mim]+, [C<inf>6</inf>mim]+ and [C<inf>8</inf>mim]+, and IL-anions [BF<inf>4</inf>]-, [PF<inf>6</inf>]- and [Tf<inf>2</inf>N]- were taken from literature in order to predict the thermodynamic derivative properties. The pure-component ePC-SAFT parameters for the IL-cations [C<inf>3</inf>mim]+, [C<inf>5</inf>mim]+, [C<inf>7</inf>mim]+ and [C<inf>10</inf>mim]+ were predicted based on linear molecular-weight-dependent relations. These estimated ePC-SAFT parameters were verified by comparing so-predicted pure-IL density as well as predicted CO<inf>2</inf> solubility in ILs with respective experimental data. Further, these parameters were used to predict the second order thermodynamic derivative properties. The comparison of model prediction with experimental data showed that ePC-SAFT predictions were reliable in a wide temperature and pressure range. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.fluid.2015.07.018
  • 2015 • 86 Molecular motions in supercooled and glassy ibuprofen: Deuteron magnetic resonance and high-resolution rheology study
    Bauer, S. and Storek, M. and Gainaru, C. and Zimmermann, H.b and Böhmer, R.
    Journal of Physical Chemistry B 119 5087-5095 (2015)
    Using deuteron nuclear magnetic resonance, the molecular motions of specifically isotope-labeled ibuprofen were probed at the carboxylic group and at the methin group next to it. Spin relaxometry revealed slight differences between the molecular motions of the two isotopomers that are rationalized with reference to the hydrogen bonding of the COOH moiety. In the glassy state, a small-angle jump process among about four sites, related to the so-called γ-process, was identified using stimulated-echo spectroscopy. Indications for a Debye-like process, previously found to leave a weak signature in the dielectric loss, could not unambiguously be detected in magnetic resonance or shear mechanical experiments carried out for supercooled liquid ibuprofen. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b01072
  • 2015 • 85 Partition coefficients of pharmaceuticals as functions of temperature and pH
    Laube, F. and Klein, T. and Sadowski, G.
    Industrial and Engineering Chemistry Research 54 3968-3975 (2015)
    Liquid-liquid extraction is a potential separation process for the purification of active pharmaceutical ingredients (APIs). The design of an extraction step requires knowledge of the API partition coefficient, which strongly depends on the solvent system and process conditions. Usually, cost-intensive experiments have to be performed to select the most suitable solvent system and the best process conditions. The number of experiments can be reduced by predicting the partition coefficient using perturbed chain statistical associating theory (PC-SAFT). In this work, modeling results and experimental data were compared for the partition coefficients of the APIs nicotinamide and salicylamide in different solvent systems at temperatures from 293.15 to 328.15 K and at pH values varying between 5.2 and 10.3. The results show that PC-SAFT is able to predict the API partition coefficients for different solvent systems as functions of temperature and pH. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.5b00068
  • 2015 • 84 Phase Equilibria in Systems of Morpholine, Acetonitrile, and n -Alkanes
    Riechert, O. and Zeiner, T. and Sadowski, G.
    Journal of Chemical and Engineering Data 60 2098-2103 (2015)
    This work presents investigations on the liquid-liquid equilibria (LLE) of ternary systems composed of morpholine, acetonitrile, and an n-alkane at 298.15 K and atmospheric pressure. The investigated n-alkanes were n-hexane, n-heptane, and n-octane. The experimental data were compared to predictions using the perturbed chain-statistical associating fluid theory (PC-SAFT). The predictions are based on pure-component parameters fitted to vapor pressures and liquid densities as well as on binary parameters fitted to binary systems' phase equilibria. For that purpose, the vapor-liquid equilibrium of the morpholine/acetonitrile system was measured at 100 mbar and modeled with PC-SAFT. Binary interaction parameters for acetonitrile/n-alkane systems were obtained from a correlation as a function of the n-alkane carbon number. This correlation, together with the other pure-component and binary parameters, was used to make predictions on ternary systems with n-alkanes longer than n-octane, for which data were taken from literature. All ternary LLE predictions were in satisfactory agreement with experimental data. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.5b00175
  • 2015 • 83 Salt induced reduction of lysozyme adsorption at charged interfaces
    Göhring, H. and Paulus, M. and Salmen, P. and Wirkert, F. and Kruse, T. and Degen, P. and Stuhr, S. and Rehage, H. and Tolan, M.
    Journal of Physics Condensed Matter 27 (2015)
    A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L-1 and 1000 mmol L-1. X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/23/235103
  • 2015 • 82 Size control and supporting of palladium nanoparticles made by laser ablation in saline solution as a facile route to heterogeneous catalysts
    Marzun, G. and Nakamura, J. and Zhang, X. and Barcikowski, S. and Wagener, P.
    Applied Surface Science 348 75-84 (2015)
    In the literature many investigations on colloidal stability and size control of gold nanoparticles are shown but less for ligand-free palladium nanoparticles, which can be promising materials in various applications. Palladium nanoparticles are perspective materials for a manifold of energy application like photo- and electrocatalysis or hydrogen storage. For this purpose, size-controlled nanoparticles with clean surfaces and facile immobilization on catalyst supports are wanted. Laser ablation in saline solution yields ligand-free, charged colloidal palladium nanoparticles that are supported by titania and graphene nanosheets as model systems for photo- and electrocatalysis, respectively. By adjusting the ionic strength during laser ablation in liquid, it is possible to control stability and particle size without compromising subsequent nanoparticle adsorption of supporting materials. A quantitative deposition of nearly 100% yield with up to 18 wt% nanoparticle load was achieved. The average size of the laser-generated nanoparticles remains the same after immobilization on a support material, in contrast to other preparation methods of catalysts. The characterization by X-ray photoelectron spectroscopy reveals a redox reaction between the immobilized nanoparticles and the graphene support. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.01.108
  • 2015 • 81 The effect of the Au loading on the liquid-phase aerobic oxidation of ethanol over Au/TiO2 catalysts prepared by pulsed laser ablation
    Dong, W. and Reichenberger, S. and Chu, S. and Weide, P. and Ruland, H. and Barcikowski, S. and Wagener, P. and Muhler, M.
    Journal of Catalysis 330 497-506 (2015)
    Gold nanoparticles (NPs) synthesized by pulsed laser ablation of a gold target in water were efficiently deposited on TiO<inf>2</inf> (P25) without any post-treatment yielding catalysts with Au loadings up to 10 wt%. Regardless of the loading, the Au NPs had a mean diameter of 8 nm before and after deposition. The ligand-free Au NPs strongly bind to TiO<inf>2</inf> surface oxygen vacancies and maintain a homogeneous distribution with loadings up to 4 wt%, while a further increase in Au content up to 10 wt% results in additional weakly adsorbed Au NPs. The catalytic tests of the Au/TiO<inf>2</inf> samples in the selective oxidation of ethanol in the liquid phase identified an optimal loading of 4 wt% resulting in the highest yield of acetic acid, which is ascribed to the homogeneous Au distribution and the adequate occupation of surface oxygen vacancies by strongly bound Au NPs without significant Au sintering during reaction. © 2015 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.jcat.2015.07.033
  • 2015 • 80 The glass transition in high-density amorphous ice
    Loerting, T. and Fuentes-Landete, V. and Handle, P.H. and Seidl, M. and Amann-Winkel, K. and Gainaru, C. and Böhmer, R.
    Journal of Non-Crystalline Solids 407 423-430 (2015)
    There has been a long controversy regarding the glass transition in low-density amorphous ice (LDA). The central question is whether or not it transforms to an ultraviscous liquid state above 136 K at ambient pressure prior to crystallization. Currently, the most widespread interpretation of the experimental findings is in terms of a transformation to a superstrong liquid above 136 K. In the last decade some work has also been devoted to the study of the glass transition in high-density amorphous ice (HDA) which is in the focus of the present review. At ambient pressure HDA is metastable against both ice I and LDA, whereas at > 0.2 GPa HDA is no longer metastable against LDA, but merely against high-pressure forms of crystalline ice. The first experimental observation interpreted as the glass transition of HDA was made using in situ methods by Mishima, who reported a glass transition temperature Tinfg/inf of 160 K at 0.40 GPa. Soon thereafter Andersson and Inaba reported a much lower glass transition temperature of 122 K at 1.0 GPa. Based on the pressure dependence of HDA's Tinfg/inf measured in Innsbruck, we suggest that they were in fact probing the distinct glass transition of very high-density amorphous ice (VHDA). Very recently the glass transition in HDA was also observed at ambient pressure at 116 K. That is, LDA and HDA show two distinct glass transitions, clearly separated by about 20 K at ambient pressure. In summary, this suggests that three glass transition lines can be defined in the p-T plane for LDA, HDA, and VHDA. © 2015 Published by Elsevier Inc.
    view abstractdoi: 10.1016/j.jnoncrysol.2014.09.003
  • 2015 • 79 Tuning the photonic properties of chiral nematic mesoporous organosilica with hydrogen-bonded liquid-crystalline assemblies
    Giese, M. and Krappitz, T. and Dong, R.Y. and Michal, C.A. and Hamad, W.Y. and Patrick, B.O. and MacLachlan, M.J.
    Journal of Materials Chemistry C 3 1537-1545 (2015)
    A series of novel hydrogen-bonded assemblies was synthesized and characterized with respect to their liquid-crystalline behaviour. Solid-state NMR spectroscopy gave insight into the columnar nematic mesophase and the corresponding ordering and alignment. Infiltrating the pores of chiral nematic mesoporous organosilica films with the liquid-crystalline compounds gives composite samples that undergo reversible phase changes with temperature, leading to tuneable photonic properties. The unique combination of liquid crystallinity arising from supramolecular interactions (hydrogen bonding) and chiral nematic organisation in a solid-state host is a promising new concept for developing optical sensors. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4tc02602k
  • 2014 • 78 Alloying colloidal silver nanoparticles with gold disproportionally controls antibacterial and toxic effects
    Grade, S. and Eberhard, J. and Jakobi, J. and Winkel, A. and Stiesch, M. and Barcikowski, S.
    Gold Bulletin 47 83-93 (2014)
    Elemental silver nanoparticles are an effective antibacterial substance and are found as additive in various medical applications. Gold nanoparticles are used due to their optical properties in microscopy and cancer therapy. These advantages might be combined within alloyed nanoparticles of both elements and thereby open new fields of interest in research and medical treatment. In this context, laser ablation of solid alloys in liquid gives access to colloidal silver-gold alloy nanoparticles with a homogeneous ultrastructure. Elemental and alloy silver-gold nanoparticles with increasing molar fractions of silver (50, 80, and 100 %) were produced and stabilized with citrate or albumin (BSA). Particles were embedded in agar at concentrations of 3-100 μg cm-3 and tested on clinical relevant Staphylococcus aureus regarding their antibacterial properties. Cytotoxic effects were measured within the same particle concentration range using human gingival fibroblasts (HGFib). As expected, a reduced fraction of silver in the nanoalloys decreased the antibacterial effect on S. aureus according to the evaluated minimal inhibitory concentrations. However, this decrease turned out stronger than expected by its relative mass per particle, due to the electrochemical, disproportionally high effect of gold on the bioresponse to silver within silver-gold nanoalloy particles. BSA was able to stabilize all colloids and maintain antibacterial activity, whereas sodium citrate reduced antibacterial effects and cytotoxicity even at high nanoparticle concentrations. The alloying of silver with gold by laser ablation in liquid produced nanoparticles with both reduced antibacterial and cytotoxic properties in comparison to silver nanoparticles but still retains the application spectrum of both elements combined in one colloid. In particular, alloying with gold may render silver nanoparticles more biocompatible, and allows bioconjugation via established thiol chemistry. © 2013 The Author(s).
    view abstractdoi: 10.1007/s13404-013-0125-6
  • 2014 • 77 Bi-Zn bond formation in liquid ammonia solution: [Bi-Zn-Bi]4-, a linear polyanion that is Iso(valence)-electronic to CO2
    Benda, C.B. and Köchner, T. and Schäper, R. and Schulz, S. and Fässler, T.F.
    Angewandte Chemie - International Edition 53 8944-8948 (2014)
    Reactions of the zinc(I) complex [Zn2(Mesnacnac)2] (Mesnacnac=[(2,4,6-Me3C6H2)NC(Me)] 2CH) with solid K3Bi2 dissolved in liquid ammonia yield crystals of the compound K4[ZnBi2] ·(NH3)12 (1), which contains the molecular, linear heteroatomic [Bi-Zn-Bi]4- polyanion (1a). This anion represents the first example of a three-atomic molecular ion of metal atoms being iso(valence)-electronic to CO2 and being synthesized in solution. The analogy of the discrete [Bi-Zn-Bi]4- anion and the polymeric 1 ∞ [(ZnBi4/2)4-] unit to monomeric CO2 and polymeric SiS2 is rationalized. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201404343
  • 2014 • 76 Calculation of complex phase equilibria of DMF/alkane systems using the PCP-SAFT equation of state
    Schäfer, E. and Sadowski, G. and Enders, S.
    Chemical Engineering Science 115 49-57 (2014)
    The Perturbed Chain Polar Statistical Association Fluid Theory (PCP-SAFT) equation of state is applied to calculate phase equilibria data for several DMF/alkane systems. DMF/alkane systems exhibit a peculiar phase behavior, due to overlapping liquid-liquid and vapor-liquid coexistence regions. Hetero- and homoazeotropes appear and disappear depending on temperature/pressure conditions and the chain length of the alkane. The vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria of DMF/alkanes systems including alkanes from C5 to C10 were studied in detail over wide temperature and pressure ranges. A single, linear expression for the binary interaction parameter kij as function of temperature and alkane chain length was used to model the experimental data. This expression was determined only by using experimental liquid-liquid equilibria (LLE) data of DMF/alkane systems. Using that, the PCP-SAFT equation of state provides very satisfying results for the description of the phase behavior in all considered systems. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.ces.2013.04.053
  • 2014 • 75 Evaluation of pulsed laser ablation in liquids generated gold nanoparticles as novel transfection tools: Efficiency and cytotoxicity
    Willenbrock, S. and Durán, M.C. and Barchanski, A. and Barcikowski, S. and Feige, K. and Nolte, I. and Murua Escobar, H.
    Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)
    Varying transfection efficiencies and cytotoxicity are crucial aspects in cell manipulation. The utilization of gold nanoparticles (AuNP) has lately attracted special interest to enhance transfection efficiency. Conventional AuNP are usually generated by chemical reactions or gas pyrolysis requiring often cell-toxic stabilizers or coatings to conserve their characteristics. Alternatively, stabilizer- and coating-free, highly pure, colloidal AuNP can be generated by pulsed laser ablation in liquids (PLAL). Mammalian cells were transfected efficiently by addition of PLAL-AuNP, but data systematically evaluating the cell-toxic potential are lacking. Herein, the transfection efficiency and cytotoxicity of PLAL AuNP was evaluated by transfection of a mammalian cell line with a recombinant HMGB1/GFP DNA expression vector. Different methods were compared using two sizes of PLAL-AuNP, commercialized AuNP, two magnetic NP-based protocols and a conventional transfection reagent (FuGENE HD; FHD). PLAL-AuNP were generated using a Spitfire Pro femtosecond laser system delivering 120 fs laser pulses at a wavelength of 800 nm focusing the fs-laser beam on a 99.99% pure gold target placed in ddH2O. Transfection efficiencies were analyzed after 24h using fluorescence microscopy and flow cytometry. Toxicity was assessed measuring cell proliferation and percentage of necrotic, propidium iodide positive cells (PI %). The addition of PLAL-AuNP significantly enhanced transfection efficiencies (FHD: 31 %; PLAL-AuNP size-1: 46 %; size-2: 50 %) with increased PI% but no reduced cell proliferation. Commercial AuNP-transfection showed significantly lower efficiency (23 %), slightly increased PI % and reduced cell proliferation. Magnetic NP based methods were less effective but showing also lowest cytotoxicity. In conclusion, addition of PLAL-AuNP provides a novel tool for transfection efficiency enhancement with acceptable cytotoxic side-effects. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2038453
  • 2014 • 74 FE-simulation and validation of liquid-bi-orientation-
    Zimmer, J. and Chauvin, G. and Stommel, M.
    AIP Conference Proceedings 1593 90-95 (2014)
    An established method to produce thin walled bottles is Stretch Blow Molding (SBM). Polyethylene terephthalate (PET)-preforms are first heated above their glass transition temperature and subsequently transferred into a closed cavity. In a second step the hot preforms are axially elongated by a stretch rod and simultaneously inflated by pressurized air until a contact with the cavity wall is reached (blowing stage). After a cooling phase, the resulting bottle is ejected and further transferred to a filling station, where the desired liquid content is poured in (filling stage). Alternatively to this sequential procedure, a new process combines the blowing and filling phases. This is done by using the desired liquid content as a pressure medium to inflate the hot preforms. Hence, no separated filling station is required. Moreover the filling time is drastically reduced and the cooling is increased through the heat transfer between hot preform and cold liquid. In the following this process is denoted as liquid-bi-orientation (LBO). Despite of its obvious advantages, LBO is not yet used for industrial series production because SBM is well controlled and established. In this paper the LBO process is investigated by experiments and FE-simulations to obtain a deeper insight and to increase process knowledge. The experiments are conducted at a prototype machine. Hereby, a high speed camera in combination with a transparent cavity enables a recording of the preform deformation. Furthermore, FE-simulations with coupled fluid-structure interactions are conducted to predict the process. In comparison to the high speed video the capabilities of the process model are evaluated. © 2014 American Institute of Physics.
    view abstractdoi: 10.1063/1.4873741
  • 2014 • 73 FSI-simulation of Liquid supported Stretch Blow Molding (LBO): Model validation and study of series production scenario
    Zimmer, J. and Stommel, M.
    Key Engineering Materials 611-612 892-900 (2014)
    Liquid-Driven Stretch Blow Molding is a new and innovative method to produce PET bottles [1]. In the well-established Stretch Blow Molding (SBM) process, preforms are biaxially deformed by pressurized air into a cavity. The resulting bottles are transferred to a separate machine, where the desired product is filled in. In contrast to that, Liquid-Driven Stretch Blow Molding is characterized by employing the liquid product to deform the material. The former separated blowing and filling steps are thus combined to a single forming stage leading to numerous advantages in energy consumption, cycle time and machine footprint. In this paper, a numerical simulation of the new process is presented. An additional challenge compared to SBM simulations is thereby the consideration of the interaction between liquid and preform. The load application cannot be solely represented by the pressure because the influx behavior as well as gravity and inertia forces influence the preform deformation. A smoothed particle hydrodynamics (SPH) approach is applied to the simulation to incorporate the additional effects. The process model is evaluated by prototype experiments. In addition, a feasibility study shows the applicability of a rotary forming system to the new process. © 2014 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2014 • 72 Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel
    Lefor, K. and Walter, M. and Weddeling, A. and Hryha, E. and Huth, S. and Weber, S. and Nyborg, L. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 46 1154-1167 (2014)
    Ni-free austenitic steels alloyed with Cr and Mn are an alternative to conventional Ni-containing steels. Nitrogen alloying of these steel grades is beneficial for several reasons such as increased strength and corrosion resistance. Low solubility in liquid and δ-ferrite restricts the maximal N-content that can be achieved via conventional metallurgy. Higher contents can be alloyed by powder-metallurgical (PM) production via gas–solid interaction. The performance of sintered parts is determined by appropriate sintering parameters. Three major PM-processing routes, hot isostatic pressing, supersolidus liquid phase sintering (SLPS), and solid-state sintering, were performed to study the influence of PM-processing route and N-content on densification, fracture, and mechanical properties. Sintering routes are designed with the assistance of thermodynamic calculations, differential thermal analysis, and residual gas analysis. Fracture surfaces were studied by X-ray photoelectron spectroscopy, secondary electron microscopy, and energy dispersive X-ray spectroscopy. Tensile tests and X-ray diffraction were performed to study mechanical properties and austenite stability. This study demonstrates that SLPS process reaches high densification of the high-Mn-containing powder material while the desired N-contents were successfully alloyed via gas–solid interaction. Produced specimens show tensile strengths >1000 MPa combined with strain to fracture of 60 pct and thus overcome the other tested production routes as well as conventional stainless austenitic or martensitic grades. © 2014, The Author(s).
    view abstractdoi: 10.1007/s11661-014-2701-7
  • 2014 • 71 Liquid 1-propanol studied by neutron scattering, near-infrared, and dielectric spectroscopy
    Sillrén, P. and Matic, A. and Karlsson, M. and Koza, M. and MacCarini, M. and Fouquet, P. and Götz, M. and Bauer, T. and Gulich, R. and Lunkenheimer, P. and Loidl, A. and Mattsson, J. and Gainaru, C. and Vynokur, E. and Schildma...
    Journal of Chemical Physics 140 (2014)
    Liquid monohydroxy alcohols exhibit unusual dynamics related to their hydrogen bonding induced structures. The connection between structure and dynamics is studied for liquid 1-propanol using quasi-elastic neutron scattering, combining time-of-flight and neutron spin-echo techniques, with a focus on the dynamics at length scales corresponding to the main peak and the pre-peak of the structure factor. At the main peak, the structural relaxation times are probed. These correspond well to mechanical relaxation times calculated from literature data. At the pre-peak, corresponding to length scales related to H-bonded structures, the relaxation times are almost an order of magnitude longer. According to previous work [C. Gainaru, R. Meier, S. Schildmann, C. Lederle, W. Hiller, E. Rössler, and R. Böhmer, Phys. Rev. Lett. 105, 258303 (2010)] this time scale difference is connected to the average size of H-bonded clusters. The relation between the relaxation times from neutron scattering and those determined from dielectric spectroscopy is discussed on the basis of broad-band permittivity data of 1-propanol. Moreover, in 1-propanol the dielectric relaxation strength as well as the near-infrared absorbance reveal anomalous behavior below ambient temperature. A corresponding feature could not be found in the polyalcohols propylene glycol and glycerol. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4868556
  • 2014 • 70 Mimicking exposures to acute and lifetime concentrations of inhaled silver nanoparticles by two different in vitro approaches
    Herzog, F. and Loza, K. and Balog, S. and Clift, M.J.D. and Epple, M. and Gehr, P. and Petri-Fink, A. and Rothen-Rutishauser, B.
    Beilstein Journal of Nanotechnology 5 1357-1370 (2014)
    In the emerging market of nano-sized products, silver nanoparticles (Ag NPs) are widely used due to their antimicrobial properties. Human interaction with Ag NPs can occur through the lung, skin, gastrointestinal tract, and bloodstream. However, the inhalation of Ag NP aerosols is a primary concern. To study the possible effects of inhaled Ag NPs, an in vitro triple cell co-culture model of the human alveolar/airway barrier (A549 epithelial cells, human peripheral blood monocyte derived dendritic and macrophage cells) together with an air-liquid interface cell exposure (ALICE) system was used in order to reflect a real-life exposure scenario. Cells were exposed at the air-liquid interface (ALI) to 0.03, 0.3, and 3 μg Ag/cm2 of Ag NPs (diameter 100 nm; coated with polyvinylpyrrolidone: PVP). Ag NPs were found to be highly aggregated within ALI exposed cells with no impairment of cell morphology. Furthermore, a significant increase in release of cytotoxic (LDH), oxidative stress (SOD-1, HMOX-1) or pro-inflammatory markers (TNF-α, IL-8) was absent. As a comparison, cells were exposed to Ag NPs in submerged conditions to 10, 20, and 30 μg Ag/mL. The deposited dose per surface area was estimated by using a dosimetry model (ISDD) to directly compare submerged vs ALI exposure concentrations after 4 and 24 h. Unlike ALI exposures, the two highest concentrations under submerged conditions promoted a cytotoxic and pro-inflammatory response after 24 h. Interestingly, when cell cultures were co-incubated with lipopolysaccharide (LPS), no synergistic inflammatory effects were observed. By using two different exposure scenarios it has been shown that the ALI as well as the suspension conditions for the lower concentrations after 4 h, reflecting reallife concentrations of an acute 24 h exposure, did not induce any adverse effects in a complex 3D model mimicking the human alveolar/airway barrier. However, the highest concentrations used in the ALI setup, as well as all concentrations under submerged conditions after 24 h, reflecting more of a chronic lifetime exposure concentration, showed cytotoxic as well as pro-inflammatory effects. In conclusion, more studies need to address long-term and chronic Ag NP exposure effects. © 2014 Herzog et al.
    view abstractdoi: 10.3762/bjnano.5.149
  • 2014 • 69 Numerical modeling of artificial ground freezing: Multiphase modeling and strength upscaling
    Zhou, M.-M. and Meschke, G.
    Geotechnical Special Publication 209-219 (2014)
    In geotechnical applications of artificial ground freezing, safe design and execution require a correct prediction of the coupled thermo-hydro-mechanical behavior of soils subjected to freezing. In the context of thermo-poro- plasticity (Coussy, 2005), a three-phase finite element model of freezing soils is presented: (1) considering solid particles, liquid water and crystal ice as separate phases; and (2) mixture temperature, liquid pressure, and solid displacement as primary field variables. Through three fundamental physical laws (overall entropy balance, mass balance of liquid water and crystal ice, and overall momentum balance) and corresponding state relations, the model captures the most relevant couplings between the phase transition, the liquid transport within the pores, and the accompanying mechanical deformation. Particularly for the description of the poro-plastic mechanical behavior of the soil model, the enhanced Barcelona Basic Model (Nishimura et al., 2009) is adopted within a unified effective-stress-based framework. The macroscopic strength criterion of the freezing soil composite is improved through multi-scale strength homogenization based upon the linear comparison composite method (Ortega et al. 2011). The performance of the proposed model is demonstrated by re-analysis of a soil freezing test and AGF processes during tunneling. © ASCE 2014.
    view abstractdoi: 10.1061/9780784413401.021
  • 2014 • 68 Reactive transient liquid phase bonding of ceramic to steel using Zr-Cu-Zr- and Zr-Ni-Cu-Zr-interlayers for high temperature applications
    Tillmann, W. and Pfeiffer, J. and Wojarski, L. and Indacochea, J.-E.
    Materialwissenschaft und Werkstofftechnik 45 512-521 (2014)
    Joints manufactured by transient liquid phase bonding feature comparable properties as diffusion weldements, but considerably lower process temperatures and pressures have to be applied. The liquid phase, which is hereby used, occurs due to interdiffusion between the base and/or the filler materials at a constant temperature, which lies below the melting temperature of the substrates. An essential requirement for this diffusion-based melting is that the involved materials have low melting alloy-constitution areas, such as eutectics. The aim of the study, presented in this contribution, is to evaluate an approach, in which an active transient liquid is created by suitable interlayers, in order to facilitate the wetting of ceramics. The potential of this attempt will be illustrated on zirconia/stainless-steel-joints for high temperature applications, such as solid oxide fuel cells. In such applications, the used materials have to withstand harsh conditions, e.g. high operating temperatures, oxidizing or reducing environments, which represent a demanding challenge for joining technologies, even at the latest state of research. In this study interlayers, consisting of Zirconium, as the active element, in combination with Copper and/or Nickel, have been investigated. These systems exhibit a wide range of alloy-constitutions with low melting temperatures, which can be used for the formation of the transient liquid phase. For the application of the interlayers, physical vapor deposition as well as 75 μm-thick Nickel-foils have been used. The joining was carried out in high vacuum with changing holding times and temperatures. Additionally, the ratio of the thickness of the used interlayers was changed. Results of microstructural investigations, nano-hardness measurements of the joining area as well as shear strength and fractography are presented. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201400267
  • 2013 • 67 A three-phase thermo-hydro-mechanical finite element model for freezing soils
    Zhou, M.M. and Meschke, G.
    International Journal for Numerical and Analytical Methods in Geomechanics 37 3173-3193 (2013)
    Artificial ground freezing (AGF) is a commonly used technique in geotechnical engineering for ground improvement such as ground water control and temporary excavation support during tunnel construction in soft soils. The main potential problem connected with this technique is that it may produce heave and settlement at the ground surface, which may cause damage to the surface infrastructure. Additionally, the freezing process and the energy needed to obtain a stable frozen ground may be significantly influenced by seepage flow. Evidently, safe design and execution of AGF require a reliable prediction of the coupled thermo-hydro-mechanical behavior of freezing soils. With the theory of poromechanics, a three-phase finite element soil model is proposed, considering solid particles, liquid water, and crystal ice as separate phases and mixture temperature, liquid pressure, and solid displacement as the primary field variables. In addition to the volume expansion of water transforming into ice, the contribution of the micro-cryo-suction mechanism to the frost heave phenomenon is described in the model using the theory of premelting dynamics. Through fundamental physical laws and corresponding state relations, the model captures various couplings among the phase transition, the liquid transport within the pore space, and the accompanying mechanical deformation. The verification and validation of the model are accomplished by means of selected analyses. An application example is related to AGF during tunnel excavation, investigating the influence of seepage flow on the freezing process and the time required to establish a closed supporting frozen arch. © 2013 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/nag.2184
  • 2013 • 66 Extension of the PC-SAFT based group contribution method for polymers to aromatic, oxygen- and silicon-based polymers
    Peters, F.T. and Herhut, M. and Sadowski, G.
    Fluid Phase Equilibria 339 89-104 (2013)
    A PC-SAFT group contribution method (GCM) for polymers developed earlier [10] is extended to aromatic, oxygen- and silicon-based (co-)polymers. Polymer parameters are determined using group contributions and applying simple arithmetic and geometric combination rules. Group contributions for six new groups are identified and parameterized: &gt;CHAr, &gt;CAr, O, &gt;CO, OH and &gt;Si< . The parameterization method is applied to liquid density and binary liquid-liquid equilibria and vapor-liquid equilibria as well as to excess enthalpies of polymers containing aromatic, oxygen- and silicon-containing monomer units in an extended spectrum of nonpolar, polar and associating solvents. Modeling results using both, GCM and fitted polymer parameters, show equally-good agreement with experimental data. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2012.11.031
  • 2013 • 65 Laser-based generation of nanocomposites without matrix-coupling agents for bioactive medical devices
    Schwenke, A. and Wagener, P. and Weiß, A. and Klimenta, K. and Wiegel, H. and Sajti, L. and Barcikowski, S.
    Chemie-Ingenieur-Technik 85 740-746 (2013)
    New production technologies are required to benefit of the full potential of nanocomposites by homogeneous dispersion of nanoparticles along the process chain. Synthesis of silver nanoparticles by laser ablation in liquid and their integration into polymers are presented. Antibacterial properties of these materials and processability into prototypes for medical devices with antibacterial protection are demonstrated. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201200035
  • 2013 • 64 Massive anisotropic thermal expansion and thermo-responsive breathing in metal-organic frameworks modulated by linker functionalization
    Henke, S. and Schneemann, A. and Fischer, R.A.
    Advanced Functional Materials 23 5990-5996 (2013)
    Functionalized metal-organic frameworks (fu-MOFs) of general formula [Zn2(fu-L)2dabco]n show unprecedentedly large uniaxial positive and negative thermal expansion (fu-L = alkoxy functionalized 1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. The alkoxy side chains of fu-L are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid state phase transitions to drastically expanded structures. The thermo-responsive properties of these hybrid solid-liquid materials are precisely controlled by the choice and combination of fu-Ls and depend on functional moieties and chain lengths. In principle, this combinatorial approach allows for a targeted design of extreme thermo-mechanical properties of MOFs addressing the regime between crystalline solid matter and the liquid state. Extremely large thermal expansion is shown by pillared-layered metal-organic frameworks (MOFs) exhibiting alkoxy-functionalized 1,4-benzenedicarboxylate linkers. At a certain threshold temperature the materials reversibly switch from a narrow pore to large pore form. This unprecedented thermo-mechanical behavior is an intrinsic property of the materials and can be modulated substantially by mixing differently functionalized linkers to obtain mixed linker MOF solid solutions. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201301256
  • 2013 • 63 Numerical solutions of population balance equations within liquid/gas-liquid flow simulations
    Bayraktar, E. and Mierka, O. and Turek, S.
    Chemie-Ingenieur-Technik 85 1137-1145 (2013)
    Numerical solvers based on population balance equations (PBE) coupled with flow equations are a promising approach to simulate liquid/gas-liquid dispersed flows, which are very commonly observed in nature and in industrial processes. The challenges for the numerical solution of the coupled equation systems are discussed and detailed numerical recipes are presented whose main ingredients are the method of classes, positivity-preserving linearization and the high-order FEM-AFC schemes, additional to the FeatFlow in-house flow solver package. Liquid-liquid flows through static mixers and dispersed phase systems in a flat bubble column are studied with the accordingly developed computational tool. The suggested recipes were validated by comparing the numerical results against experimental data. The state of the art on the numerical simulation of liquid/gas-liquid dispersed flows with population balance equations coupled to computational fluid dynamics, and the related numerical challenges are explained. A novel approach is presented to numerically simulate bubbly flows at moderate gas holdups. The suggested recipes are validated with experimental data. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201200225
  • 2013 • 62 Set of acidic resin catalysts to correlate structure and reactivity in fructose conversion to 5-hydroxymethylfurfural
    Richter, F.H. and Pupovac, K. and Palkovits, R. and Schüth, F.
    ACS Catalysis 3 123-127 (2013)
    A new synthetic route to acidic polystyrene-co-divinylbenzene resin catalysts allows systematic variation of cross-linker content, porosity, and acid site density. These resins are prepared in the form of powders by nanocasting, and the acid site density and the distribution of the acid sites in the prepared catalysts is controlled by liquid phase sulfonation with adjusted mixtures of sulfuric acid and oleum. This method allows identical synthesis conditions for the entire range of cross-linker content. With this set of model catalysts, the cross-linker content of the resin was found to be the most influential factor for the liquid phase dehydration of fructose to 5-hydroxymethylfurfural. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cs3007439
  • 2012 • 61 Adsorption of aromatic trace compounds from organic solvents on activated carbons-experimental results and modeling of adsorption equilibria
    Gräf, T. and Pasel, C. and Luckas, M. and Bathen, D.
    Adsorption 18 127-141 (2012)
    Liquid phase adsorption is an important process for the removal of trace compounds from liquid matrices. Until today, research on liquid phase adsorption is less substantial than work on other thermal separation processes. The description of relevant mechanisms and interactions is difficult mainly because of lacking experimental data. This paper presents extensive isotherm measurements for the adsorption of organic trace compounds from organic solvents on activated carbons. A systematic variation of molecular structure of adsorptives and solvents enabled the identification of main structural factors dominating adsorption in these systems. The factors are polarity, extension and density of π electrons and sterical complexity. An analysis of the measured isotherms revealed incremental effects of functional groups and structural elements being characteristic for the adsorption capacities on activated carbons. Three consecutive empirical prediction models of adsorption equilibria are developed and compared. The empirical Freundlich equation appeared to be best suited for fitting the experimental data. The models apply an incremental concept permitting the calculation of adsorption isotherms on the basis of the structural increments of solvent and adsorptive molecules. The three models have a different extent of underlying data, a different number of parameters and a different range of application. The experimental data are predicted with satisfying accuracy for many engineering applications. The most sophisticated model has the most extensive range of application and manages on the smallest number of parameters. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10450-012-9388-0
  • 2012 • 60 Direct determination of minority carrier diffusion lengths at axial GaAs nanowire p-n junctions
    Gutsche, C. and Niepelt, R. and Gnauck, M. and Lysov, A. and Prost, W. and Ronning, C. and Tegude, F.-J.
    Nano Letters 12 1453-1458 (2012)
    Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204126n
  • 2012 • 59 Fabrication of a CO2-selective membrane by stepwise liquid-phase deposition of an alkylether functionalized pillared-layered metal-organic framework [Cu2L2P]n on a macroporous support
    Bétard, A. and Bux, H. and Henke, S. and Zacher, D. and Caro, J. and Fischer, R.A.
    Microporous and Mesoporous Materials 150 76-82 (2012)
    Metal-organic framework (MOF) membranes were prepared by stepwise deposition of reactants. Two pillared layered MOFs with the general formula [Cu2L2P]n (L = dicarboxylate linker, P = pillaring ligand) were selected. Within this family, fine tuning of adsorption affinity and pore size is possible by variation or functionalization of the L and P linkers. Compound 1 was chosen to be non-polar (L = 1,4- naphtalenedicarboxylate = ndc, P = 1,4-diazabicyclo(2.2.2)octane = dabco); in contrast, compound 2 included a polar linker L with two conformationally flexible ether side chains (L = 2,5-bis(2-methoxyethoxy)-1,4-benzene- dicarboxylate = BME-bdc, P = dabco). The polar functionalization is expected to increase the framework affinity for CO2 compared to CH4. The step-by-step, liquid phase deposition of 1 and 2 resulted in pore-plugging of macroporous ceramic supports. The performances of the two MOF membranes were evaluated in gas separation experiments of equimolar CO2/CH 4 mixtures using a modified Wicke-Kallenbach technique. Anti-Knudsen CO2/CH4 separation factors in the range of ∼4-4.5 were obtained for the membrane consisting of the polar 2, whereas the separation of the membrane formed from the non-polar 1 was found to be Knudsen-like. © 2011 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2011.09.006
  • 2012 • 58 Gas phase oxidation as a tool to introduce oxygen containing groups on metal-loaded carbon nanofibers
    Gosselink, R.W. and Van Den Berg, R. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    Carbon 50 4424-4431 (2012)
    Oxygen containing groups were introduced, onto carbon nanofibers (CNFs) that were previously loaded with palladium, using HNO 3 vapor. Using traditional liquid-phase oxidations this is not possible due to severe metal leaching. For the samples oxidized using HNO 3 vapor temperature programmed desorption and X-ray photoelectron spectroscopy revealed the presence of two major classes of oxygen containing groups, i.e. carboxylic acid groups which are thermally stable up to 300 °C and less acidic (e.g. phenol) and basic groups which were stable up to 700 °C. The amount of acidic oxygen containing groups introduced by this gas-phase treatment ranged from 0.1 to 0.3 mmol/g, as determined by titration. The latter amount is comparable to that introduced by traditional liquid-phase treatment in 65% HNO 3 on bare CNFs. Transmission electron microscopy and H 2-chemisorption measurements show a gradual increase of the average metal particle size from 2.1 nm for the starting Pd/CNF to 4.5 nm for Pd/CNF treated for 75 h in HNO 3 vapor indicating that the extent of sintering with gas-phase treatment is limited. Elemental analysis showed that no leaching occurred upon gas-phase oxidation, whereas 90% of the metal was lost with a liquid-phase reflux HNO 3 treatment. © 2012 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2012.05.020
  • 2012 • 57 Modeling liquid-liquid equilibria of polyimide solutions
    Hesse, L. and Sadowski, G.
    Industrial and Engineering Chemistry Research 51 539-546 (2012)
    The mutual affinity of volatile organic compounds (VOCs) and the active polymer layer of dense organic solvent nanofiltration (OSN) membranes is an important property that influences the separation efficiency. Therefore, the affinity between active layer materials and VOCs was investigated by measuring the liquid-liquid equilibria (LLE) between neat polymers used as active layer materials and VOCs. The measured phase equilibria are modeled using the thermodynamic PC-SAFT (perturbed chain statistical associating fluid theory) model. This model allows for estimation of the pressure- and temperature-dependent chemical potentials of VOCs and membrane materials in feeds, membranes, and permeates. Two polyimides (P84 and Matrimid 5218) were selected as typical OSN membrane materials, and five different VOCs (n-hexane, ethyl acetate, 2-propanol, ethanol, and toluene) were considered. It was found that all miscibility gaps between the polyimides and VOCs are open miscibility gaps. The miscibility gaps of Matrimid 5218 and VOCs are much smaller than those of P84 with all VOCs. By means of the PC-SAFT equation of state, the phase behavior of the two polyimides with all considered VOCs could be well described. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ie2011142
  • 2012 • 56 Modelling of dendritic growth and bubble formation
    Wu, W. and Zhu, M.F. and Sun, D.K. and Dai, T. and Han, Q.Y. and Raabe, D.
    IOP Conference Series: Materials Science and Engineering 33 (2012)
    A two-dimensional lattice Boltzmann method (LBM)-cellular automaton (CA) model is developed for the simulation of dendritic growth and bubble formation during alloy solidification. In the model, a kinetic LBM, which describes flow dynamics through the evolution of distribution functions of moving pseudo-particles, is adopted to numerically solve the gas-liquid two-phase flow based on the Shan-Chen multiphase scheme. The kinetics of dendritic growth is determined according to a local solute equilibrium approach. The present model takes into account the effect of liquid-solid phase transformation on the nucleation and growth of bubbles. The interaction mechanism between dendrites and bubbles is also embedded in the model. The wettability of a bubble on a smooth solid surface is simulated. The simulated contact angles with various interaction coefficients agree well with the data calculated from an empirical formula derived from the Young's equation. The proposed model is applied to simulate dendritic growth and bubble formation under directional solidification conditions. The simulated results are compared with those observed experimentally during solidification of a transparent organic material. The simulation results reveal some dynamic features of bubble nucleation, growth, and motion, as well as the interaction between the dendritic growth and bubble formation during solidification. © Published under licence by IOP Publishing Ltd.
    view abstractdoi: 10.1088/1757-899X/33/1/012103
  • 2012 • 55 Nanocomposite fibre fabrication via in situ monomer grafting and bonding on laser-generated nanoparticles
    van't Zand, D.D. and Nachev, P. and Rosenfeld, R. and Wagener, P. and Pich, A. and klee, D. and Barcikowski, S.
    Journal of Laser Micro Nanoengineering 7 21-27 (2012)
    Bioactive nanocomposites may become an important material if both the carrier matrix and the nanoparticle are biocompatible, like is known for zinc oxide and lactones. The fabrication of such nanocomposite made of polycaprolactone nanofibres with embedded nanoparticles is studied during laser ablation in liquid monomer and polymer solution. The in situ conjugation of zinc oxide nano-particles with ε-caprolactone followed by zinc-initiated polymerization was studied. Indication for covalent bonding between the zinc oxide nanoparticles and the carboxylic units of the oligomers is observed. In addition to the study of the intended nanohybrid formation, possible formation of unintended byproducts was investigated. Laser-induced pyrolysis of solvent was studied for nanosecond, picosecond, and femtosecond laser pulse durations at the same energy input, where all pulse durations caused unintended solvent modification and picosecond pulses were most efficient for nano-particle production. Heading towards fabrication of macroscopic bioactive fibre pads, the laser-generated zinc oxide polymer nanocomposite have been successfully spun into nanofibres using electrospinning. Polymer-embedding is demonstrated at the example of macroscopic nanocomposite fibre pads with various bio-relevant nanoparticles fabricated by laser ablation of magnesium, iron, and tantalum in polycaprolactone solution.
    view abstractdoi: 10.2961/jlmn.2012.01.0004
  • 2012 • 54 Nanoparticle formation in a cavitation bubble after pulsed laser ablation in liquid studied with high time resolution small angle x-ray scattering
    Ibrahimkutty, S. and Wagener, P. and Menzel, A. and Plech, A. and Barcikowski, S.
    Applied Physics Letters 101 (2012)
    We investigated nanoparticle formation after pulsed laser ablation in liquid using time-resolved small angle x-ray scattering. Laser ablation of a gold target in water induces a cavitation bubble in which two different particle species could be identified at maximum bubble extension: (i) primary particles of about 8-10 nm diameter, which show a smooth concentration gradient starting from the target and can also be found outside the cavitation bubble in the free liquid and (ii) secondary particles in the range of 45 nm diameter which have highest concentration in the upper part of the cavitation bubble but do not penetrate into the liquid. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4750250
  • 2012 • 53 On the spontaneous formation of clathrate hydrates at water-guest interfaces
    Boewer, L. and Nase, J. and Paulus, M. and Lehmkühler, F. and Tiemeyer, S. and Holz, S. and Pontoni, D. and Tolan, M.
    Journal of Physical Chemistry C 116 8548-8553 (2012)
    The formation of hydrates, cage-like water-gas structures, is of tremendous importance both in industries and research. Although of major significance, the formation process is not completely understood so far. We present a comprehensive study of hydrate formation at liquid-liquid interfaces between water and isobutane, propane, carbon dioxide, and at the liquid-gas interface between water and xenon. We investigated the structure of these interfaces under quiescent conditions in situ by means of X-ray reflectivity measurements both inside and outside the zone of hydrate stability. At the interfaces between water and liquid alkanes, no evidence for a structural change was found. In contrast, the accumulation of guest molecules inside nanothick interfacial layers was observed at the water-xenon and liquid-liquid water-CO 2 interfaces. We show that only those systems initially exhibiting such guest-enriched interfacial layers developed into macroscopic gas hydrates within our observation times (∼12 h). Therefore, these layers act as triggers for the spontaneous formation of macroscopic hydrates. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp211784w
  • 2012 • 52 Pulsed-field gradient NMR measurements on hydrogels from phosphocholine
    Linders, J. and Mayer, C. and Sekine, T. and Hoffmann, H.
    Journal of Physical Chemistry B 116 11459-11465 (2012)
    Gels from diacylphosphatidylcholine in glycerol/butylene glycol mixtures were investigated by pulsed-field gradient NMR measurements. Previous measurements had shown that the gels are formed by networks from crystalline multilamellar vesicles (MLV). The obtained self-diffusion coefficients for water and butylene glycol molecules indicate that both molecules occur in two different environments, even at temperatures above the phase transition T m where the system is still in a liquid crystalline state. While the larger fraction of the molecules shows a free self-diffusion process like in a homogeneous phase, the smaller fraction seems to be encapsulated in closed domains and undergoes only hindered self-diffusion. It is concluded that the hindered diffusions are due to the solvent molecules trapped between the bilayers of the multilamellar vesicles, while the free diffusion is assigned to the solvent molecules outside of the MLV. Since the fraction of the entrapped molecules does not change during phase transition, we assume that the structure of the network in the samples remains the same when gelation occurs. The gelation process is simply due to the transformation of the vesicle bilayers from the liquid crystalline to the crystalline state. The permeability of the bilayer for the solvent molecules is drastically changed by this transition. The exchange of water molecules through the bilayers slows down significantly below Tm: while the average residence time of water molecules inside the vesicles is smaller than 50 ms in the liquid crystalline state, this value increases to more than 1 s for the gel state. In the case of pure butylene glycol, no vesicles are present, and it is likely that these gels are formed from crystalline fibers. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp3046565
  • 2012 • 51 SintClad: A new approach for the production of wear-resistant tools
    Blüm, M. and Hill, H. and Moll, H. and Weber, S. and Theisen, W.
    Journal of Materials Engineering and Performance 21 756-763 (2012)
    Tools used in the mineral processing industry are required to feature high wear resistance to facilitate an adequate cost efficiency. These kinds of tools are made of composite materials based on a low-alloyed substrate material and a high-alloyed coating. The coatings can be applied in different ways using production processes like HIP cladding, deposit welding, and composite casting. The article is concerned with the problem of a novel and cost-effective coating alternative: sinter cladding, using the principle of supersolidus liquid-phase sintering (SLPS). Usually SLPS represents a sintering technique, which is used for the compaction of high-alloyed metal powders. However, no recognizable efforts were made to use the SLPSprocess for applying a PM-coating on a bulk substrate material. Sinter cladding for the first time uses SLPS to combine the process of powder compaction with the application of a coating to a solid steel substrate into one single step. Another advantage of the process is the possibility to produce massive bulk coatings with thicknesses exceeding 20 mm. This article is original in the scope of question and investigation methods in terms of microstructure, hardness profiles, EDX measurements, diffusion calculations, and computational thermodynamics. © ASM International.
    view abstractdoi: 10.1007/s11665-012-0199-y
  • 2012 • 50 Supersolidus liquid-phase sintering of ultrahigh-boron high-carbon steels for wear-protection applications
    Röttger, A. and Weber, S. and Theisen, W.
    Materials Science and Engineering A 532 511-521 (2012)
    Powder metallurgy (PM) represents an alternative to conventional casting processes for the production of wear-resistant materials. PM hard alloys for wear-protection applications feature both higher strength and fracture toughness compared to cast hard alloys due to their more finely grained microstructure. However, densification by hot-isostatic pressing (HIP), the conventional PM-compaction method, is relatively expensive and thus partially counteracts low-cost processing. To increase the economic efficiency of the processing route, supersolidus liquid-phase sintering (SLPS) was investigated. In addition, expensive Ni- and Co-base hard alloys were substituted by boron-rich Fe-base hard-facing alloys.In this study, three ultrahigh-boron hard-facing alloy powders were densified by SLPS and HIP. The sintering temperatures were optimized by means of sintering experiments that were supported by thermodynamic calculations. Both densification states were investigated and compared with respect to the microstructure and the tribological and mechanical properties of the compacted hard-facing alloys. It was shown that the mechanical and tribological properties are strongly influenced by the microstructure. Although the microstructure is affected by the chemical composition, it can also be adapted by the densification process. SLPS-densified hard-facing alloys have a coarse microstructure that imparts not only a high wear resistance but also a detrimental effect on the mechanical properties. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.msea.2011.10.118
  • 2012 • 49 Tailoring of CNT surface oxygen groups by gas-phase oxidation and its implications for lithium ion batteries
    Klink, S. and Ventosa, E. and Xia, W. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    Electrochemistry Communications 15 10-13 (2012)
    Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2011.11.012
  • 2012 • 48 Ultrafast transitions from solid to liquid and plasma states of graphite induced by X-ray free-electron laser pulses
    Hau-Riege, S.P. and Graf, A. and Döppner, T. and London, R.A. and Krzywinski, J. and Fortmann, C. and Glenzer, S.H. and Frank, M. and Sokolowski-Tinten, K. and Messerschmidt, M. and Bostedt, C. and Schorb, S. and Bradley, J.A. an...
    Physical Review Letters 108 (2012)
    We used photon pulses from an x-ray free-electron laser to study ultrafast x-ray-induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg-peak degradation is smaller and the ion-heating rate is faster than current x-ray-matter interaction models predict. © 2012 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.108.217402
  • 2012 • 47 Visualization of the gas flow in fuel cell bipolar plates using molecular flow seeding and micro-particle image velocimetry
    Hecht, C. and Van Der Schoot, N. and Kronemayer, H. and Wlokas, I. and Lindken, R. and Schulz, C.
    Experiments in Fluids 52 743-748 (2012)
    Main components of proton exchange membrane fuel cells are bipolar plates that electrically connect the electrodes and provide a gas flow to the membrane. We investigate the flow in the channel structures of bipolar plates. Flow seeding is used to visualize the propagating and mixing gas stream. It is shown that a part of the gas is transported perpendicularly to the channel structure. An analysis of the diffusion compared with the convection shows different transport behavior for both flow directions. Additionally, the convective flow field is investigated in detail near the channel wall using Micro-PIV in a Reynolds-number-scaled liquid fluid system. For a more exact comparison of the experimental setups, flow seeding in both gas and liquid systems is performed. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00348-011-1112-4
  • 2011 • 46 Alginate/poly-l-lysine capsules: Mechanical properties and drug release characteristics
    Leick, S. and Kemper, A. and Rehage, H.
    Soft Matter 7 6684-6694 (2011)
    In this paper we studied the mechanical stability and the release kinetics of different types of liquid-filled calcium alginate/poly-l-lysine capsules. The aqueous cores of these particles were filled with anthocyanins which have antioxidant abilities and may, as additives in foods, provide several benefits of health. By means of spinning capsule experiments it was possible to measure the deformation of the gel particles at various centrifugal forces. These investigations provided insight into the elastic properties of the capsule membranes. In a series of experiments we measured the capsule deformation as a function of the poly-l-lysine concentration and the adsorption time. From these data we calculated the surface Young moduli. In addition, the use of a pH-dependent UV/VIS-absorption spectroscopy method gained access to the diffusive drug-release performance of the encapsulated anthocyanins. From these kinetic measurements we could evaluate the effective diffusion constants of the encapsulated compounds. The performed experiments showed that the mechanical properties of liquid-filled alginate capsules could be changed and adjusted selectively by the addition of poly-l-lysine. The drug release properties, however, did not change significantly for different compositions of the multi-component capsules. In addition, it could be shown that a high amount of anthocyanin molecules was immobilized in the capsules. This phenomenon could be explained by adsorption or polymerization processes of the colored ingredients. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1sm05676j
  • 2011 • 45 Corrosion properties of a plastic mould steel with special focus on the processing route
    Hill, H. and Huth, S. and Weber, S. and Theisen, W.
    Materials and Corrosion 62 436-443 (2011)
    Applications in plastics processing bear increased requirements for the used materials, especially with respect to their corrosion and wear resistance. For this reason, special powder metallurgical tools steels were developed that fulfil these demands. The common processing route for their production is hot isostatic pressing (HIP) of pre-alloyed powders which is followed by hot working if semi-finished parts are to be produced. As an alternative to HIP, super solidus liquid phase sintering (SLPS) permits the consolidation of pre-alloyed tool steel powders to near net-shape parts. It can be performed in different sintering atmospheres. In this work, the plastic mould steel X190CrVMo20-4 was processed by SLPS in vacuum as well as under nitrogen atmosphere. The resulting materials were analysed with respect to their microstructure, tempering behaviour and corrosion resistance in 0.5 molar sulphuric acid in dependence of the heat treatment. As a reference, the HIPed and the HIPed and worked state were also investigated. The results show that different heat treatments alter the ranking of the sintered and the HIPed state with respect to corrosion resistance. As expected, a high tempering for maximum secondary hardness causes a significant loss of corrosion resistance. The experimental findings were supported by thermodynamic calculations based on slight alterations in chemical composition that result from the different manufacturing processes. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.200905570
  • 2011 • 44 Critical investigation of high temperature gas nitriding of a PM tool steel
    Weber, S. and Theisen, W.
    International Journal of Materials Research 102 17-24 (2011)
    The nitrogen uptake of high-alloyed steels, especially tool steels, from gaseous atmospheres at elevated temperatures is a well known effect. Besides, for instance, the nitrogen-based case hardening, one application arises in the field of powder metallurgy. Here, introducing nitrogen containing atmospheres during sintering of high-alloyed powder metal-lurgically produced tool steels could lead to an optimization of the sintering process and an improvement of the materials properties. In this context, several works deal with the application of computational thermodynamics for considering the nitrogen uptake. The scope of this contribution is to investigate in detail the agreement of calculated and experimentally found nitrogen contents. Therefore, one typical gas-atomized powder of a ledeburitic cold work steel was sintered at a temperature of 1230 °C, varying the nitrogen partial pressure during sintering between 0.02 MPa and 0.2 MPa. The measured nitrogen contents are compared with values obtained from equilibrium calculations. Additionally, the distribution of nitrogen in the microstructure is investigated by time-resolved optical emission spectrometry and energy dispersive X-ray spectrometry measurements. The results exhibit good agreement between calculated and measured values for low partial pressures of less than 0.1 MPa and an increasing deviation for higher partial pressures. Furthermore, the transformation of vanadium-containing MC carbides to M(C, N) carbonitrides was verified. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110447
  • 2011 • 43 Delay time and concentration effects during bioconjugation of nanosecond laser-generated nanoparticles in a liquid flow
    Sajti, C.L. and Barchanski, A. and Wagener, P. and Klein, S. and Barcikowski, S.
    Journal of Physical Chemistry C 115 5094-5101 (2011)
    Fast ex situ functionalization of gold nanoparticles with fluorophore-labeled cell-penetrating peptides is investigated with a novel liquid flow cascade injection system. Successful conjugation is proved by various methods, such as UV-vis spectrometry and electron microscopy, whereas nanoparticle size-quenching is clearly observed. By variation of the peptide concentration introduced promptly after particle generation, gold nanoparticle bioconjugates with different degrees of cluster formation and/or aggregation and different peptide surface coverage values are obtained. The sizes of synthesized inorganic-organic gold nanoparticle bioconjugate show obvious correlation with time-delayed conjugation, giving evidence that laser-generated nanoparticles continue growing outside the cavitation bubble in the multisecond time scale until achieving their final size. Introducing 6.6 μM bioactive ligands, the highest conjugation efficiency of 93% and ? potential of 27.5 mV is reached at the shortest delay time (200 ms), resulting in 20 nm average sized bioconjugates. Finally, in a preliminary biological application, laser scanning confocal microscopy clearly revealed an amplified cellular uptake using HIV-1 transactivator peptide-conjugated gold nanoparticles compared with nonconjugated entities within embryonic fibroblasts after a short coincubation time of 1 h. The generation of high amounts of highly pure cell-penetrating nanomarkers by the nanosecond laser-assisted fast ex situ conjugation is thus a promising method to probe biological activities such as nanodrug internalization mechanisms. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp1093405
  • 2011 • 42 Ice formation in porous media
    Bluhm, J. and Ricken, T. and Bloßfeld, M.
    Lecture Notes in Applied and Computational Mechanics 59 LNACM 153-174 (2011)
    Ice formation in porous media results from coupled heat and mass transport and is accompanied by ice expansion. The volume increase in space and time corresponds to the moving freezing front inside the porous solid. In this contribution, a macroscopic model based on the Theory of Porous Media (TPM) is presented toward the description of freezing and thawing processes in saturated porous media. Therefore, a quadruple model consisting of the constituents solid, ice, liquid and gas is used. Attention is paid to the description of capillary suction, liquid- and gas pressure on the surrounding surfaces, volume deformations due to ice formation, temperature distribution as well as influence of heat of fusion under thermal loading. For detection of energetic effects regarding the control of phase transition of water and ice, a physically motivated evolution equation for the mass exchange based on the local divergence of the heat flux is used. Numerical examples are presented to the applications of the model. © 2011 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/978-3-642-22738-7_8
  • 2011 • 41 Identification of the active species generated from supported Pd catalysts in heck reactions: An in situ quick scanning EXAFS investigation
    Reimann, S. and Stötzel, J. and Frahm, R. and Kleist, W. and Grunwaldt, J.-D. and Baiker, A.
    Journal of the American Chemical Society 133 3921-3930 (2011)
    Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd0 clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr4]2-, [Pd2Br6]2-). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja108636u
  • 2011 • 40 Influence of Na on the structure of Bi4Ti3O 12 films deposited by liquid-delivery spin MOCVD
    Schwarzkopf, J. and Dirsyte, R. and Devi, A. and Kwasniewski, A. and Schmidbauer, M. and Wagner, G. and Michling, M. and Schmeisser, D. and Fornari, R.
    Thin Solid Films 519 5754-5759 (2011)
    Thin Na-substituted Bi4Ti3O12 films were grown by the liquid-delivery spin metal-organic chemical vapor deposition (MOCVD) method with different concentrations of sodium bis(trimethylsilyl)amide [Na(TMSA)] as Na precursor. At a substrate temperature of 600 °C the original Aurivillius structure was preserved, however high resolution x-ray diffraction (HRXRD) studies indicate that the Na-substituted phase exhibits a slightly smaller lattice parameter compared to the pure Bi4Ti 3O12 phase. From additional x-ray photoemission spectroscopy (XPS) results, we have concluded that monovalent Na+ ions have been incorporated on Bi3+ sites in the perovskite units. The proposed charge compensation for this aliovalent substitution is explained by a shift of the valence state of Bi3+ ions in the vicinity of the incorporated Na+ ions from 3+ to 5+. Due to the small ionic radius of the Bi5+ ions, the incorporation efficiency amounts to a few atomic percent only. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2010.12.206
  • 2011 • 39 Investigation of corrosion and wear properties of fe based metal matrix composites consolidated by sintering and hot isostatic pressing
    Hill, H. and Weber, S. and Siebert, S. and Theisen, W.
    Powder Metallurgy 54 455-462 (2011)
    Many industrial applications, e.g. processing of polymers, suffer from high costs caused by corrosion and wear. Particularly the combination of both increases the requirements for the materials used. Corrosion resistant cold work steels were developed to withstand the combined attack. Resistance is achieved by a sufficient content of chromium in the metal matrix and by carbides dispersed in a martensitic matrix. A further gain in wear resistance is possible by adding hard phases to the steel to produce a particulate reinforced metal matrix composite (MMC). The common consolidation process for such MMCs is hot isostatic pressing, but they can also be processed by solid state or liquid phase sintering. This work focuses on detailed investigations of the properties in dependence on the processing route. The results show that the resulting corrosion and wear resistance depend not only on the processing method, but also on the incorporated hard phases in combination with the manufacturing method. In addition, the unreinforced metal matrices were compared to the MMC. © 2011 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/003258910X12678035166656
  • 2011 • 38 Janus cylinders at liquid-liquid interfaces
    Ruhland, T.M. and Gröschel, A.H. and Walther, A. and Müller, A.H.E.
    Langmuir 27 9807-9814 (2011)
    We describe the first study on the self-assembly behavior of Janus cylinders at liquid/liquid interfaces. The Janus cylinders are characterized by a phase separation along the major axis into two hemicylinders of different wettability. The pendant drop technique and microscopic imaging were used to characterize the adsorption behavior and self-assembly of Janus cylinders at perfluorinated oil/dioxane and perfluorinated oil/dimethyl sulfoxide interfaces. According to the evolution of the interfacial tension and a series of TEM images taken during the cylinder adsorption, we will specify the characteristics of early to late stages of the Janus cylinder adsorption at a liquid-liquid interface and discuss the effect of Janus cylinder length and their concentration. We also establish that the broken symmetry of the corona leads to significantly higher interfacial activity as compared to homogeneous core-shell cylinders. The adsorption is characterized by three different adsorption stages: first, free diffusion to the interface, followed by continuous adsorption of cylinders including ordering and domain formation and, finally, additional packing with a rearrangement of domains and formation of a loose multilayer system. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la201863x
  • 2011 • 37 Laser-based diagnostics for the measurement of liquid water film thickness
    Greszik, D. and Yang, H. and Dreier, T. and Schulz, C.
    Applied Optics 50 A60-A67 (2011)
    Three different diagnostic techniques are investigated for measurement of the thickness of liquid water films deposited on a transparent quartz plate. The methods are based on laser-induced fluorescence (LIF) from low concentrations of a dissolved tracer substance and spontaneous Raman scattering of liquid water, respectively, both excited with 266nm of radiation, and diode laser absorption spectroscopy (DLAS) in the near-infrared spectral region. Signal intensities are calibrated using liquid layers of known thickness between 0 and 1000 μm. When applied to evaporating liquid water films, the thickness values derived from the direct DLAS and Raman scattering measurements correlate well with each other as a function of time after the start of data recording, while the LIF signal derived thickness values decrease faster with time due to selective tracer evaporation from the liquid. The simultaneous application of the LIF with a tracer-free detection technique can serve as an in situ reference for quantitative film thickness measurements. © 2010 Optical Society of America.
    view abstractdoi: 10.1364/AO.50.000A60
  • 2011 • 36 Lattice Boltzmann modeling of dendritic growth in forced and natural convection
    Sun, D.K. and Zhu, M.F. and Pan, S.Y. and Yang, C.R. and Raabe, D.
    Computers and Mathematics with Applications 61 3585-3592 (2011)
    A two-dimensional (2D) coupled model is developed for the simulation of dendritic growth during alloy solidification in the presence of forced and natural convection. Instead of conventional continuum-based NavierStokes (NS) solvers, the present model adopts a kinetic-based lattice Boltzmann method (LBM), which describes flow dynamics by the evolution of distribution functions of moving pseudo-particles, for the numerical computations of flow dynamics as well as thermal and solutal transport. The dendritic growth is modeled using a solutal equilibrium approach previously proposed by Zhu and Stefanescu (ZS), in which the evolution of the solid/liquid interface is driven by the difference between the local equilibrium composition and the local actual liquid composition. The local equilibrium composition is calculated from the local temperature and curvature. The local temperature and actual liquid composition, controlled by both diffusion and convection, are obtained by solving the LB equations using the lattice BhatnagarGrossKrook (LBGK) scheme. Detailed model validation is performed by comparing the simulations with analytical predictions, which demonstrates the quantitative capability of the proposed model. Furthermore, the convective dendritic growth features predicted by the present model are compared with those obtained from the ZhuStefanescu and NavierStokes (ZSNS) model, in which the fluid flow is calculated using an NS solver. It is found that the evolution of the solid fraction of dendritic growth calculated by both models coincides well. However, the present model has the significant advantages of numerical stability and computational efficiency for the simulation of dendritic growth with melt convection. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.camwa.2010.11.001
  • 2011 • 35 Liquid-phase epitaxy of multicomponent layer-based porous coordination polymer thin films of [M(L)(P)0.5] type: Importance of deposition sequence on the oriented growth
    Zacher, D. and Yusenko, K. and Bétard, A. and Henke, S. and Molon, M. and Ladnorg, T. and Shekhah, O. and Schüpbach, B. and Dea Losa Arcos, T. and Krasnopolski, M. and Meilikhov, M. and Winter, J. and Terfort, A. and Wöll, C. a...
    Chemistry - A European Journal 17 1448-1455 (2011)
    The progressive liquid-phase layer-by-layer (LbL) growth of anisotropic multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P)0.5] (M: Cu2+, Zn2+; L: dicarboxylate linker; P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers (SAMs) on gold substrates as templates. It was found that the deposition of smooth, highly crystalline, and oriented multilayer films of these PCPs depends on the conditions at the early growth cycles. In the case of a two-step process with an equimolar mixture of L and P, growth along the [001] direction is strongly preferred. However, employing a three-step scheme with full separation of all components allows deposition along the [100] direction on carboxyl-terminated SAMs. Interestingly, the growth of additional layers on top of previously grown oriented seeding layers proved to be insensitive to the particular growth scheme and full retention of the initial orientation, either along the [001] or [100] direction, was observed. This homo- and heteroepitaxial LbL growth allows full control over the orientation and the layer sequence, including introduction of functionalized linkers and pillars. One layer at a time: The stepwise liquid-phase layer-by-layer growth of anisotropic, multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P) 0.5] (M: Cu2+, Zn2+; L: dicarboxylate linker, P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers as templates. Highly oriented PCP multilayers were selectively grown along the [100] and [001] directions (see figure). © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201002381
  • 2011 • 34 Microfluidic emulsion separation - Simultaneous separation and sensing by multilayer nanofilm structures
    Uhlmann, P. and Varnik, F. and Truman, P. and Zikos, G. and Moulin, J.-F. and Müller-Buschbaum, P. and Stamm, M.
    Journal of Physics Condensed Matter 23 (2011)
    Emulsion separation is of high relevance for filtration applications, liquid-liquid-partitioning of biomolecules like proteins and recovery of products from droplet microreactors. Selective interaction of various components of an emulsion with substrates is used to design microfluidic flow chambers for efficient separation of emulsions into their individual components. Our lab-on-a-chip device consists of an emulsion separation cell with an integrated silicon sensor chip, the latter allowing the detection of liquid motion via the field-effect signal. Thus, within our lab-on-a-chip device, emulsions can be separated while the separation process is monitored simultaneously. For emulsion separation a surface energy step gradient, namely a sharp interface between the hydrophobic and hydrophilic parts of the separation chamber, is used. The key component of the lab-on-a-chip system is a multilayer and multifunctional nanofilm structure which not only provides the surface energy step gradient for emulsion separation but also constitutes the functional parts of the field-effect transistors. The proof-of-principle was performed using a model emulsion consisting of immiscible aqueous and organic solvent components. Droplet coalescence was identified as a key aspect influencing the separation process, with quite different effects during separation on open surfaces as compared to slit geometry. For a detailed description of this observation, an analytical model was derived and lattice Boltzmann computer simulations were performed. By use of grazing incidence small angle x-ray scattering (GISAXS) interfacial nanostructures during gold nanoparticle deposition in a flow field were probed to demonstrate the potential of GISAXS for insitu investigations during flow. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184123
  • 2011 • 33 Modelling of dendritic growth in ternary alloy solidification with melt convection
    Sun, D.-K. and Zhu, M.-F. and Dai, T. and Cao, W.-S. and Chen, S.-L. and Raabe, D. and Hong, C.-P.
    International Journal of Cast Metals Research 24 177-183 (2011)
    A two-dimensional lattice Boltzmann-cellular automaton model is coupled with the CALPHAD (Calculation of Phase Diagrams) method for simulating dendritic growth during ternary alloy solidification with convection. In the model, the kinetics of dendritic growth is determined by the difference between the equilibrium liquidus temperature and the actual temperature at the solid/liquid interface, incorporating the effects of the interface curvature and the preferred dendritic growth orientation. The lattice Boltzmann method is used for evaluating the local liquid compositions of the two solutes impacted by diffusion and convection. Based on the local liquid compositions, the equilibrium liquidus temperature and the solid concentrations of the two solutes are obtained by the CALPHAD method. The model is applied to simulate dendritic growth of an Al-4?0 wt-%Cu-1?0 wt-%Mg ternary alloy with melt convection. The results demonstrate the high numerical convergence and stability, as well as computational efficiency, of the proposed model. Melt convection is found to influence the dendritic morphologies and microsegregation patterns in the solidification of ternary alloys. © 2011 W. S. Maney & Son Ltd.
    view abstractdoi: 10.1179/136404611X13001912813988
  • 2011 • 32 n-Type Doping of Vapor-Liquid-Solid Grown GaAs Nanowires
    Gutsche, C. and Lysov, A. and Regolin, I. and Blekker, K. and Prost, W. and Tegude, F.-J.
    Nanoscale Research Letters 6 1-6 (2011)
    In this letter, n-type doping of GaAs nanowires grown by metal-organic vapor phase epitaxy in the vapor-liquid-solid growth mode on (111)B GaAs substrates is reported. A low growth temperature of 400°C is adjusted in order to exclude shell growth. The impact of doping precursors on the morphology of GaAs nanowires was investigated. Tetraethyl tin as doping precursor enables heavily n-type doped GaAs nanowires in a relatively small process window while no doping effect could be found for ditertiarybutylsilane. Electrical measurements carried out on single nanowires reveal an axially non-uniform doping profile. Within a number of wires from the same run, the donor concentrations N D of GaAs nanowires are found to vary from 7 × 10 17 cm -3 to 2 × 10 18 cm -3. The n-type conductivity is proven by the transfer characteristics of fabricated nanowire metal-insulator-semiconductor field-effect transistor devices. © 2010 The Author(s).
    view abstractdoi: 10.1007/s11671-010-9815-7
  • 2011 • 31 Organic nanoparticles generated by combination of laser fragmentation and ultrasonication in liquid
    Wagener, P. and Jakobi, J. and Barcikowski, S.
    Journal of Laser Micro Nanoengineering 6 59-63 (2011)
    Melamincyanurate microcrystals suspended in water were converted into colloidal nanoparticles by a novel approach approach of ultrasound-assisted laser fragmentation in a free liquid jet. A crucial step within the laser-based synthesis is the sufficient stabilization of nascent nanoparticles with an adequate stabilization agent. Electron microscopy of stabilized and unstabilized nanoparticle colloids revealed that insufficient stabilized colloids contain a huge fraction of agglomerates consisting of nanoparticles adsorbed on microparticles. Those agglomerates could be destroyed by ultrasound sonication. Therefore, an implementation of ultrasonication into the laser fragmentation process enhances efficiency which was quantified by absorption spectra. By using a high-power nanosecond laser we demonstrated that the technique of laser-fragmentation in free liquid jet could be suitable for scale-up because nanoparticle properties like hydrodynamic size or zeta potential did not depend on laser power or process time and laser-fabricated nanoparticle yield continuously increases during process duration.
    view abstractdoi: 10.2961/jlmn.2011.01.0013
  • 2011 • 30 PePC-SAFT: Modeling of polyelectrolyte systems 2. Aqueous two-phase systems
    Naeem, S. and Sadowski, G.
    Fluid Phase Equilibria 306 67-75 (2011)
    This work considers aqueous two-phase systems (ATPS) containing one polymer-polyelectrolyte as well as one salt. To model the liquid-liquid equilibria (LLE) of these systems, the recently presented model pePC-SAFT has been employed. ATPS containing poly(acrylic acid) of different degrees of neutralization or poly(vinyl pyrrolidone), respectively, were considered. The binary interaction parameters used between water-poly(acrylic acid) and water-poly(vinyl pyrrolidone) were adjusted to vapor-liquid equilibrium (VLE) data of these systems. ATPS consisting of poly(vinyl pyrrolidone)-water-sodium sulfate were predicted as function of temperature as well as of molar mass of the polymer. For poly(acrylic acid) systems, ATPS were predicted as function of charge density (degree of neutralization) for different types of salt. For these calculations, the polyelectrolyte model parameters were determined from the non-charged polymer whereas the effect of increasing charge density has been purely predicted by the model. Using this approach, it is possible to predict the shrinking of the liquid-liquid equilibrium region with increasing charging of the polyelectrolyte. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2011.02.024
  • 2011 • 29 Polymerisation of butyl acrylate in the two phase slug flow regime of parallel microcapillary reactors
    Mendorf, M. and Moenter, A. and Moll, T. and Agar, D.W. and Tiller, J.
    Macromolecular Symposia 302 245-256 (2011)
    Summary: The potential and problems of conducting a free radical polymerisation in parallel capillary reactors are presented. By operating in the so-called slug flow regime of immiscible liquid-liquid flow, one can achieve perfectly uniform residence times which are inaccessible using single phase flow. The excellent performance available in microreactors can be exploited for higher throughputs through the simple expedient of numbering-up, i.e. operation of multiple similar reactors in parallel under identical hydrodynamic conditions. In practice this approach often comes to grief on the coupling between hydrodynamics and chemical reaction, for example due to the strong influence of polymerisation on viscosity. Rigorous modeling reveals that the operating conditions sought are actually unstable. Furthermore, the uniformity of flow distribution between parallel capillaries was found to be very sensitive to the manufacturing tolerances of the capillaries used in the presence of polymerisation. Two strategies for resolving such problems are discussed. In the first case, coupling between reaction and the flow distribution is suppressed by a sufficiently high pressure drop upstream of the temperature regulated reactor segments. The pressure drop necessary to achieve this decoupling was estimated by the model. An alternative technique involves an appropriately inexpensive flow control system for each individual capillary. Since commercially available microvalves and flow measurement equipment are too costly for parallelisation purposes, it is necessary to develop new components to fulfill these functions. An optical monitoring technique is presented that meets both the technical and economic criteria, and which can be readily combined with recently developed new micro valves.1 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/masy.201000067
  • 2011 • 28 Structure and flow of droplets on solid surfaces
    Müller-Buschbaum, P. and Magerl, D. and Hengstler, R. and Moulin, J.-F. and Körstgens, V. and Diethert, A. and Perlich, J. and Roth, S.V. and Burghammer, M. and Riekel, C. and Gross, M. and Varnik, F. and Uhlmann, P. and Stamm, ...
    Journal of Physics Condensed Matter 23 (2011)
    The structure and flow of droplets on solid surfaces is investigated with imaging and scattering techniques and compared to simulations. To access nanostructures at the liquid-solid interface advanced scattering techniques such as grazing incidence small-angle x-ray scattering (GISAXS) with micro-and nanometer-sized beams, GISAXS and insitu imaging ellipsometry and GISAXS tomography are used. Using gold nanoparticle suspensions, structures observed in the wetting area due to deposition are probed insitu during the drying of the droplets. After drying, nanostructures in the wetting area and inside the dried droplets are monitored. In addition to drying, a macroscopic movement of droplets is caused by body forces acting on an inclined substrate. The complexity of the solid surfaces is increased from simple silicon substrates to binary polymer brushes, which undergo a switching due to the liquid in the droplet. Nanostructures introduced in the polymer brush due to the movement of droplets are observed. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184111
  • 2011 • 27 Surface chemistry of metal-organic frameworks at the liquid-solid interface
    Zacher, D. and Schmid, R. and Wöll, C. and Fischer, R.A.
    Angewandte Chemie - International Edition 50 176-199 (2011)
    MOFs on surfaces: Many parameters need to be considered in the formation of metal-organic frameworks (MOFs; see structures) at the liquid-solid interface. The methods and growth mechanisms for the layer-by-layer deposition of MOFs on functional materials, the homo- and heteroepitaxial deposition of MOF heterocrystals, and the coordination modulation of MOF surfaces are reviewed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201002451
  • 2011 • 26 Temperature-induced structural changes of tetrahydrofuran clathrate and of the liquid water/tetrahydrofuran mixture
    Lehmkühler, F. and Sakko, A. and Steinke, I. and Sternemann, C. and Hakala, M. and Sahle, C.J. and Buslaps, T. and Simonelli, L. and Galambosi, S. and Paulus, M. and Pylkkänen, T. and Tolan, M. and Hämäläinen, K.
    Journal of Physical Chemistry C 115 21009-21015 (2011)
    We present two complementary inelastic X-ray scattering studies on the structure of tetrahydrofuran (THF) clathrate hydrate and the supercooled stoichiometric liquid mixture of water and THF. Compton scattering experiments of the liquid mixture show that formation of hydrate precursors is unlikely. By comparing experimental spectra of THF hydrate and water/THF mixtures at temperatures above 250 K with density functional theory calculations, structural changes that manifest in OH bond length changes are observed. X-ray Raman scattering measurements of the oxygen K-edge in the same temperature range corroborate these results. The experimental results of THF hydrate at temperatures between 20 and 244 K can be modeled best by assuming thermal expansion only. Therefore, dependency on the system's temperature different structural behavior of THF hydrate is reported. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp207027p
  • 2011 • 25 The impact of processing on microstructure, single-phase properties and wear resistance of MMCs
    Hill, H. and Weber, S. and Huth, S. and Niederhofer, P. and Theisen, W.
    Wear 271 1895-1902 (2011)
    High mechanical loads and abrasive wear are boundary conditions for many tooling materials used in modern economy. One of the standard materials used in high abrasive environments is tool steel, or more specifically cold work tool steel. Wear resistance can be increased by adding hard phases like titanium carbides (TiC) to obtain a particle-reinforced metal-matrix composite (MMC) that can be produced either by hot isostatic pressing (HIP) or super solidus liquid-phase sintering (SLPS). Starting from the same raw materials, these two processes lead to different microstructures. The amount and dispersion of the hard phases as well as their hardness and fracture toughness control the wear resistance. In this study the hardness, Young's modulus and the fracture toughness of titanium carbides in Fe-base MMCs have been investigated by nanoindentation measurements. Owing to the small size of the TiC particles, the matrix properties have a pronounced effect on the results. This effect was studied in dependence on the indentation depth to enable the discussion of both the apparent and the real changes in the properties resulting from the production process and the heat treatment. This contribution also discusses the link between the microstructural properties and the macroscopic wear behavior. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.wear.2010.11.031
  • 2011 • 24 Thermodynamic modelling of the Ag-Cu-Ti ternary system
    Dezellus, O. and Arroyave, R. and Fries, S.G.
    International Journal of Materials Research 102 286-297 (2011)
    The Ag-Cu-Ti system is important for brazing applications, particularly for ceramic joining. This system is characterized by numerous intermetallics in the Cu-Ti binary and the existence of a miscibility gap in the liquid phase. For applications, knowledge of the phase equilibria, invariant reactions in the temperature range of interest and thermodynamic activity values (mainly of Ti) are important. Thermodynamic model parameters for all the stable phases in the Ag-Cu, Cu-Ti and Ag-Ti systems, previously obtained using the Calphad method and available in the literature are used. A new thermodynamic description for the ternary interaction parameter of the liquid is obtained from experimental informations. Ti 2Cu and Ti 2Ag which have the same crystallographic structure were modelled as a single phase. The same was done for TiCu and TiAg. Finally, solid solubility of Ag in the Ti-Cu intermetallics is taken into account. The parameters obtained in this assessment are later used for the calculation of selected sections that can be useful for research and applications in the field of joining with Ti-activated Ag-Cu braze. © Carl Hanser Verlag GmbH & Co. KG.
    view abstractdoi: 10.3139/146.110472
  • 2011 • 23 Transfer-matrix method for efficient ablation by pulsed laser ablation and nanoparticle generation in liquids
    Menéndez-Manjón, A. and Wagener, P. and Barcikowski, S.
    Journal of Physical Chemistry C 115 5108-5114 (2011)
    Comparable low nanoparticle production is a weakness of femtosecond-pulsed laser ablation in liquids, but the process ablation rate can be maximized at optimal focusing conditions and liquid levels. Refraction at the air-liquid boundary, vaporization of the liquid, self-focusing, and optical breakdown in the liquid complicate the determination of these optimal parameters. A semiempirical method has been developed, allowing an a priori determination of the appropriate experimental setup (liquid layer over the target, focal length, and lens position) for efficient ablation. The presented work can be applied with high accuracy for tightly focused beams, whereas loosely focused ultrashort lasers should be avoided to induce effective fabrication of colloids via laser ablation in liquids. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp109370q
  • 2011 • 22 Tunable diode laser absorption sensor for the simultaneous measurement of water film thickness, liquid- and vapor-phase temperature
    Yang, H. and Greszik, D. and Wlokas, I. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 104 21-27 (2011)
    A four-wavelength near-infrared (NIR) tunable diode laser sensor has been developed for the simultaneous measurement of liquid water film thickness, liquid-phase temperature and vapor-phase temperature above the film. This work is an important improvement of a three-wavelength concept previously introduced by Yang et al. (Appl. Phys. B 99:385, 2010), which measured the film thickness in environments with known temperature only. In the new sensor, an optimized combination of four wavelengths is chosen based on a sensitivity analysis with regard to the temperature dependence of the liquid water absorption cross section around 1.4 μm. The temperature of liquid water and the film thickness are calculated from absorbance ratios taken at three wavelength positions assessing the broad-band spectral signature of liquid water. The vapor-phase temperature is determined from the absorbance ratio of two lasers rapidly tuned across two narrow-band gas-phase water absorption transitions. The performance of the sensor was demonstrated in a calibration cell providing liquid layers of variable thickness and temperature with uncertainties smaller than 5% for thickness measurements and 1.5% for liquid-phase temperatures, respectively. Experiments are also presented for time-resolved thickness and temperature measurements of evaporating water films on a quartz plate. © 2011 Springer-Verlag.
    view abstractdoi: 10.1007/s00340-011-4643-8
  • 2011 • 21 Wetting morphologies and their transitions in grooved substrates
    Seemann, R. and Brinkmann, M. and Herminghaus, S. and Khare, K. and Law, B.M. and McBride, S. and Kostourou, K. and Gurevich, E. and Bommer, S. and Herrmann, C. and Michler, D.
    Journal of Physics Condensed Matter 23 (2011)
    When exposed to a partially wetting liquid, many natural and artificial surfaces equipped with complex topographies display a rich variety of liquid interfacial morphologies. In the present article, we focus on a few simple paradigmatic surface topographies and elaborate on the statics and dynamics of the resulting wetting morphologies. It is demonstrated that the spectrum of wetting morphologies increases with increasing complexity of the groove structure. On elastically deformable substrates, additional structures in the liquid morphologies can be observed, which are caused by deformations of the groove geometry in the presence of capillary forces. The emergence of certain liquid morphologies in grooves can be actively controlled by changes in wettability and geometry. For electrically conducting solid substrates, the apparent contact angle can be varied by electrowetting. This allows, depending on groove geometry, a reversible or irreversible transport of liquid along surface grooves. In the case of irreversible liquid transport in triangular grooves, the dynamics of the emerging instability is sensitive to the apparent hydrodynamic slip at the substrate. On elastic substrates, the geometry can be varied in a straightforward manner by stretching or relaxing the sample. The imbibition velocity in deformable grooves is significantly reduced compared to solid grooves, which is a result of the microscopic deformation of the elastic groove material close to the three phase contact line. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/18/184108
  • 2010 • 20 Ablation efficiency of α-Al2O3 in liquid phase and ambient air by nanosecond laser irradiation
    Sajti, C.L. and Sattari, R. and Chichkov, B. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 100 203-206 (2010)
    Ablation efficiency and influences of laser parameters on a material removal rate by a nanosecond laser irradiation of α-Al2O 3 are studied in gas and liquid phases. The laser ablation in the air yields maximum material removal rate of 12 ng/pulse using a 4.6-mJ pulse energy at 4-kHz repetition rate, compared to 88 ng/pulse in the water flow. Using a specific interpulse distance and a laser repetition rate further increase material removal rate by factor of 3 and 65, respectively, owing to an optimized lattice temperature and laser pulse interactions with the generated cavitation bubble. For the ablation in the air, these parameters do not significantly affect the ablation efficiency. © 2010 The Author(s).
    view abstractdoi: 10.1007/s00339-010-5572-9
  • 2010 • 19 Accessing ultrashort reaction times in particle formation with SAXS experiments: ZnS precipitation on the microsecond time scale
    Schmidt, W. and Bussian, P. and Lindén, M. and Amenitsch, H. and Agren, P. and Tiemann, M. and Schüth, F.
    Journal of the American Chemical Society 132 6822-6826 (2010)
    Precipitation of zinc sulfide particles is a very rapid process, and monitoring of the particle growth is experimentally very demanding. Applying a liquid jet flow cell, we were able to follow zinc sulfide particle formation on time scales down to 10 -5 s. The flow cell was designed in such a way that data acquisition on the microsecond time scale was possible under steady-state conditions along a liquid jet (tubular reactor concept), allowing SAXS data accumulation over a time scale of minutes. We were able to monitor the growth of zinc sulfide particles and found experimental evidence for very rapid particle aggregation processes within the liquid jet. Under the experimental conditions the particle growth is controlled by mass transfer: i.e., the diffusion of the hydrogen sulfide into the liquid jet. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ja101519z
  • 2010 • 18 Adsorptive removal of alkoxyphenols from ketones and esters with activated carbon - Experiments and modelling
    Gräf, T. and Pasel, C. and Bathen, D.
    Chemie-Ingenieur-Technik 82 1763-1769 (2010)
    Ultra pure chemicals are most notably used in electronics, optics, pharmaceutics and analytics. For the production of ultra pure liquids adsorptive removal of trace compounds and moisture is the preferred treatment. Adsorption experiments with the model system activated carbon/alkoxyphenol in ketones and esters as solvent are presented. Points of interest are the investigation of adsorption mechanisms as well as the measurement and modelling of adsorption isotherms. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201000086
  • 2010 • 17 Analysis of the ion distribution at a charged solid-liquid interface using X-ray standing waves
    Brücher, M. and Jacob, P. and Von Bohlen, A. and Franzke, J. and Sternemann, C. and Paulus, M. and Hergenröder, R.
    Langmuir 26 959-966 (2010)
    Functionalized solid-liquid interfaces were analyzed by X-ray standing waves (XSW) combined with, streaming current measurements to study surface charges, interfacial potential, and ion distributions. Thin films of aqueous solution containing Br- anions and Fe3+ cations at a concentration of 10 mg/L were prepared on functionalized silicon wafers. Functionalization of Si surfaces was accomplished by aminosilane groups shifting the interfacial potential, toward, positive values. The ion distribution was measured with nanometer resolution, which allows distinguishing between absorbed and mobile ions at the surface and in the diffusive layer, respectively. For Br-, different degrees of ion attraction were measured for the pH values 5.7 and 2.8. The ion Debye length values of the diffuse layer were 4 and 2 nm, respectively. © 2009 American Chemical Society.
    view abstractdoi: 10.1021/la902385d
  • 2010 • 16 Anodic oxides on a beta type Nb-Ti alloy and their characterization by electrochemical impedance spectroscopy
    Woldemedhin, M.T. and Raabe, D. and Hassel, A.W.
    Physica Status Solidi (A) Applications and Materials Science 207 812-816 (2010)
    Anodic oxides were grown on the surface of an electropolished (Ti-30at% Nb) beta-titanium (β-Ti) alloy by cyclic voltammetry. The scan rate was 100 mVs -1 between 0 and 8V in increments of lV in an acetate buffer of pH 6.0. Electrochemical impedance spectroscopy was carried out right after each anodic oxide growth increment to study the electronic properties of the oxide/electrolyte interface in a wide frequency range from 100 kHz to 10 MHz with an AC perturbation voltage of 10 mV. A film formation factor of 2.4 nm V -1 was found and a relative permittivity number (dielectric constant) of 42.4 was deter- mined for the oxide film formed. Mott-Schottky analysis on a potentiostatically formed 7 nm thick oxide film was performed to assess the semiconducting properties of the mixed anodic oxide grown on the alloy. A flat band potential of - 0.47 V (standard hydrogen electrode, SHE) was determined, connected to a donor density of 8.2 × 1017cm -3. β-Ti being highly isotropic in terms of mechanical properties should be superior to the stiffer α-Ti compound. Its application, however, requires a passivation behaviour comparable or better than α-Ti which in fact is found. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983324
  • 2010 • 15 Comprehensive investigations of the supersolidus liquid-phase sintering of two plastic mold steels
    Hill, H. and Weber, S. and Siebert, S. and Huth, S. and Theisen, W.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 41 686-695 (2010)
    The processing of plastics, particularly reinforced composites, necessitates the use of corrosion- and wear-resistant materials for tools that come into contact with the polymer. For such applications, plastic mold steels were developed that offer not only a good wear resistance due to the presence of carbides in a martensitic matrix, but also good corrosion resistance provided primarily by a sufficient amount of dissolved chromium. The common processing route for these high-alloyed materials is the hot isostatic pressing (HIP) of gas-atomized powders (PM-HIP). In this context, sintering plays an insignificant role, except for the processing of metal-matrix composites (MMCs). The development of novel wear- and corrosion-resistant MMCs based on plastic mold steels requires knowledge of the sintering behavior of prealloyed powders of such tool steels. It is well known that alloyed powders can be processed by supersolidus liquid-phase sintering (SLPS), a method leading to almost full densification and to microstructures without significant coarsening effects. In this work, two different gas-atomized powders of plastic mold steels were investigated by computational thermodynamics, thermal analysis, sintering experiments, and microstructural characterization. The results show that both powders can be sintered to almost full density (1 to 3 pct porosity) by SLPS in a vacuum or a nitrogen atmosphere. Experimental findings on the densification behavior, nitrogen uptake, and carbide volume fractions are in good agreement with calculations performed by computational thermodynamics. © 2010 The Minerals, Metals & Materials Society and ASM International.
    view abstractdoi: 10.1007/s11661-009-0148-z
  • 2010 • 14 Depositions of SrRuO3 thin films on oxide substrates with liquid-delivery spin MOCVD
    Schwarzkopf, J. and Dirsyte, R. and Devi, A. and Schmidbauer, M. and Wagner, G. and Fornari, R.
    Thin Solid Films 518 4675-4679 (2010)
    Systematic variations of the deposition conditions for thin epitaxial SrRuO3 films with a liquid-delivery spin MOCVD were performed in order to get a detailed understanding of the physical and chemical issues in the growth process. We have observed that at very low as well as at high growth rates the structural ordering of the films and the lattice strain is low, while for optimized conditions (∼ 0.14-0.2 nm/min) films can be grown under high compressive strain on SrTiO3 and under tensile strain on DyScO 3, showing an electrical resistivity of ∼ 250 μΩcm. Films on NdGaO3 are nearly totally plastically relaxed. In contrast to PLD, step-flow growth could not be detected due to significantly higher carbon incorporation. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2009.12.057
  • 2010 • 13 Diffusivities of an Al-Fe-Ni melt and their effects on the microstructure during solidification
    Zhang, L. and Du, Y. and Steinbach, I. and Chen, Q. and Huang, B.
    Acta Materialia 58 3664-3675 (2010)
    A systematical investigation of the diffusivities in an Al-Fe-Ni melt was presented. Based on the experimental and theoretical data about diffusivities, the temperature- and composition-dependent atomic mobilities were evaluated for the elements in Al-Ni, Al-Fe, Fe-Ni and Al-Fe-Ni melts via an effective approach. Most of the reported diffusivities can be reproduced well by the obtained atomic mobilities. In particular, for the first time the ternary diffusivity of the liquid in a ternary system is described in conjunction with the established atomic mobilities. The effect of the atomic mobilities in a liquid on microstructure and microsegregation during solidification was demonstrated with one Al-Ni binary alloy. The simulation results indicate that accurate databases of mobilities in the liquid phase are much needed for the quantitative simulation of microstructural evolution during solidification by using various approaches, including DICTRA and the phase-field method. © 2010 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2010.03.002
  • 2010 • 12 Efficient phase separation and product recovery in organic-aqueous bioprocessing using supercritical carbon dioxide
    Brandenbusch, C. and Bühler, B. and Hoffmann, P. and Sadowski, G. and Schmid, A.
    Biotechnology and Bioengineering 107 642-651 (2010)
    Biphasic hydrocarbon functionalizations catalyzed by recombinant microorganisms have been shown to be one of the most promising approaches for replacing common chemical synthesis routes on an industrial scale. However, the formation of stable emulsions complicates downstream processing, especially phase separation. This fact has turned out to be a major hurdle for industrial implementation. To overcome this limitation, we used supercritical carbon dioxide (scCO2) for both phase separation and product purification. The stable emulsion, originating from a stereospecific epoxidation of styrene to (S)-styrene oxide, a reaction catalyzed by recombinant Escherichia coli, could be destabilized efficiently and irreversibly, enabling complete phase separation within minutes. By further use of scCO2 as extraction agent, the product (S)-styrene oxide could be obtained with a purity of 81% (w/w) in one single extraction step. By combining phase separation and product purification using scCO2, the number of necessary workup steps can be reduced to one. This efficient and easy to use technique is generally applicable for the workup of biphasic biocatalytic hydrocarbon functionalizations and enables a cost effective downstream processing even on a large scale. Biotechnol. Bioeng. 2010;107:642-651. © 2010 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.22846
  • 2010 • 11 Field-induced Tomonaga-Luttinger liquid phase of a two-leg spin-1/2 ladder with strong leg interactions
    Hong, T. and Kim, Y.H. and Hotta, C. and Takano, Y. and Tremelling, G. and Turnbull, M.M. and Landee, C.P. and Kang, H.-J. and Christensen, N.B. and Lefmann, K. and Schmidt, K.P. and Uhrig, G.S. and Broholm, C.
    Physical Review Letters 105 (2010)
    We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). At temperatures below about 1a K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an S=1/2 two-leg spin ladder in the strong-leg regime. © 2010 The American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.105.137207
  • 2010 • 10 Growth and characterization of ti-ta-o thin films on si substrates by liquid injection MOCVD for high-k applications from modified titanium and tantalum precursors
    Devi, A. and Hellwig, M. and Barreca, D. and Parala, H. and Thomas, R. and Becker, H.-W. and Katiyar, R.S. and Fischer, R.A. and Tondello, E.
    Chemical Vapor Deposition 16 157-165 (2010)
    Titanium oxide (TiO2) and titanium-tantalum oxide (Ti-Ta-O) thin films are deposited by liquid injection (LI) metal-organic (MO) CVD using metal amide-malonate complexes, [Ti(NR2)2 (dbml) 2], and tantalum, [Ta(NMe2)2 (dbml)] (R Me, Et; dbml di-tert-butylmalonato). TiO2 and Ti-Ta-O films are deposited on Si(100) in the temperature ranges 350-650°C and 500-700°C, respectively. The structure, morphology, and chemical composition of the films are evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Rutherford backscattering spectroscopy (RBS), and X-ray photoelectron spectroscopy (XPS). The electrical properties of the films, namely the dielectric properties, are assessed by carrying out capacitance-voltage (C-V) measurements on metal-oxide-semiconductor (MOS) capacitor structures.
    view abstractdoi: 10.1002/cvde.200906813
  • 2010 • 9 Influence of water temperature on the hydrodynamic diameter of gold nanoparticles from laser ablation
    Menéndez-Manjón, A. and Chichkov, B.N. and Barcikowski, S.
    Journal of Physical Chemistry C 114 2499-2504 (2010)
    Defined hydrodynamic properties of nanoparticle colloids are required for applications in dosimetry, rheology, or biosensing studies. During the generation of nanoparticles by laser ablation of a solid target in liquids, the temperature of the liquid increases, which may effect cavitation bubble and particle formation. We demonstrate that this temperature variation influences the hydrodynamic diameter of the resulting colloidal nanoparticles when a gold target is ablated by an IR femtosecond laser in water at different stabilized liquid temperatures in the range of 283-353 K. The maximum hydrodynamic diameter was observed at 330 K, the temperature at which the compressibility of water reaches its minimum. The formation of particles by condensation of ablated species in the liquid matrix or inside the confined cavitation bubble is discussed, as well as the influence of the physical properties of the liquid that vary with temperature, such as viscosity and compressibility. The reduction of the hydrodynamic particle diameter at the higher compressible state of water indicates that a lower number of agglomerates are dispersed in the liquid, reducing the polydispersity index of the gold colloid. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp909897v
  • 2010 • 8 In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation
    Sajti, C.L. and Petersen, S. and Menéndez-Manjón, A. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 101 259-264 (2010)
    In-situ functionalization of gold nanoparticles with fluorophore-tagged oligonucleotides is studied by comparing femtosecond laser ablation in stationary liquid and in biomolecule flow. Femtosecond laser pulses induce significant degradation to sensitive biomolecules when ablating gold in a stationary solution of oligonucleotides. Contrary, in-situ conjugation of nanoparticles in biomolecule flow considerably reduces the degree of degradation studied by gel electrophoresis and UV-Vis spectrometry. Ablating gold with 100 μJ femtosecond laser pulses DNA sequence does not degrade, while the degree of fluorophore tag degradation was 84% in stationary solution compared to 5% for 1 mL/min liquid flow. It is concluded that femtosecond laser-induced degradation of biomolecules is triggered by absorption of nanoparticle conjugates suspended in the colloid and not by ablation of the target. Quenching of nanoparticle size appears from 0.5 μM biomolecule concentration for 0.3 μg/s nanoparticle productivity indicating the successful surface functionalization. Finally, increasing the liquid flow rate from stationary to 450 mL/min enhances nanoparticle productivity from 0.2 μg/s to 1.5 μg/s, as increasing liquid flow allows removal of light absorbing nanoparticles from the ablation zone, avoiding attenuation of subsequent laser photons. © 2010 The Author(s).
    view abstractdoi: 10.1007/s00339-010-5813-y
  • 2010 • 7 Laser fragmentation of organic microparticles into colloidal nanoparticles in a free liquid jet
    Wagener, P. and Barcikowski, S.
    Applied Physics A: Materials Science and Processing 101 435-439 (2010)
    We present a novel approach for laser fragmentation of melamine cyanurate microcrystals suspended in liquid into colloidal nanoparticles. Laser fragmentation is done by irradiating a liquid jet of melamine cyanurate suspended in water with intense picosecond pulses. The free liquid jet is generated by a nozzle with small diameter and provides a thin liquid filament (d fil< 1 mm) perpendicular to the focused laser beam. This geometry allows tight focusing resulting in high intensities without the danger of damaging an optical element like windows necessary in conventional flow cells or cuvettes. It reduces losses of excitation light by avoiding scattering or absorption in front of the focus. We stabilized the nanoparticles electrosterically in-situ with neutral and polyelectrolytic polymers preventing agglomeration and precipitation. The threshold for sufficient stabilization of laser-fragmented nanoparticles (d hydrodyn≈200 nm) is reached at a mass fraction of 0.25 wt% dextrin as a neutral polymer and 0.01 wt% polyacrylic acid as a polyelectrolytic polymer. Hydrodynamic size and zeta-potential of the nanoparticles can be controlled by mass fraction of the stabilization agent. © 2010 The Author(s).
    view abstractdoi: 10.1007/s00339-010-5814-x
  • 2010 • 6 Liquid injection MOCVD grown binary oxides and ternary rare-earth oxide as alternate gate-oxides for logic devices
    Thomas, R. and Ehrhart, P. and Waser, R. and Schubert, J. and Devi, A. and Katiyar, R.S.
    ECS Transactions 33 211-219 (2010)
    ZrO2, HfO2 and DyScO3 thin films having thickness in the range 2-20 nm were grown on SiOx/Si(100) substrates in a multi-wafer planetary MOCVD reactor combined with a liquid delivery system using engineered precursors. Growth rate, surface morphology, crystal structure, crystal density of the as-deposited films were analysed as a function of deposition temperature. The influence of post deposition annealing on the densification and crystallization was studied. Electrical properties of MIS capacitor structures are also discussed. Results on the optimised gate stack of Pt/ZrO2/SiOx/Si, Pt/HfO2/SiOx/Si, Pt/DyScO3/SiOx/Si are finally compared; and DyScO 3 seems to be promising high-k material candidate compared to Group-IVB oxides for the coming technology nodes. ©The Electrochemical Society.
    view abstractdoi: 10.1149/1.3481608
  • 2010 • 5 Microstructure and wear properties of novel sintered cold work steel and related particle reinforced composite materials
    Weber, S. and Li, J.R. and Theisen, W.
    Materials Science and Technology 26 1494-1502 (2010)
    In the field of wear resistant materials it is known, that for certain applications steel based composites produced by powder metallurgy are beneficial due to a higher wear resistance compared to conventional cast materials. Early developments of these high wear resistant MMC were dependent on hot isostatic pressing but latest experimental findings nowadays also allow for a production by liquid phase sintering. Several materials systems have already been investigated which, however, lack of a sufficient hardness in the as sintered state. Especially for wear resistant coatings it would be beneficial to avoid a separate hardening of a coated component. The low hardness in the as sintered state is related to the transformation kinetics of the metallic matrix of the coating materials, typically leading to a comparatively soft pearlitic microstructure, if the cooling rate is too low. The use of a nickel alloyed PM cold work steel as matrix material avoids this restriction, as the formation of pearlite and bainite is delayed significantly, improving the hardenability of the steel. Adding coarse hard particles of chromium carbide (Cr 3C2), aluminium zirconium oxide (AlZrO) or titanium carbide (TiC) to the steel powder, composite materials with a high abrasive wear resistance can be obtained by liquid phase sintering. The development of these materials, supported by thermodynamic calculations, is presented here together with results of the microstructural investigation and wear tests. © 2010 Institute of Materials, Minerals and Mining.
    view abstractdoi: 10.1179/026708309X12506933872982
  • 2010 • 4 Nonextensive entropy of quantum liquid in fractal dimension space
    Tayurskii, D.A. and Lysogorskii, Y.V.
    Journal of Low Temperature Physics 158 237-243 (2010)
    There are several approaches to describe the behavior of superfluid helium-4. For example, two-fluid model, the microscopic description based on the Gross-Pitaevskii equation and one-fluid theory in the framework of extended thermodynamics. Recently the observable peculiarities of quantum liquids behavior in the confined geometries (nanopores, aerogels, etc.) have caused the interest to the correct description of quantum liquids at nanoscale. The fractal geometry and the effects of huge inner surface area should be taken into account to describe dynamics and thermodynamics of liquid helium-4 inside nanoporous media. In the present paper we propose a two-fluid hydrodynamic model in fractal dimension space on the basis of a nonextensive entropy and energy approach. In the framework of this model the coupling between temperature and pressure oscillations ("sound modes conversion") due to fractal geometry is found. © 2009 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10909-009-0035-4
  • 2010 • 3 Optimum between purification and colloidal stability of ZnO nanoparticles
    Marczak, R. and Segets, D. and Voigt, M. and Peukert, W.
    Advanced Powder Technology 21 41-49 (2010)
    Crystalline ZnO quantum dots have been synthesized by hydrolysis of zinc acetate dihydrate with lithium hydroxide in ethanolic solution. By varying different parameters of the synthesis process, the size of the ZnO particles can be controlled. Detailed investigation of the ripening of the nanoparticles evidenced that despite of the well-known influence of ageing temperature and time, the presence of the reaction byproduct lithium acetate strongly affects the ripening behaviour. In particular, the particle size can be almost completely arrested by the removal of this byproduct via reversible flocculation of the ZnO nanoparticles using heptane as an antisolvent. A closer analysis of the repeated washing process shows an initial improvement of the colloidal stability of the ZnO nanoparticles during the first purification cycle as it mainly removes the lithium acetate from the suspension and not the stabilizing acetate groups directly bound to the particle surface. With further washing the remaining acetate ligands are unable to maintain the stabilization against agglomeration of the ZnO nanoparticles. Thus, there exists an optimum between purification progress and colloidal stability. These findings are also confirmed by calculations according to the DLVO theory, which show that there exists nearly no primary minimum of small ZnO nanoparticles below 5 nm in the presence of stabilizing acetate ions whereas the decrease in acetate ions bound to the particle surface leads to a more and more pronounced primary minimum. The present work is of particular significance for the preparation of purified colloidal ZnO nanoparticles for studies of their electrical and optical properties with respect to their wide range of potential applications. © 2009 The Society of Powder Technology Japan.
    view abstractdoi: 10.1016/j.apt.2009.10.005
  • 2010 • 2 Solubility, crystallization and oiling-out behavior of PEGDME: 1. Pure-solvent systems
    Kiesow, K. and Ruether, F. and Sadowski, G.
    Fluid Phase Equilibria 298 253-261 (2010)
    Oiling out denotes a (metastable) liquid-liquid demixing during cooling crystallization prior to formation of the first crystals. This in most cases unwanted effect deteriorates the properties of the desired solid product. On the basis of the crystallization of the model substance polyethylenglycoldimethylether (PEGDME) from pure solvents, the influence of the molecular size of the solute and the type of solvent on the oiling-out behavior was systematically investigated. In this study the solubility data were determined gravimetrically as well as by using differential scanning calorimetry. The crystallization and oiling-out temperatures were detected visually in batch crystallization experiments. Oiling out was observed during the crystallization of PEGDME with a molar mass of 2000. g/mol (PEGDME2000) from diethylketone, ethyl acetate and 2-propanol, whereas no oiling out was detected during the cooling process of PEGDME with a molar mass of 1000. g/mol (PEGDME1000) from all solvents considered. Furthermore the oiling-out temperature for PEGDME2000 was not significantly influenced by the chosen solvents diethylketone, ethyl acetate and 2-propanol. In the second part of this study, it is shown that the appearance and absence of oiling out in all considered solvents can be qualitatively predicted by the pertubed chain statistical association theory (PC-SAFT) only using solubility data. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2010.08.005
  • 2010 • 1 Which controls the depolymerization of cellulose in ionic liquids: The solid acid catalyst or cellulose?
    Rinaldi, R. and Meine, N. and vom Stein, J. and Palkovits, R. and Schüth, F.
    ChemSusChem 3 266-276 (2010)
    Cellulose is a renewable and widely available feedstock. It is a biopolymer that is typically found in wood, straw, grass, municipal solid waste, and crop residues. Its use as raw material for biofuel production opens up the possibility of sustainable biorefinery schemes that do not compete with food supply. Tapping into this feedstock for the production of biofuels and chemicals requires-as the first-step-its depolymerization or its hydrolysis into intermediates that are more susceptible to chemical and/or biological transformations. We have shown earlier that solid acids selectively catalyze the depolymerization of cellulose solubilized in 1-butyl-3-methylimidazolium chloride (BMIMCl) at 100°C. Here, we address the factors responsible for the control of this reaction. Both cellulose and solid acid catalysts have distinct and important roles in the process. Describing the depolymerization of cellulose by the equivalent number of scissions occurring in the cellulosic chains allows a direct correlation between the product yields and the extent of the polymer breakdown. The effect of the acid strength on the depolymerization of cellulose is discussed in detail. Practical aspects of the reaction, concerning the homogeneous nature of the catalysis in spite of the use of a solid acid catalyst, are thoroughly addressed. The effect of impurities present in the imidazolium-based ionic liquids on the reaction performance, the suitability of different ionic liquids as solvents, and the recyclability of Amberlyst 15DRY and BMIMCl are also presented. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.200900281