Universität Stuttgart
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Item Open Access Tuning the properties and microstructuring of ionic liquid mixtures at surfaces through atomistic modeling(2021) Kobayashi, Takeshi; Fyta, Maria (Prof. Dr.)Item Open Access Simulation of novel magnetic materials in the field of soft matter(2014) Weeber, Rudolf; Holm, Christian (Prof. Dr.)This thesis has dealt with the tailoring of magnetic soft matter. Two strategies are available to achieve this goal. First, it is possible to alter the magnetic nanoparticles, in order to change their interactions. Second, it is possible to exchange the carrier fluid into which the magnetic particles are embedded by a more complex matrix. For each of these two possibilities, an example was studied, namely shifted-dipole particles and magnetic gels. Shifted-dipole particles (sd-particles) are a special kind of model magnetic particles which can be used to explain findings for particles with magnetic caps as well as for particles with magnetic inclusions. Magnetic gels, on the other hand, derive their particular properties from an interplay of the magnetic properties of the nanoparticles and the elastic behavior of the polymer matrix. In the thesis, the findings for these two systems will be discussed, and further research questions will be identified.Item Open Access Modeling the translocation of DNA structures through nanopores(2021) Szuttor, Kai; Holm, Christian (Prof. Dr.)Item Open Access Lattice Boltzmann simulations of fluid flow in the vicinity of rough and hydrophobic boundaries(2010) Kunert, Christian; Harting, Jens (P.D. Dr.)In recent years, it became possible to perform very well controlled experiments that have shown a violation of the no-slip boundary condition in sub-micron sized geometries. Since then, mostly experimental, but also theoretical works, as well as computer simulations, have been performed to improve our understanding of boundary slip. The topic is of fundamental interest because it has practical consequences in the physical and engineering sciences as well as for medical and industrial applications. This work focuses on numerical investigations of the slip phenomenon by means of lattice Boltzmann simulations with a strong focus on roughness and the interplay between roughness and wetting phenomena. To do so, two different slip measurement methods are simulated. One is to apply a Poiseuille flow between two patterned boundaries, and to record the flow profile. Then, the profile can be compared to the theoretical one, which assumes a slip boundary condition. The second method records the drag force that is acting on a sphere which is moved with a constant velocity towards the observed surface. Due to the influence of the boundary, the drag force acting on the sphere is disturbed and a correction function is needed to describe the measured force.Item Open Access Hocheffiziente Präzisionsalgorithmen zur Modellierung und Analyse granularer poröser Medien(2020) Zauner, Thomas; Hilfer, Rudolf (Prof. Dr. Dr.)Item Open Access The electrophoretic mobility of bare and soft spherical colloids : a molecular dynamics study(2014) Raafatnia, Shervin; Holm, Christian (Prof. Dr.)In this work, the electrophoretic mobility of colloids in salt solutions are studied by means of coarse-grained Molecular Dynamics simulations. Two different types of colloids are considered; bare colloids and polyelectrolyte-grafted colloids. A novel model for simulation of large bare colloids in the presence of explicit ions is developed. Comparison of the results with experimental data helps gain a better understanding of the mechanisms responsible for the interesting phenomenon of mobility reversal. Furthermore, a hitherto unknown electrokinetic behavior of polyelectrolyte-grafted colloids is found from simulations including full hydrodynamic interactions. The validity of the existing theories is verified via comparison with simulation results.Item Open Access Entropic segregation of polymers under confinement(2016) Minina, Elena; Holm, Christan (Prof. Dr.)Overlapping polymers confined in a cylinder experience strong repulsion that drives them towards segregation. This has biological relevance to chromosome segregation in single-celled elongated bacteria such as Escherichia coli because in principle, chromosomes can segregate for purely entropic reasons without any help from active mechanisms. In this thesis, we investigated entropic segregation of polymers under cylindrical confinement of infinite length where the confining cylinder is so narrow that its diameter is significantly smaller than the radius of gyration of the unconfined polymers.Item Open Access From the inhomogeneous electron gas to classical force fields : a multi-scale model for ionic liquids(2013) Dommert, Florian; Holm, Christian (Prof. Dr.)Ionic liquids are a class of solvents that have attracted a broad interest in the recent decade. They are often liquid at room temperature, but consist only of cations and anions without any additional solvent. The characteristic properties of these liquids are the negligible volatility and their tunability. Thus they are often referred to as designer solvents, which can be adapted in order to increase the efficiency of many applications like catalysis, capturing of flue gases, or solar cells. The number of different ionic liquids is quite large and an experimental screening of all ionic liquids would be very expensive and time-consuming. At this stage, classical molecular dynamics simulations are a very suitable tool that allows one to study thermodynamic, structural, and dynamic properties of liquids, but they depend on an accurate force field. For ionic liquids, reliable and transferable force fields are rare. For this reason, the main object of this work is to establish a method to optimize or generate a set of force field parameters. This is achieved by mapping the electronic and geometric structure information of the liquid phase gathered on the DFT level to the classical scale. Apart from our test system dimethylimidazolium chloride [MMIM][Cl] , which was used to develop corresponding methods, we investigated a broad spectrum of imidazolium-based cations combined with the anions thiocyanate [SCN]-, dicyanamide [DCA]-, and chloride [Cl]-. These studies were based on CPMD simulation snapshots, which allowed one to gain insight into the electronic structure of ionic liquids, especially in mechanisms of partial charge distribution and the net—charge reduction, which has been identified as a model for implicit polarization and charge transfer. These features are not only characteristic for an IL, but it has also been shown, that they occur already on a very local scale, which is the reason for the good performance of partial charges that were derived from small ion pair clusters. It was shown that the predicted charge transfer and polarization is in agreement with NMR experiments and measurements of the refractive index. From the computational side, consistency of the dipole moments given by the partial charges or by a Wannier analysis of the CPMD results was achieved. Though the width of dipole moment distribution could not be completely reproduced by the static partial charges, the increase in the dipole moment for an increasing number of interacting ion pairs (IPs) coincides very well. This shows that the proposed set of partial charges is perfectly suitable for the bulk phase of an IL. Finally the short—range (SR) interactions were adapted in respect to our proposed set of partial charges. An algorithm was implemented that optimizes the SR parameters based on an iterative adaption to match static properties given by the experimental mass density and the radial distribution functions derived from CPMD simulations. The routine has been successfully tested for [MMIM][Cl] and it has been shown that the implicit consideration of polarization and charge transfer models the dynamics already well, such that one can rely on static properties during the SR parameter optimization. This alleviates the tuning process, because the amount of simulation time required to sample a highly viscous IL decreases and, apart from the experimental mass density, only computational results are required. The latter aspect is of great importance for ILs, because a broad spectrum of experimental knowledge is not a given. Thus, during this work, the technique and computational framework to optimize FF parameters for ionic liquids was established and a large data set was accumulated. Thus, a solid basis for future work is provided, the development of a generic and transferable force field for ILs.Item Open Access Coarse-grained simulations of molecular catalysis in confined spaces(2023) Tischler, Ingo; Holm, Christian (Prof. Dr.)Item Open Access Simulation and modeling of polyelectrolyte gels(2020) Landsgesell, Jonas; Holm, Christian (Prof. Dr.)Diese Dissertation behandelt die Entwicklung von Computermodellen zur Beschreibung von Polyelektrolytnetzwerken. Basierend auf dem periodische Gelmodell entwickeln wir zwei aufeinander aufbauende Computermodelle, welche zur Beschreibung der elastischen Eigenschaften von Polyelektrolytgelen dienen: das Einzelketten-Zellen-Gelmodell (ZGM) und das Poisson-Boltzmann Zellen-Gelmodell (PB ZGM). Da viele Polyelektrolytgele aus Bausteinen bestehen, welche chemisch reaktiv sind, ist es wichtig diese Eigenschaft korrekt in Computermodellen abzubilden. Zur Untersuchung dieser schwachen Polyelektrolytgele führen wir eine Methode zur Simulation von Ionisationsgleichgewichten in solchen Systemen ein. Der pH-Wert und die Salzkonzentration werden durch die Zusammensetzung der Überstandslösung definiert. Unsere Implementierung des Teilchenaustausches mit der Überstandslösung vermeidet bekannte Artefakte und unphysikalische Parameterkombinationen.
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