Universität Stuttgart
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Item Open Access Efficient algorithms for electrostatic interactions including dielectric contrasts(2013) Arnold, Axel; Breitsprecher, Konrad; Fahrenberger, Florian; Kesselheim, Stefan; Lenz, Olaf; Holm, ChristianCoarse grained models of soft matter are usually combined with implicit solvent models that take the electrostatic polarizability into account via a dielectric background. In biophysical or nanoscale simulations that include water, this constant can vary greatly within the system. Performing molecular dynamics or other simulations that need compute exact electrostatic interactions between charges in those systems is computationally demanding. We review here several algorithms developped by us that perform exactly this task. For planar dielectric surfaces in partial periodic boundary conditions, the arising image charges can be either treated with the MMM2D algorithm in a very efficient and accurate way, or with the ELC term that enables the user to use his favorite 3D periodic Coulomb solver . Arbitrarily shaped interfaces can be dealt with using induced surface charges with the ICC algorithm. Finally, the local electrostatics algorithm MEMD (Maxwell Equations Molecular Dynamics) allows even to employ a smoothly varying dielectric constant in the systems. We introduce the concepts of these three algorithms, and an extension for the inclusion of boundaries that are to be held fixed at constant potential (metal conditions). For each method, we present a showcase application to highlight the importance of dielectric interfaces.Item Open Access Simulation of peloids(2007) Hecht, Martin; Herrmann, Hans J. (Prof. Dr.)In this work we investigate dense colloidal suspensions of alumina particles, which we regard as a model system for clay-like soils (peloids). Beyond soil mechanics these suspensions are important for wet processing of work-pieces in ceramics. In the present work we investigate the rheological properties of the suspensions and connect them to the underlying microstructure. Experimental findings depending on the pH value and the salt concentration in the sample (expressed as ionic strength) are reviewed (viscosity versus shear rate, shear thinning, oedometer data, sedimentation experiments, cyclic loading...) and computer simulations are performed to carry out further research work. For this purposes a coupled Molecular Dynamics (MD) and Stochastic Rotation Dynamics (SRD) code has been developed and the simulation results are compared to the experimental data. To describe the surface charge of the particles a charge regulation model, which describes adsorption and desorption of the charge determining ions on the particle surface, has been developed within Debye Huckel theory. This model has been calibrated to measurements of the zeta potential. Starting from the known values, the model allows us to extrapolate to different experimental conditions. Based on characteristic time scales and dimensionless numbers like the Reynolds number and the Peclet number, we apply a scaling scheme to determine the simulation parameters, so that we can achieve a quantitative comparability of simulation and experiment. Using the shear viscosity, shear force, pair correlation function, density fluctuations and structure factor we can identify three different regimes: a clustered regime, a stable suspension, a repulsive structure similar to the structure known from glassy systems. The microstructures are plotted in a stability diagram depending on pH value an ionic strength. The microstructures found in the simulations provide a possibility to explain the relations found in the experiments.Item Open Access Untersuchung der Pfropfendynamik und -stabilität bei der vertikalen und horizontalen Pfropfenförderung(2005) Strauß, Martin; Herrman, Hans J. (Prof. Dr.)Die Pfropfenförderung stellt ein produktschonendes Transportverfahren für granulare Medien dar, dass in der Industrie breite Verwendung findet. Eine zuverlässige Voraussage des Massentransportes bei der Auslegung industrieller Anlagen ist beim derzeitigen Wissensstand jedoch nicht möglich, da Grundkenntnisse über diesen Fördertyp fehlen. Ursache ist, dass sich die Untersuchung der bei der Pfropfenförderung mittels Druckluft durch Transportröhren getriebenen Granulatpfropfen sich weitestgehend experimentellen Messmethoden entzieht. Um Zugang zu den Prozessen während des Transportes zu erhalten, wurde für diese Arbeit die Förderung mittels einer Kombination aus Molekulardynamiksimulation und einem Löser für den Druckverlust am System auf dem Computer nachgebildet. Die Gültigkeit des Ansatzes wurde mittels experimenteller Messdaten an einer Anlage im Maßstab 1 zu 1 nachgewiesen. Es wurden sowohl Parameterstudien für die vertikale als auch die horizontale Förderung durchgeführt. Die Ergebnisse zeigen unter anderem, dass die Stabilität der Pfropfen durch entgegengesetzt auf den Pfropfen wirkende Kräfte bewirkt wird. Maßgeblich sind dabei die vorantreibende Druckluft und die auf den Pfropfen treffenden bremsenden Granulatteilchen zwischen den Pfropfen. Der Effekt ist unabhängig von der Transportrichtung. Unterschiede im Längenwachstum und der inneren Dynamik der Pfropfen zwischen vertikaler und horizontaler Förderung zeigen, dass eine einheitliche Abhandlung der Förderungsrichtungen bei der Auslegung von industriellen Anlagen nicht sinnvoll ist.Item Open Access Sand dunes on Mars and on Earth(2007) Ribeiro Parteli, Eric Josef; Herrmann, Hans Jürgen (Prof. Dr.)In this work the dune model introduced by Sauermann et al. (2001) is extended and applied to investigate the formation of different dune shapes on Mars and on Earth as function of wind directionality and sand availability. The formation of sand dunes on Mars under the present atmospheric conditions of the red planet is studied and conclusions about wind speed, migration velocity of dunes and changing wind regimes on Mars are presented. Field measurements of the shape of coastal transverse dunes are presented and the formation of coastal dune fields is explained. Finally, the formation of linear dunes by bimodal wind regimes is calculated. The simulations explain the appearance of exotic bimodal dune shapes in areas of low sand availability on Mars and on Earth.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 Vegetated dunes and barchan dune fields(2007) Durán, Orencio; Herrmann, Hans J. (Prof. Dr.)Desertification is closely related to aeolian sediment transport, including sand dunes formation, evolution and migration. Sand dunes propagate in huge clusters of thousand of dunes with an internal complex dynamics that determines their size and spatial distribution. Furthermore, it is known that the vegetation growing on sand dunes is an active agent that modifies dune mobility and sand distribution in a desert area, ultimately leading to an inactive sand landscape. This work addresses three crucial problems related to aeolian sand transport, barchan dune fields and vegetated dunes. First, what are the phenomenological parameters behind our sand transport model, and how they are related with the physical properties of the system. Second, how are barchan dunes distributed over desert? Which factors determine the size and spatial distribution of dune fields? And, third, how can vegetation deactivate a barchan dune? Which conditions are behind this inactivation process? As a result, we were able to extend the phenomenological parameters of our sand transport model to different physical conditions, such as under water or in the Mars atmosphere. We found that the size distribution of barchan dune fields is log-normal and dunes are regularly spaced. Therefore, barchan dune fields are fully described by the mean dune size, the standard deviation and the inter-dune spacing. Furthermore, these properties are not independent but are related by a simple constitutive equation and are dynamically selected by the upwind boundary conditions in the dune field. Finally, we found that active barchan dunes are transformed into inactive parabolic dunes under the effect of vegetation growth. This inactivation process occurs if the fixation index, a parameter fully determined by initial conditions, is below a critical value. This is a simple condition that leads to the first quantitative solution of the crucial dune inactivation problem.Item Open Access Die Bedeutung perkolierender und nichtperkolierender Phasen bei Mehrphasenströmungen in porösen Medien auf Laborskala(2011) Doster, Florian; Hilfer, Rudolf (Prof. Dr. Dr.)Das Verständnis makroskopischer Phänomene bei Mehrphasenströmungen in porösen Medien ist sowohl von wissenschaftlicher Seite als auch für Anwendungen von großem Interesse. Obwohl seit über einem Jahrhundert intensiv daran geforscht wird und die Phänomene auf der Porenskala durch die Gleichungen der klassischen Hydrodynamik beschrieben werden, ist bis heute keine Theorie vorhanden, die auf Labor- und Feldskala Hysterese und residuale Fluidkonfigurationen bei Mehrphasenströmungen umfassend und physikalisch richtig beschreibt. Die Berücksichtigung der unterschiedlichen hydrodynamischen Eigenschaften von perkolierenden und nichtperkolierenden Fluidanteilen auf makroskopischen Skalen könnte der Schlüssel zu einem besseren Modell sein. Hilfer schlägt ein Modell vor (Phys. Rev. E. 73, 016307 (2006)), in welchem diese Unterschiede nicht ignoriert werden. Erste quasistationäre Lösungen dieses Modells geben Hoffnung, dass durch das Einbeziehen dieser Unterschiede in der Modellbildung Schwachpunkte traditioneller Ansätze behoben werden können. Weitergehende Untersuchungen dieses Modells und seiner Gleichungen bilden die Aufgabenstellung dieser Dissertation. Um die Gleichungen zu studieren, wurden Umformungen und Näherungen formuliert, die analytische Lösungen ermöglichen. Außerdem wurden Schließbedingungen formuliert, die die Lösung zeitabhängiger Fragestellungen erlauben. Vier Anfangs- und Randwertprobleme wurden analytisch bzw. quasianalytisch gelöst. Sie sind Verallgemeinerungen des Buckley-Leverett-Problems, der schwerkraftgetriebenen Umverteilung, des McWhorter-Sunada-Problems und des Philip-Problems. Ferner wurden vier numerische Algorithmen entwickelt, die Anfangs- und Randwertprobleme für unterschiedliche mathematische Formulierungen und physikalische Näherungen des Modells lösen. Mit diesen Algorithmen wurden Laborexperimente simuliert. Die Experimente können drei Klassen zugeordnet werden. Die erste Klasse bilden Experimente mit einer geschlossenen porösen Säule. In diesen Experimenten bewirken allein die Schwerkraft und Kapillar- und Grenzflächenkräfte eine Umverteilung der Fluide. Die Ergebnisse zeigen, dass das Modell hysteretisches Verhalten in Sättigungsverteilungen beschreiben kann. Sie illustrieren außerdem Gemeinsamkeiten und Unterschiede zu bestehenden Modellen. Die gewonnenen Aussagen können im Labor überprüft werden. Die zweite Klasse bilden Experimente, bei denen eine poröse Säule von einem von außen aufgeprägten Fluss durchströmt wird. Die Simulationen zeigen, dass die Beschreibung der Dynamik residualer Sättigungen mit diesem Modell möglich ist. Die dritte Klasse bilden Experimente mit Druckrandbedingungen. Solche Experimente werden im Labor zur Bestimmung und Überprüfung von Kapillardrucksättigungsbeziehungen durchgeführt. Die Simulation eines solchen Experiments stimmt gut mit Messwerten aus der Literatur überein. Die Ergebnisse dieser Dissertation zeigen, dass zumindest einige Schwächen traditioneller Ansätze durch die Berücksichtigung der unterschiedlichen hydrodynamischen Eigenschaften von perkolierenden und nichtperkolierenden Fluidanteilen behoben werden können. Für das neue Modell wurden analytische und quasianalytische Lösungen bestimmt und Verfahren entwickelt, numerische Lösungen zu berechnen. Einige der numerischen Lösungen wurden mit experimentellen Daten verglichen. Es konnte eine gute Übereinstimmung der Ergebnisse gezeigt werden. Die übrigen Lösungen dienen als Vorschlag für Experimentatoren. Eine Durchführung dieser Experimente im Labor würde wichtige Erkenntnisse zur Qualität des Modells liefern.Item Open Access Simulations on evolutionary phenomena with ageing models(2006) Schwämmle, Veit; Herrmann, Hans (Prof.)The idea of reducing the characteristics of highly complex systems to some basic features without losing the essential informations has been successful for the comprehension of not only physical systems. The experience in treating systems which consist of many elements that interact over different time scales as well as over wide distances, has opened many interdisciplinary fields to statistical physicists. Although animals and human beings behave in a much more complex way than for instance atoms or molecules, simple statistical models are able to reproduce many biological phenomena. The goal is to reproduce the collective behaviour of a large number of organisms: their individual properties are not of crucial importance. Due to the strongly increasing computer power along the last fifty years, computational models have gained more and more importance for research in many areas. The limitation of analytical models to describe accurately non-linear systems with critical behavior makes computational models nearly irreplaceable.Item Open Access Fibre models for shear failure and plasticity(2007) Raischel, Frank; Herrmann, Hans J. (Prof. Dr.)The fracture and failure of fiber reinforced materials such as wood, reinforced concrete, GFRP (glass fiber reinforced polymers) or CFRP (carbon fiber reinforced polymers) in considerably determined by the disordered properties of these materials and their constituents. A comprehensive understanding of their degradation and failure therefore requires the inclusion of statistical methods and models. This thesis describes the use of fibre bundle models, a well established model class used in statistical physics, to investigate the failure of fiber reinforced materials under shear, and the appearance of plasticity. We also consider the appearance of statistical precursors of imminent failure, and we improve already existing fibre bundle models for continuous damage to account for the failure of materials with a pronounced hierarchical setup.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.