Universität Stuttgart
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Item Open Access A brief review of capillary number and its use in capillary desaturation curves(2022) Guo, Hu; Song, Kaoping; Hilfer, R.Capillary number, understood as the ratio of viscous force to capillary force, is one of the most important parameters in enhanced oil recovery (EOR). It continues to attract the interest of scientists and engineers, because the nature and quantification of macroscopic capillary forces remain controversial. At least 41 different capillary numbers have been collected here from the literature. The ratio of viscous and capillary force enters crucially into capillary desaturation experiments. Although the ratio is length scale dependent, not all definitions of capillary number depend on length scale, indicating potential inconsistencies between various applications and publications. Recently, new numbers have appeared and the subject continues to be actively discussed. Therefore, a short review seems appropriate and pertinent.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 PDADMAC/PSS oligoelectrolyte multilayers : internal structure and hydration properties at early growth stages from atomistic simulations(2020) Sánchez, Pedro A.; Vögele, Martin; Smiatek, Jens; Qiao, Baofu; Sega, Marcello; Holm, ChristianWe analyze the internal structure and hydration properties of poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt) oligoelectrolyte multilayers at early stages of their layer-by-layer growth process. Our study is based on large-scale molecular dynamics simulations with atomistic resolution that we presented recently [Sánchez et al., Soft Matter 2019, 15, 9437], in which we produced the first four deposition cycles of a multilayer obtained by alternate exposure of a flat silica substrate to aqueous electrolyte solutions of such polymers at 0.1M of NaCl. In contrast to any previous work, here we perform a local structural analysis that allows us to determine the dependence of the multilayer properties on the distance to the substrate. We prove that the large accumulation of water and ions next to the substrate observed in previous overall measurements actually decreases the degree of intrinsic charge compensation, but this remains as the main mechanism within the interface region. We show that the range of influence of the substrate reaches approximately 3 nm, whereas the structure of the outer region is rather independent from the position. This detailed characterization is essential for the development of accurate mesoscale models able to reach length and time scales of technological interest.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 Hocheffiziente Präzisionsalgorithmen zur Modellierung und Analyse granularer poröser Medien(2020) Zauner, Thomas; Hilfer, Rudolf (Prof. Dr. Dr.)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 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 Existence and uniqueness of nonmonotone solutions in porous media flow(2022) Steinle, Rouven; Kleiner, Tillmann; Kumar, Pradeep; Hilfer, RudolfExistence and uniqueness of solutions for a simplified model of immiscible two-phase flow in porous media are obtained in this paper. The mathematical model is a simplified physical model with hysteresis in the flux functions. The resulting semilinear hyperbolic-parabolic equation is expected from numerical work to admit non-monotone imbibition-drainage fronts. We prove the local existence of imbibition-drainage fronts. The uniqueness, global existence, maximal regularity and boundedness of the solutions are also discussed. Methodically, the results are established by means of semigroup theory and fractional interpolation spaces.