Abstract: Interfacial pressure and density profiles are calculated from molecular dynamics and lattice Boltzmann simulations of a liquid film in equilibrium with its vapor. The set of local values of tangential pressure and density along an interface exhibits a van der Waals-type loop; starting from the stable vapor bulk phase one passes through metastable and unstable states to the stable liquid bulk phase. The minimum and maximum values of the profile of tangential pressure are related to the liquid and vapor spinodal states, respectively. The spinodal pressures turn out to be linearly related to the extreme values of the tangential pressure in the interface. The comparison with equations of state shows good agreement with the simulation results of the spinodals. In addition the properties of the metastable region are obtained. Based on this investigation a method is proposed for the estimation of the liquid spinodal from experimentally obtained interfacial properties. Estimations for water and helium are presented.
Abstract: The formation of platinum nano-particles on a polyethylene substrate is investigated by molecular dynamics simulation. As initial configuration, a polymer film is put in contact with a supersaturated platinum vapour. Argon is added in the vapour phase as carrier gas that transfers heat from the vapour phase to the polymer surface. The simulations provide a deep insight into cluster formation at the atomic level. The presence of the polymer affects cluster growth significantly. Surface growth and agglomeration are limited by the polymer matrix. The influence of supersaturation on the cluster size distribution is also different to the particle formation in the gas phase. In addition, the structure of the polymer substrate is modified during the embedding of platinum. These effects are analysed and compared to experimental investigations of the formation of metal–polymer composites. The resulting distribution of metal clusters on the surface and inside the polymer is in general agreement with available experimental results of similar polymer–metal systems.
Abstract: The formation of metal nano-particles in a homogeneous vapour phase and in a polymer matrix is investigated by molecular dynamics simulations. The different types of particle formation and growth are investigated specially with respect to the formation of atomic structure and particle morphology. We find that the structure formation proceeds via liquid-like and icosahedral structures to a close-packed structure as the particle size increases. Decoupled from the growth process the cluster solidification exhibits a similar structure formation scenario. In the case of the gas phase aggregation several coalescence processes have been analysed. Metal deposition on a polymer substrate shows a significantly different course. Due to the matrix several growth and structure formation processes are suppressed.
Abstract: Heterogeneous nucleation and growth of supersaturated argon vapor at polyethylene surfaces is investigated using molecular dynamics simulation. The specific system is chosen as model for high wettable systems. The simulations are conducted in a nonequilibrium ensemble which includes heat transfer during the condensation process. Temporary density and temperature gradients are developed in the vapor phase. The gradual transition from nearly adsorption behavior close to the binodal to heterogeneous nucleation of a metastable vapor is investigated by stepwise varying the initial saturation of the vapor. By increasing the supersaturation along an isotherm a continuous transition from the layer-by-layer growth to the islands-on-layers one is observed. The results are compared to classical nucleation theory for the heterogeneous two- and three-dimensional growth models. We find for this system that a two-dimensional version of the classical heterogeneous nucleation theory is most suitable to describe the nucleation rate data vs saturation obtained from simulation over a wide range of supersaturation.
Abstract: For a broad range of applications, the most important transport property of porous media is permeability. Here we calculate the permeability of pore network approximations of porous media as simple diagenetic or shrinking processes reduces their pore spaces. We use a simple random bond-shrinkage mechanism by which porosity is decreased; a tube is selected at random and its radius is reduced by a fixed factor, the process is repeated until porosity is reduced either to zero or a preset value. For flow simulations at selected porosity levels, we use precise Monte Carlo calculations and the lattice Boltzmann method with a 9-speed model on two-dimensional square lattices. Calculations show a simple power-law behavior. The value of the exponent relates strongly to the shrinking process and extension, and hence to the skewness of the pore size distribution, which varies with shrinking, and weakly to pore sizes and shapes. Smooth shrinking produces pore space microstructures resembling the starting primitive material; one value of the exponent suffices to describe the relation between permeability and porosity. Severe shrinking however produces pore space microstructures that apparently forget their origin; the permeability-porosity curve is only piecewise continuous. The power-law thus is not universal, a well-known fact. An effective pore length or critical pore size parameter, lc, characterizes pore space microstructures at any level of porosity. For severe shrinking lc becomes singular, indicating a change in the microstructure controlling permeability, and thus flow, thus explaining power-law transitions.
Abstract: ,Particle capture at membrane surfaces in cross-flow microfiltration is studied both experimentally and computationally to better understand permeability reduction as function of membrane pore sizes and particle sizes and concentrations. Microfiltration experiments for various bentonite suspension concentrations are conducted in commercial polysulphone membranes of nominal pore sizes 2 and 0.2 μm. Permeability reduction is continuously evaluated. Scanning electron microscopy (SEM) and image analysis are used to determine membrane pore size and particle size distributions. For simulation purposes, the membrane pore space is represented by a bundle of nonintersecting tubes. Pore segments in the model membranes have circular cross-sections, random locations and sizes distributed according to distributions determined experimentally. Monte Carlo simulations of cross-flow microfiltration of a well-characterized particle suspension on well-defined model membranes are presented and permeability reduction calculated. Particle capture and size exclusion at pore segments are considered the dominant mechanisms of membrane fouling. In the simulations a matching size criterion between pore size and particle size is used to define pore blocking. Simulation results accord with those obtained experimentally. Permeability decreases more rapidly at higher particle concentrations. For all particle concentrations, permeability reduction of the thinner pore membrane is more intense. Membrane permeability scales linearly with porosity in both cases of membranes. This result is discussed in the light of simple theoretical arguments.
Abstract: Der nachfolgende Artikel beschreibt die Installation eines Linux-Clusters, zuerst aus Sicht der Administratoren Thomas Lange und Constantin Hellweg. Thomas Lange ist Systemadministrator im Institut für Informatik, seit drei Jahren Debian-Developer und Autor der Software Fully Automatic Installation (FAI). Constantin Hellweg ist studentische Hilfskraft am Institut für Informatik und seit Beginn 2002 vor allem für Tests und Konfigurationen von FAI
zuständig.
