USHER: an algorithm for particle insertion in dense fluids

The insertion of solvent particles in molecular dynamics simulations of complex fluids is required in many situations involving open systems, but this challenging task has been scarcely explored in the literature. We propose a simple and fast algorithm (USHER) that inserts the new solvent particles at locations where the potential energy has the desired prespecified value. For instance, this value may be set equal to the system's excess energy per particle, in such way that the inserted particles are energetically indistinguishable from the other particles present. During the search for the insertion site, the USHER algorithm uses a steepest descent iterator with a displacement whose magnitude is adapted to the local features of the energy landscape. The only adjustable parameter in the algorithm is the maximum displacement and we show that its optimal value can be extracted from an analysis of the structure of the potential energy landscape. We present insertion tests in periodic and non-periodic systems filled with a Lennard-Jones fluid whose density ranges from moderate values to high values.Comment: 10 pages (Latex), 8 figures (postscript); J. Chem. Phys. (in press) 200

Emergence of rheological properties in lattice Boltzmann simulations of gyroid mesophases

We use a lattice Boltzmann (LB) kinetic scheme for modelling amphiphilic fluids that correctly predicts rheological effects in flow. No macroscopic parameters are included in the model. Instead, three-dimensional hydrodynamic and rheological effects are emergent from the underlying particulate conservation laws and interactions. We report evidence of shear thinning and viscoelastic flow for a self-assembled gyroid mesophase. This purely kinetic approach is of general importance for the modelling and simulation of complex fluid flows in situations when rheological properties cannot be predicted {\em a priori}.Comment: 7 pages, 5 figure

Large-scale lattice Boltzmann simulations of complex fluids: advances through the advent of computational grids

During the last two years the RealityGrid project has allowed us to be one of the few scientific groups involved in the development of computational grids. Since smoothly working production grids are not yet available, we have been able to substantially influence the direction of software development and grid deployment within the project. In this paper we review our results from large scale three-dimensional lattice Boltzmann simulations performed over the last two years. We describe how the proactive use of computational steering and advanced job migration and visualization techniques enabled us to do our scientific work more efficiently. The projects reported on in this paper are studies of complex fluid flows under shear or in porous media, as well as large-scale parameter searches, and studies of the self-organisation of liquid cubic mesophases. Movies are available at http://www.ica1.uni-stuttgart.de/~jens/pub/05/05-PhilTransReview.htmlComment: 18 pages, 9 figures, 4 movies available, accepted for publication in Phil. Trans. R. Soc. London Series

Multiscale modelling of liquids with molecular specificity

The separation between molecular and mesoscopic length and time scales poses a severe limit to molecular simulations of mesoscale phenomena. We describe a hybrid multiscale computational technique which address this problem by keeping the full molecular nature of the system where it is of interest and coarse-graining it elsewhere. This is made possible by coupling molecular dynamics with a mesoscopic description of realistic liquids based on Landau's fluctuating hydrodynamics. We show that our scheme correctly couples hydrodynamics and that fluctuations, at both the molecular and continuum levels, are thermodynamically consistent. Hybrid simulations of sound waves in bulk water and reflected by a lipid monolayer are presented as illustrations of the scheme

Lattice Boltzmann simulations of contact line motion in a liquid-gas system

We use a lattice Boltzmann algorithm for liquid-gas coexistence to investigate the steady state interface profile of a droplet held between two shearing walls. The algorithm solves the hydrodynamic equations of motion for the system. Partial wetting at the walls is implemented to agree with Cahn theory. This allows us to investigate the processes which lead to the motion of the three-phase contact line. We confirm that the profiles are a function of the capillary number and a finite size analysis shows the emergence of a dynamic contact angle, which can be defined in a region where the interfacial curvature tends to zero.Comment: 13 pages, 5 figures, to appear in Phil. Trans. Roy. Soc. A (Proceedings of the Xth International Conference on Discrete Simulation of Fluid Dynamics.