Lattice Boltzmann simulations of soft matter systems

This article concerns numerical simulations of the dynamics of particles immersed in a continuum solvent. As prototypical systems, we consider colloidal dispersions of spherical particles and solutions of uncharged polymers. After a brief explanation of the concept of hydrodynamic interactions, we give a general overview over the various simulation methods that have been developed to cope with the resulting computational problems. We then focus on the approach we have developed, which couples a system of particles to a lattice Boltzmann model representing the solvent degrees of freedom. The standard D3Q19 lattice Boltzmann model is derived and explained in depth, followed by a detailed discussion of complementary methods for the coupling of solvent and solute. Colloidal dispersions are best described in terms of extended particles with appropriate boundary conditions at the surfaces, while particles with internal degrees of freedom are easier to simulate as an arrangement of mass points with frictional coupling to the solvent. In both cases, particular care has been taken to simulate thermal fluctuations in a consistent way. The usefulness of this methodology is illustrated by studies from our own research, where the dynamics of colloidal and polymeric systems has been investigated in both equilibrium and nonequilibrium situations.Comment: Review article, submitted to Advances in Polymer Science. 16 figures, 76 page

Experimental simulations of explosive degassing of magma.

The violent release of volatiles in explosive volcanic eruptions is known to cause fragmentation of magma and acceleration of the resulting mixture of gas and pyroclasts to velocities exceeding 100 m s-1 (ref. 1). But the mechanisms underlying bubble nuclea-tion, flow acceleration and fragmentation are complex and poorly understood. To gain insight into these phenomena, we have simu-lated explosive eruptions using two model systems that generate expansion rates and flow velocities comparable to those observed in erupting volcanos. The key feature of both experiments is the generation of large supersaturations of carbon dioxide in a liquid phase, achieved either by decompressing CO2-saturated water or by rapid mixing of concentrated K2CO3 and HC1 solutions. We show that liberation of CO2 from the aqueous phase is enhanced by violent acceleration of the mixture, which induces strong exten-sional strain in the developing foam. Fragmentation then occurs when the bubble density and expansion rate are such that the bubble walls rupture. In contrast to conventional models of fragmentation1,2, we find that expansion and acceleration precede—and indeed cause—fragmentation