Brownian dynamics simulation of dipole suspensions under shear: the phase diagram

Abstract

An idealized model of a sheared suspension of particles with dipole interactions is simulated at a volume fraction of 30%. The model is motivated by the need to understand electro-rheological suspensions. In the phase separation regime of the model in equilibrium, the particles form a gel. The nonequilibrium phase diagram for the model has liquid and shear string phases and a phase of flowing layers which corresponds to the equilibrium phase separation. Transitions between the phases are driven by the competition between the Brownian, dipole and shear forces. The layered phase is bounded by Brownian and Stokesian boundaries: crossover between these is a particle size effect. Viscosity data for the Stokesian regime of the phase diagram when plotted against the Mason number dimensionless group shows an approximate data collapse at high Mason numbers, in accord with experiment, but this universality is lost in the Brownian region of the model. Details are given of the fundamental layering transition in the model and the flow mechanisms of its layered phase.