BROWNIAN DYNAMICS SIMULATIONS OF ELECTRO-RHEOLOGICAL FLUIDS, II: SCALING LAWS

Electro-rheological fluids are technologically relevant colloidal dispersions that on application of an applied electric field manifest a yield stress and dramatic increase in viscosity. To assist optimisation of these fluids we have performed molecular simulations to understand the basic mechanisms operating in these fluids. In this work a dispersion of field-aligned dipoles is simulated in shear flow by non-equilibrium Brownian dynamics. In agreement with experiment, a plot of the simultated relative, eta(r), against a dimensionless characteristic ER parameter, the Mason Number, Mn, exhibits a plateau region (at low electric fields and/or high shear rates) prior to a steep increase in viscoisty for Mn smaller. Although, the simulations exhibit only an approximate data collapse according to the Mn. Following on from the first paper in this series we relate the origins of the ER effect in more detail to the temporal structural changes that take place in the fluid. We find for example, that the fluids reorganise microscopically into layers of strings of particles in the shearing plane at low Mn, a structure which is destroyed on entry onto the plateau in eta, at higher Mn. We suggest how the model could be made more realistic in future studies.