Packing Constraints and Number Dependence in Simulations of Sheared Colloidal Suspensions

The system size dependence of the shear viscosity of a strongly interacting, electrostatically stabilized colloidal suspension was studied using nonfequilibrium Brownian dynamics simulations. Systems ranging in size from 59 to 500 particles were examined at a constant volume fraction of 0.4. Continuous shear thinning and a highly ordered string phase were found for a 256-particle system over the entire range of shear rates used. This contrasts sharply with the behavior of a 108-particle system, which undergoes a transition to a disordered, more highly viscous state at lower shear rates. Simulations of a 500-particle system over a restricted range of shear rates also revealed a highly ordered string phase, but gave viscosities clearly higher than those of the 256-particle system. At a shear rate favoring highly ordered particle arrangements, the excess shear viscosity and the type of ordered structure of the suspension varied erratically with increasing system size. This behavior was attributed to geometric restrictions on the types of ordered particle arrangements permitted. These constraints arise because the simulation box dimensions vary slowly in comparison to the relatively rapid discrete increases in the number of particles. Thus, drastic changes in suspension structure are often required in order to accommodate even one additional particle in a box whose dimensions have increased only slightly.