Shear induced ordering in simulations of colloidal suspensions: Oscillatory shear and computational artefacts

We report on the nature of the shear induced order observed in nonequilibrium Brownian dynamics simulations of particles interacting via a screened Coulomb potential. Under steady shear, the nature of the ordered phase differs depending on the temperature. Below the equilibrium melting temperature, the shear induced order takes the form of hexagonally packed strings aligned along the direction of flow. Above the melting temperature, the liquid organizes itself into unstructured layers whose normal lies parallel to the shear gradient. We find a significant and anisotropic system size dependence of the ordering transition under steady shear. The critical shear rate required for ordering increases with increasing length of the simulation cell along the direction of flow. No such size dependence is found in oscillatory shears whose amplitude is less than half the cell length. Our results suggest that the order found in simulations under steady shear is an artefact of pseudo-oscillations resulting from shearing through the periodic boundary conditions. (C) 1996 American Institute of Physics.