RHEOLOGY AND DYNAMICS OF SHEARED ARRAYS OF COLLOIDAL PARTICLES

Concentrated suspensions of colloidal particles undergo dynamical micr ostructural transitions under shear. During the transition from oscill ating face-centered cubic twin structures to sliding layer structures, the system can exhibit hysteretic and discontinuous rheology as the s hear rate is varied. We capture this behavior with a dynamic simulatio n of a sheared lattice of non-Brownian spherical particles with screen ed electrostatic interactions and hydrodynamic Interactions determined using the Stokesian dynamics approximation. Rheological data are dete rmined for a range of volume fractions, electrostatic screening length s and shear rates or shear stresses. In controlled stress simulations, static yield stresses are observed. In controlled shear rate simulati ons of certain lattice orientations, plateau viscosities are observed at high and low shear rates with a high to low shear rate plateau visc osity ratio ranging from 1.4 to 2.2. Large viscosity transitions with hysteretic-like rheology are observed only in controlled shear rate si mulations of face-centered cubic (111) layers sheared parallel to the [211] direction with full representation of the hydrodynamic particle interactions. Rheological curves collapse when stresses are scaled by the elastic modulus and shear rates by the elastic modulus divided by the high-shear-rate limiting viscosity. The magnitude of the hystereti c viscosity jump and the scaled critical stresses match experimental v alues. (C) 1998 The Society of Rheology.