RHEOLOGY OF COLLOIDAL PARTICLES IN A CONFINED CHANNEL UNDER SHEAR-FLOW BY BROWNIAN DYNAMIC SIMULATIONS

A colloidal system of strongly charged particles, confined between two charged walls, is studied under static conditions and in the presence of shear flows. Brownian Dynamics simulations (ED) are used to determ ine the concentration profiles for several separations of charged wall s. Results show good agreement with those obtained under static condit ions using the Hypernetted Chain Approximation (HNC) and Monte Carlo s imulations. Results obtained with ED and HNC for neutral walls show mo re similarities than those between ED and Monte Carlo simulations depe nding on the initial state of the colloidal particles. The presence of a shear flow field perturbs the equilibrium concentration profiles an d the distribution function in the flow direction, generating a struct ureless system, as confirmed by the absence of peaks in the radial dis tribution function. The mobility of the particles in the transversal d irection decreases rapidly and becomes practically frozen. The flow ex hibits a non-Newtonian behavior with shear-thinning viscosity. Due to the interparticle interactions and particle-wall interactions, the vis cosity is lower as the wall separation decreases, giving rise to an ap parent slip in the colloidal suspension. The slip velocity for repulsi ve walls is higher than that obtained with neutral walls and increases with the shear stress according to a power law, as observed in polyme r solutions. The shear viscosity and the normal stress differences dep end strongly on the combined effect of confinement, concentration of p articles, magnitudes of inter-particle interactions and wall-particle repulsion. (C) 1997 Academic Press.