Configurational viscosity of dilute colloidal suspensions

The colloid configurational contribution to the excess stress tensor of a dilute suspension interacting via a screened Coulomb potential in a fluid undergoing Couette flow is analyzed. The structure factor for the nonequilibrium system obtained by solving a fluctuating diffusion equation with a wave-vector-dependent diffusion constant is used to obtain shear-rate-dependent viscosity and normal stress functions. The viscometric functions are nonanalytic in the limit of vanishing shear rate, although the nonanalytic parts are very small. At high shear rates, the viscosity due to the colloid is inversely proportional to the shear rate. In general, the principle of material objectivity, often evoked in rheology, is not satisfied.