SQUARE-ROOT SINGULARITY IN THE VISCOSITY OF NEUTRAL COLLOIDAL SUSPENSIONS AT LARGE FREQUENCIES

The asymptotic frequency, omega, dependence of the dynamic viscosity o f neutral hard-sphere colloidal suspensions is shown to be of the form eta(0)A(phi)(omega tau(p))(-1/2), where A(phi) has been determined as a function of the volume fraction phi, for all concentrations in the fluid range, eta(0) is the solvent viscosity, and tau(p) is the Peclet time. For a soft potential it is shown that, to leading order in the steepness, the asymptotic behavior is the same as that for the hard-sp here potential and a condition for the crossover behavior to 1/omega t au(p) is given. Our result for the hardsphere potential generalizes a result of Cichocki and Felderhof obtained at low concentrations and ag rees well with the experiments of van der Werff et al. if the usual St okes-Einstein diffusion coefficient D-0 in the Smoluchowski operator i s consistently replaced by the short-time self-diffusion coefficient D -s(phi) for nondilute colloidal suspensions.