ELECTRORHEOLOGY OF A MODEL COLLOIDAL FLUID

We report experimental and theoretical results on the steady and oscil latory shear viscoelasticity of a model electrorheological fluid. Deta ils of the fluid synthesis, via the nucleation and growth of monodispe rse silica spheres, and light scattering studies of the spheres are al so presented. The shear-thinning viscosity mu shows a power-law depend ence mu similar to gamma(-Delta) on the inverse strain rate gamma, wit h Delta in the range 0.68-0.93. Likewise, the frequency-dependent visc osity mu(omega) shows a power-law dependence mu(omega) similar to omeg a(-($) over bar Delta) with ($) over bar Delta in the range 0.73-1.0. A theoretical treatment of steady-state cluster formation in applied e lectric and shear fields predicts the observed power-law dependences o f the steady and oscillatory shear viscosities, albeit with exponents Delta = ($) over bar Delta = 2/3. (C) 1994 Academic Press, Inc.