STRUCTURE AND DYNAMICS OF ELECTRORHEOLOGICAL FLUIDS

We have used two-dimensional light scattering to study the structure a nd dynamics of a single-scattering electrorheological fluid in the qui escent state and in steady and oscillatory shear. Studies of the quies cent fluid show that particle columns grow in two stages; Particles fi rst chain along the electric field, causing scattering lobes to appear orthogonal to the field, and then aggregate into columns, causing the scattering lobes to move to smaller angles. Column formation can be u nderstood in terms of a thermal coarsening model we present, whereas t he early-time scattering in the direction parallel to the field can be compared to the theory of line liquids. In simple shear the scatterin g lobes are inclined in the direction of fluid vorticity, in detailed agreement with the independent droplet model of the shear thinning vis cosity. In oscillatory shear the orientation of the scattering lobes v aries nonsinusoidally. This nonlinear dynamics is described by a kinet ic chain model, which provides a theory of the nonlinear shear rheolog y in arbitrary shear flows.