Shear-induced microstructure distortion and its relaxation for colloids very close to the critical point

Abstract

Light scattering experiments on a stationary sheared colloidal system very close to the gas-liquid critical point, beyond the mean-field region, show that there is a pronounced shear-induced distortion of the structure factor in directions perpendicular to the flow direction for very small shear rates. This is contrary to what is found in the mean-field region, further away from the critical point, where the structure in these directions is unaffected. Light scattering experiments are presented for a colloid-polymer mixture and possible origins for this unexpected effect are discussed. After cessation of the shear flow we find an unusual relaxation phenomenon where the scattered intensity develops a ringlike structure, implying that there is an optimum relaxation rate at some intermediate wave vector. A theoretical explanation for this phenomenon is given, which shows that the scattering ring is the result of the interplay between a driving force and a rate limiting diffusion process. The phenomena that are observed experimentally are extensively compared to theoretical predictions. The necessary theoretical background is discussed in some detail.