SHEAR-INDUCED MICROSTRUCTURE DISTORTION AND ITS RELAXATION FOR COLLOIDS VERY CLOSE TO THE CRITICAL-POINT

Light scattering experiments on a stationary sheared colloidal system very close to the gas-liquid critical point, beyond the mean-field reg ion, show that there is a pronounced shear-induced distortion of the s tructure 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 structur e 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 de velops a ringlike structure, implying that there is an optimum relaxat ion rate at some intermediate wave vector. A theoretical explanation f or this phenomenon is given, which shows that the scattering ring is t he 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 theore tical background is discussed in some detail. [S1063-651X(98)13810-2].