Microstructural response of a near-critical colloid-polymer mixture to shear flow

Long-ranged microstructural order of a near-critical colloid-polymer mixture under stationary and oscillatory shear flow is studied by means of time-resolved small-angle light scattering. The distance from the critical point, the shear rate, and the frequency of oscillation are systematically varied. Unexpected shear-induced distortions of critical microstructural order is observed in directions perpendicular to the flow direction, more so on closer approach of the gas-liquid critical point. The measured distortion of long-ranged order can be quantitatively understood on the basis of an approximate solution of the Smoluchowski equation, provided that a nonanalytical distortion of short-ranged microstructural order is assumed. These short-ranged microstructural changes, induced by the flow, account for the observed distortion perpendicular to the flow direction, and are responsible for the shear-induced shift of the location of the gas-liquid critical point. The importance of short-ranged correlations renders the behavior of critical systems under shear flow nonuniversal. The origin of the nonanalytic dependence of short-ranged distortions on the shear rate remains unclear. Including these short-ranged microstructural distortions, we find a good agreement between theory and experiment for both stationary and oscillatory shear flow.