Flow dichroism in critical colloidal fluids - art. no. 061401

Due to long-range correlations and slow dynamics of concentration fluctuati ons in the vicinity of the gas-liquid critical point, shear flow is very ef fective in distorting the microstructure of near-critical fluids. The aniso tropic nature of the shear-field renders the microstructure highly anisotro pic, leading to dichroism. Experiments on the dichroic behavior can thus be used to test theoretical predictions on microstructural order under shear flow conditions. We performed both static and dynamic dichroism and turbidi ty measurements on a colloid-polymer mixture, existing of silica spheres (r adius 51 nm) and polydimethylsiloxane polymer (molar weight 204 kg/mol). Su fficiently far away from the critical point, in the mean-field region, the experimental data are in good agreement with theory. Very close to the crit ical point, beyond mean field, for which no theory exists yet, an unexpecte d decrease of dichroism on approach of the critical point is observed. More over, we do not observe critical slowing down of shear-induced dichroism, r ight up to the critical point, in contrast to the turbidity.