Normal stress differences in attractive colloid/polymer mixtures

Hard-sphere particles in flowing suspensions can move across streamlines to locally increase concentration above that of the bulk through shear-induced migration, which is theoretically proposed to arise from the particle normal stresses. How polymers in solution alter mechanisms affecting migration, including flocculation and normal stresses, is poorly understood. We develop a model system well-suited for normal stress measurements and microscopic imaging on colloid/polymer mixtures. Colloidal (trifluoroethyl methacrylate)-co-(tert-butyl methacrylate) particles were nearly refractive-index and density-matched in glycerol/water mixtures, and poly(acrylamide) was added to induce attractive depletion interactions. We measure shear-rate-dependent viscosity and first normal stress differences for mixtures at constant particle concentration and image suspensions before, and after shear using confocal microscopy. These measurements allow us to test various physical pictures for mechanisms leading to nonzero normal stress differences in colloid/polymer mixtures, providing insight into physical processing underlying migration in complex mixtures.