REVERSIBLE SHEAR THICKENING IN MONODISPERSE AND BIDISPERSE COLLOIDAL DISPERSIONS

Reversible shear thickening is measured in model colloidal suspensions of silica that exhibit near hard-sphere behavior. Flow dichroism meas urements and the colloidal stress-optical relationship prove that hydr odynamic interactions drive the shear thickening transition. Turbidity and flow-small angle neutron scattering (flow-SANS) demonstrate that particles cluster reversibly in the shear thickened state. Further, SA NS measurements show that shear thickening occurs without any shear-in duced order-disorder transition, in contrast to observations for dispe rsions of charged colloids. A simple force balance predicts the scalin g of the critical stress for the onset of shear thickening with partic le size and volume fraction. Measurements on bimodal mixtures verify t he scaling laws derived from the force balance and provide a strategy for controlling the shear thickening transition. (C) 1996 Society of R heology.