Abstract

At very high solid volume fractions, many dense suspensions undergo shear thickening. (Wet corn starch is a good example.) Such suspensions flow freely at low stresses but jam up completely if the stress is increased. I will outline a recent theory that explains this sudden transition in terms of a smooth stress dependence of the fraction of interparticle contacts at which friction acts. The resulting flow curves (stress vs shear rate) at first sight suggest that the system could form steady shear-bands oriented along the vorticity direction with coexistence of high stress and low stress material at equal shear rate. Experimentally this is not seen and I will outline tentative arguments, based on the same model, for why the steady shear-banded state might always be replaced by an unsteady flow.