Cornstarch Suspensions: Thinning, Thickening, Hysteresis and Lubrication

Most non-Brownian suspensions shear-thin, but starch suspensions discontinuously shear-thicken above a threshold strain rate. I will begin with a simple mean field model of shear thinning to explain Hoffman's classic observations. In empirical studies of shear thickening we have observed large amplitude hysteresis: A suspension jammed throughout may remain in the jammed thickened state as the strain rate is reduced, but an unjamming front may propagate from unjammed regions. Transient thickening is observed at strain rates below the DST threshold, and above it the stress fluctuates, resembling stick-slip friction. I will discuss simple frictional interpretations, and suggest that the critical strain rate for thickening may be determined by contact line hysteresis and the stress in the unjammed state.