Propagating high stress fronts, fluid migration and concentration variations in shear thickening cornstarch suspensions.

Rheological measurements of dense suspensions often reveal dramatic shear thickening, but only provide measurements of average quantities. Evidence is accumulating that shear thickening is associated with large spatiotemporal fluctuations. Here we describe direct measurements of spatially resolved stress at the boundary of a sheared cornstarch suspensions, showing persistent regions of high local stress that propagate in the flow direction. The persistence of these regions enables a detailed description of their structure, including the profile of boundary stress, dramatic non-affine velocity of particles at the boundary of the suspension, and a measurement of the relative flow between the particlute phase and the suspending fluid, indicating that the fronts are associated with substantial variations in the concentration of the particulate phase. The propagating fronts fully account for then increase in viscosity with applied stress reported by the rheometer.