Dectection of heterogeneities in a shear thickening suspension through a local normal stress measurement

We study the shear-thickening of a concentrated suspension of non-brownian particles (cornstarch) using an in-house sensor array. The sensor is placed on the bottom plate of a torsional rheometer, and gives access to the pressure on 25 regularly spaced measuring points (of surface 4x4 mm² each). It is used to map the local normal stresses in the flow.
In the shear thickening regime, we demonstrate that the flow of the cornstarch suspension becomes heterogeneous. The sensors show very regular pressure peaks, with an amplitude increasing with the applied shear stress. Intriguingly, this evidences the presence of a single, stable object rotating with the surrounding fluid. We characterise in detail this heterogeneity: we show how its shape and its relative velocity varies with the solid fraction of the suspension, the gap height and the nature of the particules. We finally discuss its origin.