Insight into Shear Thickening Suspensions using Boundary Stress Microscopy

The bulk rheological response of shear thickening (ST) suspensions is well documented but the microscopic origin of ST remains poorly understood. Using direct measurement of spatially resolved surface stresses in ST using boundary stress microscopy (BSM), we recently reported the existence of clearly defined dynamic regions of substantially increased stress that appear intermittently at stresses above a critical value. Here we present measurements using a smaller system and lower magnification so that the whole suspension is visualized during shear and show that the localized high stress events observed in steady shear have a finite lifetime. In large amplitude oscillatory measurements, we do not observe any localized high stress events if the peak shear rate is less than a critical shear rate, but the events appear when the peak shear rate is large enough. At very high values, the heterogeneous events become well established.