Controlling shear thickening transition in confined dense suspensions through boundary stresses

We experimentally investigate the shear thickening behaviors of a dense granular suspension in a confined geometry. Our study specifically focuses on the slow relaxation dynamics and stress fluctuations near the thickening onset. By systematically varying the measuring time at each shear rate, we observe a dramatic change of the shear thickening behaviors in a confined geometry. With a gradual increase of the system size, this waiting-time dependence of suspension rheology will reduce and eventually disappear. By combining steady state rheological measurements with in-situ rheo-optical imaging, we quantitatively show that strong shear thickening response in a confined suspension is associated with a significant enhancement of both temporal stress fluctuations and spatial stress inhomogeneities. Further increase of shear rate or shear stress beyond the onset, however, will reduce the stress fluctuations. Our results clearly demonstrated how to control the statistical mechanics and shear rheology of confined suspensions through boundary stresses.