We study the rheology of cornstarch suspensions, a non-Brownian particle system that exhibits shear thickening. From magnetic resonance imaging velocimetry and classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid, and then solid again when it shear thickens. For the onset of thickening we find that the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate.

We propose a new view on yield stress materials. Dense suspensions and many other materials have a yield stress-they flow only if a large enough shear stress is exerted on them. There has been an ongoing debate in the literature on whether true yield stress fluids exist, and even whether the concept is useful. This is mainly due to the experimental difficulties in determining the yield stress. We show that most if not all of these difficulties disappear when a clear distinction is made between two types of yield stress fluids: thixotropic and simple ones. For the former, adequate experimental protocols need to be employed that take into account the time evolution of these materials: ageing and shear rejuvenation. This solves the problem of experimental determination of the yield stress. Also, we show that true yield stress materials indeed exist, and in addition, we account for shear banding that is generically observed in yield stress fluids.

We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometry experiments. In steady state, we observe that the material is heterogeneous, and we find that that the local rheology presents a continuous transition at low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with shear stresses proportional to the shear rate squared, as predicted by a scaling analysis. We show that the heterogeneity results from an unexpectedly fast migration of grains, which we attribute to the emergence of the Bagnoldian rheology. The migration process is observed to be accompanied by macroscopic transient discontinuous shear thickening, which is consequently not an intrinsic property of granular suspensions.