  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.
