  We study the rheology of cornstarch suspensions, a dense system of
non-Brownian particles that exhibits shear thickening, i.e. a viscosity that
increases with increasing shear rate. Using MRI velocimetry we show that the
suspension has a yield stress. From 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. The onset shear rate
for thickening is found to depend on the measurement geometry: 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 the Reynolds
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.
