  We study the rheology of cornstarch suspensions, a non-Brownian particle
system that exhibits discontinuous shear thickening. Using magnetic resonance
imaging (MRI), the local properties of the flow are obtained by the
determination of local velocity profiles and concentrations in a Couette cell.
For low rotational rates, we observe shear localization characteristic of yield
stress fluids. When the overall shear rate is increased, the width of the
sheared region increases. The discontinuous shear thickening is found to set in
at the end of this shear localization regime when all of the fluid is sheared:
the existence of a nonflowing region, thus, seems to prevent or delay shear
thickening. Macroscopic observations using different measurement geometries
show that the smaller the gap of the shear cell, the lower the shear rate at
which shear thickening sets in. We, thus, propose that the discontinuous shear
thickening of cornstarch suspensions is a consequence of dilatancy: the system
under flow attempts to dilate but instead undergoes a jamming transition,
because it is confined. This proposition is confirmed by an independent
measurement of the dilation of the suspension as a function of the shear rate.
It is also explains the MRI observations: when flow is localized, the
nonflowing region plays the role of a "dilatancy reservoir" which allows the
material to be sheared without jamming.
