  Dense granular matter has to expand in order to flow, a phenomenon known as
dilatancy. Here we perform, by means of Magnetic Resonance Imaging, direct
measurements of the evolution of the local packing density of a slow and smooth
granular shear flow generated in a split-bottomed geometry. The dilatancy is
found to be surprisingly strong. The dilated zone follow the region of large
strain rate and slowly spreads as a function of time. This suggests that the
local packing density is governed by the total amount of local strain
experienced since the start of the experiment.
