Abstract
Dense granular matter has to expand in order to flow, a phenomenon known as dilatancy. Here we perform, by means of Magnetic Resonance Imaging (MRI), 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 degree of dilatancy is found to be surprisingly strong. For flows without appreciable transient, the dilated zone follows the region of large strain rate, while for flows with a strong transient, the dilated zone extends also into the region where transient flow took place. In all cases, the dilated zone slowly spreads as a function of time. These findings suggest that the local packing density is governed by the total amount of local strain experienced since the start of the experiment.
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