  The rheological properties of granular matter within a two-dimensional flow
around a moving disk is investigated experimentally. Using a combination of
photoelastic and standard tessellation techniques, the strain and stress
tensors are estimated at the grain scale in the time-averaged flow field around
a large disk pulled at constant velocity in an assembly of smaller disks. On
the one hand, one observes inhomogeneous shear rate and strongly localized
shear stress and pressure fields. On the other hand, a significant dilation
rate, which has the same magnitude as the shear strain rate, is reported.
Significant deviations are observed with local rheology that justify the need
of searching for a non-local rheology.
