Dissipation in granular materials

For two simple set-ups, we discuss how dissipative grain—grain interactions give rise to unexpected properties on large scales. In the first exaniple, a spherical particle rolling along a rough surface experiences an effective viscous friction on large time scales. It is due to temporal correlations among collisions with incomplete normal restitution. The second example is a sheared granular packing. There, spatial correlations among non-sliding and sliding contacts with Coulomb friction suggest that the deviatoric stress, although responsible for the dissipation, is localized on bonds that are non-sliding and hence non-dissipative.