Dissipation in Granular Materials

For two simple setups it is discussed, how dissipative grain-grain interactions give rise to unexpected properties on large scales. In the rst example, a spherical particle rolling along a rough surface experiences an eeective 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 nonsliding and sliding contacts with Coulomb friction suggest that the deviatoric stress, although responsible for the dissipation, is localized on bonds that are nonsliding and hence nondissipative.