Some remarks on the rheology of dense granular flows.

The paper reviews some peculiar properties exhibited by granular flows. We emphasize the inability of kinetic theory and of Bagnold's heuristic approach to describe the rapid regime of densely packed flows, characterized by the breakdown of the binary collision picture and by multibody long-lasting contacts. We suggest that deformation waves through the continuous paths of contacts can be effective to transport momentum and energy through the bulk, in a time very short compared to the inverse of the shear rate. This mechanism could explain some key rheological features encountered in dense granular materials.