Two mechanisms of momentum transfer in granular flows.

This Rapid Communication highlights the physical processes at the origin of the constitutive law of dense granular flows. In simulated plane shear flows, we present a micro-mechanical expression for the phenomenological friction law μ(I). The expression highlights two distinct pathways for momentum transport-through either balanced contact forces or grain micro-acceleration. We show that these two rate-dependent processes control and explain the friction law. This understanding may help advance rheological models for granular materials and other soft materials such as emulsions and suspensions.