Granular Friction in a Wide Range of Shear Rates

We conduct an experiment on the frictional properties of granular matter over a wide range of shear rate that covers both the quasistatic and the inertial regimes. We show that the friction coefficient exhibits negative shear-rate dependence in the quasistatic regime, whereas the shear-rate dependence is positive in the inertial regime. The crossover shear rate is determined in terms of the competition between two physical processes, namely frictional healing and anelasticity. We confirm that these results are independent of the grain shape by using angular sand and spherical beads. We also show that the behavior in the inertial regime is quantitatively the same as that in numerical simulations. As an application, we derive the exponential velocity profiles that are often observed in the quasistatic heap flow.