Crossover from negative to positive shear rate dependence in granular friction

The frictional properties of granular matter are important for analyzing various phenomena in geosciences. Here, we conduct an experiment on the shear-rate dependence of the friction coefficient, which determines the stability of granular flow. By changing the shear rate over four orders of magnitude, we find the characteristic rate at which the least friction is realized associated with the crossover from negative to positive shear rate dependence. The least friction is explained in terms of the competition between two very different physical processes, namely frictional healing and anelasticity. We determine the expression of the characteristic shear rate, which is proportional to the square root of the normal stress.