Influence of friction on the dynamics of sheared granular materials*

The presence of friction is one of the most important aspects of granular materials. Although the influence of friction on the static properties of the jammed state has been studied well, much less is known about the friction-dependence of the dynamics.  Using particle-based computer simulations, we probe how friction influences the relaxation dynamics of frictional particles that undergo oscillatory shear in 2d. We find that increasing friction gives rise to a liquid-solid-liquid transition in the translational mean squared displacement and at the same time a diffusion-ballistics-diffusion transition in the rotational dynamics. We show that these reentrant transitions can be rationalized with two competing mechanisms: i) Slowing down of the dynamics because of an increasing coupling between the translational degrees of freedom with the rotational ones and ii) Acceleration of the dynamics because of the emergence of domains that move collectively due to high friction.