Superlubricity in granular shear flows under external vibrations

We investigate the use of external vibrations to reduce macroscopic friction in pressure-imposed granular flows sheared between bumpy planes, in the absence of gravity, using the discrete element method. We observe that the system becomes superlubric, i.e., the macroscopic friction is less than 0.01, if one of the bumpy planes oscillates with a sufficiently large velocity amplitude. We quantify the reduction in the energy dissipated by the system through the work of the shear stress induced by the reduction in macroscopic friction and the external energy required to make the bumpy plane oscillate for different combinations of amplitude and frequency, and imposed pressure. We propose a phase diagram and criteria in terms of imposed pressure and velocity amplitude of oscillations to predict the resistance to shear.