Friction in Granular Layers: Hysteresis and Precursors

Sensitive and fast force measurements are performed on sheared granular layers with simultaneous optical imaging. Discrete displacements that can be much smaller than the particle diameter occur for low imposed velocity gradients, with a transition to continuous motion at a higher gradient. The instantaneous frictional force measured within individual slip events is a multivalued function of the instantaneous velocity. Localized microscopic rearrangements precede (and follow) macroscopic slip events; the accumulation of localized rearrangements leads to macroscopic creep.