Slip events and dilatancy in a sheared fine noncohesive powder

We present experimental results of the transition from steady-state sliding to oscillatory motion for a fine noncohesive powder, sheared in an annular cell. The onset of instability is compared to the Dieterich-Ruina model for solid friction. We present data showing that at low velocity and close to the transition, the major sliding jumps are preceded by a short or long period of unstable plastic yielding of the granular matter. This ambivalent behavior suggests that the jumps are initiated when the sliding overcomes a critical velocity. During the stick-slip motion, the dilatancy of the powder bed has been also observed: the slippage is associated with a compaction whose value increases with the jump in the friction coefficient.