TIME-RESOLVED STUDIES OF STICK-SLIP FRICTION IN SHEARED GRANULAR LAYERS

Sensitive and fast force measurements are performed on sheared granula r layers undergoing stick-slip motion, along with simultaneous optical imaging. A full study has been done for spherical glass particles wit h a 20% size distribution. Stick-slip motion due to repetitive fluidiz ation of the granular layer occurs for low driving velocities. Between major slip events, slight creep occurs that is highly variable from o ne event to the next. The effects of varying the stiffness k of the dr iving system and the driving velocity V are studied in detail. The sti ck-slip motion is almost periodic for spherical particles over a wide range of parameters, whereas it becomes irregular when k is large and Vis relatively small. At larger V, the motion becomes smoother and is affected by the inertia of the upper plate bounding the layer. Measure ments of the period and amplitude of the relative motion are presented as a function of V. At a critical value V, a transition to continuous Sliding motion occurs. The transition is discontinuous for k not too large, and large fluctuations occur in the neighborhood of the transit ion. The time dependence of the instantaneous velocity of the upper pl ate and the frictional force produced by the granular layer are determ ined within individual slipping events. The frictional force is found to be a multivalued function of the instantaneous velocity during slip , with pronounced hysteresis and a sudden drop just prior to restickin g. Measurements of vertical displacement reveal a very small dilation of the material (about one-tenth of the mean particle size in a layer 20 particles deep) associated with each slip event; the dilation reach es its maximum amplitude close to the time of maximum acceleration. Fi nally, optical imaging reveals that localized microscopic rearrangemen ts precede land follow) each macroscopic slip event; their number is h ighly variable and the accumulation of these local displacements is as sociated with macroscopic creep. The behavior of smooth particles is c ontrasted qualitatively with that of rough particles.