Fluctuations and dynamics for a two-dimensional sheared granular material

We describe an experiment designed to probe the effects of fluctuations on relatively slow granular shear flows. In this ongoing experiment, we visualize stress fluctuations and the motion of individual grains–i.e. disks–using a 2D Couette geometry. The disks are made of a photoelastic material, and regions of large stress appear bright when viewed through crossed polarizers. This system undergoes a kind of dilatancy transistion which differs somewhat from the usual Reynolds dilatancy principle, since the grains are deformable. Visual observations show hat appears to be a statistically stationary state under steady shear which is nevertheless characterized by large stress fluctuations in space and time.