Friction of a slider on a granular layer

Abstract: We investigate the effective friction encountered by a mass sliding on a granular layer as a function of bed thickness and boundary roughness conditions. The observed friction has minima for a small number of layers before it increases and saturates to a value that depends on the roughness of the sliding surface. We use an index-matched interstitial liquid to probe the internal motion of the grains with fluorescence imaging in a regime where the liquid has no significant effect on the measured friction. The shear profiles obtained as a function of depth show a decrease in slip near the sliding surface as the layer thickness is increased. We propose that the friction depends on the degree of grain confinement relative to the sliding surfaces.

Movies of sheared granular using a refractive index matching technique for various granular bed thickness h. This technique allows us to visualize grains away from side walls and in the bulk of the granular bed. We show that at slow shear rates, the index matching liquid has no measureable effect on the flow and friction properties.