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We investigate the effective friction encountered by a mass sliding on agranular layer as a function of bed thickness and boundary roughnessconditions. The observed friction has minima for a small number of layersbefore it increases and saturates to a value which depends on the roughness ofthe sliding surface. We use an index-matched interstitial liquid to probe theinternal motion of the grains with fluorescence imaging in a regime where theliquid has no significant effect on the measured friction. The shear profilesobtained as a function of depth show decrease in slip near the sliding surfaceas the layer thickness is increased. We propose that the friction depends onthe degree of grain confinement relative to the sliding surfaces.Comment: 4 pages, 6 figure

Friction of a slider on a granular layer: Nonmonotonic thickness dependence and effect of boundary conditions