ABSTRACT
The frictional response of granular materials to an applied shear stress is studied experimentally by sliding a rough plate across a horizontal granular layer. We monitor both the vertical and horizontal displacements of the sliding plate. We obtain additional information on the granular flow within the layer by imaging the motion of individual grains from the side of the sample with the help of a digital fast video camera. The experiments are performed on glass beads, either dry or immersed, and on binary mixtures of both glass beads and sand. As observed in the case of solid friction, the granular frictional force is proportional to the normal stress, and does not depend significantly on the shear rate. The granular flow occurs within a few layers of grains and the velocity profile is roughly exponential. We also observe aging of the granular material: When the sliding plate remains at rest for a long time between experiments, the initial resistance to the applied shear stress increases roughly logarithmically with the resting time provided that a static shear stress is applied. In the case of binary mixtures, we find that the frictional force depends on the relative concentration of the two granular components in a non-trivial way and that the frictional properties of mixtures cannot be simply extrapolated from the frictional properties of the individual components.
