Friction and rotation modes in a packing of cylinders under shear stress

We study the influence of collective processes such as rotations on the effective friction of sheared granular media made of two-dimensional array of cylinders. An original experimental device allows us to measure simultaneously grain rotations and the global friction force between the packing and the basal plane. It is shown that the correlation between these two quantities can be analyzed at two different time scales: 1. Averaging over the duration of a whole experiment, the mean sliding behavior of the first row on the base of the packing describes satisfactorily the global friction force. 2. At short-time, description of this correlation requires the knowledge of the propagation of rotation perpendicularly to the shear direction