Granular media involve many particles, so that it is tempting to describe them thermodynamically. Unfortunately, energy is lost through friction and has to be brought by a non-thermal source. As a consequence, the dynamical equations do not leave any obvious ensemble invariant.
However it has been proposed that these systems could have a statistical mechanics of their own. Here, we present experimental results on the statistical properties of 2D disordered granular media under cyclic shear in an attempt to check the possibility of a thermodynamic construction for dense granular materials.
Following tracers, we show that the particle motion is subdiffusive and that the trajectories exhibit strong cage effect of the kind observed in colloidal glasses. By direct visual observations, we investigate the cage properties (size, elapsed time in a cage...) in order to relate them to the global properties of the material. Further statistical characterization of the motion are studied in an attempt to identify relevant lengthscales and ultimately the physical nature of a ”cage”.
Finally, we expect to measure the response function to a small driving force in order to investigate the fluctuation-dissipation relation and, eventually, measure an effective temperature.