ABSTRACT
Many materials are characterized by the presence of structural disorder at the mesoscopic scale causing metastability, nonergodicity and slow dynamical evolution. One of them, a granular medium submitted to external mechanical perturbation, is often presented as an ideal system to study out-of-equilibrium thermodynamics. Indeed such a medium displays a slow compaction similar to the slow dynamics of an outof-equilibrium system. A full understanding of the relation between the observed macroscopic relaxation and the grain motion is still lacking.
