Free-volume kinetic models of granular matter

We show that the main dynamical features of granular media can be understood by means of simple models of fragile-glass-forming liquid [Kob and Andersen, Phys. Rev. E 48, 4364 (1993)] provided that gravity alone is taken into account. In such lattice-gas models of cohesionless and frictionless particles, the compaction and segregation phenomena appear as purely nonequilibrium effects unrelated to the Boltzmann-Gibbs measure, which in this case is trivial. They provide a natural framework in which slow relaxation phenomena in granular and glassy systems can be explained in terms of a common microscopic mechanism given by a free-volume kinetic constraint.