Granular systems: the complexity of non-equilibrium*

After an introduction to the realm of granular materials, their applications and their interest in physics, I will discuss some recent experimental results obtained in a vibrofluidized setup where a suspended rotator is put in random, Brownian-like, motion by collisions with a granular fluid. The setup can be explored in both the dilute and the dense regimes. In the dilute regime the experiments offer a benchmark of theoretical ideas in non-equilibrium statistical physics: I will show, in particular, the differences and similarities between conservative and nonconservative gases - put in evidence by violations of fluctuation-dissipation theorems or ratchet-like effects - and how it is possible to demonstrate the presence of dissipation even without measuring the energy lost in the collisions or cross-correlations (which would exhibit broken Onsager reciprocity but are not accessible with a single coordinate). In the dense regime the setup becomes a rich source of transport phenomena with anomalous diffusion transients, as well as a puzzle for rheology and macroscopic friction. I will show, in particular, how the long time ballistic behavior (observed at high density and low granular temperature) can be explained in terms of a kind of disorder-induced creeping ratchet effect, possibly relevant for earthquake dynamics. I will finally discuss the role that the amplitude and frequency of the vibro-fluidization have for the friction felt by the rotator when pulled by an external constant force.