Models of fluidized granular materials: examples of non-equilibrium   stationary states
  We review some models of granular materials fluidized by means of external forces such as: random homogeneous forcing with damping, vibrating plates, flow in an inclined channel and flow in a double well potential. All these systems show the presence of density correlations and non-Gaussian velocity distributions. These models are useful to understand the role of a kinetically defined ``temperature'' (in this case the so-called {\em granular temperature}) in a non-equilibrium stationary state. In the homogeneously randomly driven gas the granular temperature is {\em different} from that of the driving bath. Moreover two different granular materials mixed together may stay in a stationary state with different temperatures. At the same time, granular temperature determines (as in equilibrium systems) the escape time from a well in a double well potential. 
Andrea Puglisi, Fabio Cecconi and Angelo Vulpiani