Granular gas dynamics: how Maxwell's demon rules in a non-equilibrium system

The main characteristic of a granular gas, which makes it fundamentally different from ordinary molecular gases, is its tendency to form clusters, i.e. to spontaneously separate into dense and dilute regions. This can be interpreted as a separation in cold and hot regions, meaning that Maxwell's demon is at work: this demon - notoriously powerless in any system in thermodynamic equilibrium - makes clever use of the non-equilibrium state of affairs that reigns in a granular gas, with on the one hand an external energy source and on the other a continuous loss of energy due to the inelastic particle collisions. We focus on vibrated compartmentalised systems, because these give a particularly clear-cut view of the clustering process and also because they resemble the typical machinery used in industrial applications to sort and transport granular materials. We discuss how the clustering can be exploited to build a Brownian motor, a fountain, a granular clock, and how it gives insight into a related clustering problem of prime importance in modern society, namely the formation of traffic jams.