Experimental study of a granular gas fluidized by vibrations
  We report experimental results on the behavior of an ensemble of inelastically colliding particles, excited by a vibrated piston in a vertical cylinder. When the particle number is increased, we observe a transition from a regime where the particles have erratic motions (granular "gas") to a collective behavior where all the particles bounce like a nearly solid body. In the gaslike regime, we measure the pressure at constant volume, and the bed expansion at constant external pressure, as a function of the number N of particles. We also measure the density of particles as a function of the altitude, and find that the "atmosphere" is exponential far enough from the piston. From these three independent measurements, we determine a "state equation" between pressure, volume, particle number and the vibration amplitude and frequency. 
Eric Falcon (1), Stephan Fauve (2) and Claude Laroche (1) ((1) ENS   Lyon France, (2) ENS Paris France)