Mechanical behaviour of granular-gas and heterogeneous-fluid systems submitted to vibrations in microgravity

Abstract
Recent vibration experiments (amplitude A, frequency f=ω/2π) in microgravity have demonstrated that the "granular-gas" state exists only in the Knudsen regime. The excitation was "supersonic" and the probability density function of the pressure of the gas scaled as (Aω)^3/2. This paper draw some analogy with i) the ideal-gas theory and (ii) the thermodynamics of a single particle in a 1d box. The anomalous scaling of pressure fluctuations (Aω)^3/2is explained by the crossover from a single-collision regime during the sampling time, which imposes p ~ (Aω)² at small speed, to a multiple-collision regime imposing p ~ (Aω)². The effects of air and g-jitter are discussed and quantified. Grain-grain collisions are introduced and their effect on the speed distribution discussed. The second part of the paper addresses the general problem of pattern formation under vibration and in weightlessness condition, using the example of sedimentation.