Velocity Distribution of Vibration-driven Granular Gas in Knudsen Regime in Microgravity

Dynamics of quasi-2D dissipative granular gas is studied in micro-gravity condition (of the order of 10(-4) g) in the limit of Knudsen regime. The gas, made of 4 spheres, is confined in a square cell enforced to follow linear sinusoidal vibration in ten different vibration modes. The trajectory of one of the particles is followed for 2 hours, and is reconstructed from video data by particle tracking. From statistical analysis, we find that (i) loss due to wall friction is small, (ii) trajectory looks ergodic in space, and (iii) distribution rho(v) of speed follows an exponential distribution, i.e., rho(upsilon) approximate to exp [-v/(v(x0,y0))], with v(x0,y0) being a characteristic velocity along a direction parallel (y) or perpendicular (x) to vibration direction. This law deviates strongly from the Boltzmann distribution of speed in molecular gas. Comparisons of this result with previous measurements in earth environment, and what was found in 3D cell (Falcon et al., Europhys Lett 74: 830, 2006) performed in environment of about +/- 5 x 10(-2) g are given.