Velocity-dependent restitution coefficient and granular cooling in microgravity

We experimentally investigate the free cooling process occurring in a vibrated granular medium made of inelastic particles in a two-dimensional geometry. Experiments are realized in microgravity to cancel gravitational effects and recorded with the help of a high-speed camera. From the trajectories of the particles, obtained by image analysis, we can determine both the restitution coefficient and the time decay of the energy in the medium as soon as the vibration is cut off. We found evidence at low velocities of a positive slope of the restitution coefficient vs. the impact velocity, contrary to the usual approach where it only decreases with the velocity. We also found that the experimental cooling time is also much shorter than the one predicted on the basis of constant or decreasing restitution coefficient. A better agreement between theory and experiment is found if we take into account either the rotational degree of freedom or the velocity dependence of the coefficient of restitution.