Pseudo-thermal cooling in a bounded system of viscoelastic hard-spheres

Using computer simulations, the cooling rate of a system of inelastic hard spheres in the Couette geometry has been investigated after stopping the relative motion of boundaries. The energy of the system is initially balanced between the incoming kinetic energy from the moving boundaries and the dissipated kinetic energy due to the successive collisions of inelastic particles. When the relative motion of moving boundaries is suddenly stopped the pseudo-thermal energy of the system decays. However, the rate of decay is found to be slower than that introduced for a shear-free ensemble. Moreover, the simulations clearly show that the existence of pseudo-thermal energy is a necessary condition for diffusion in a particulate system. The simulation results may elucidate the physical origin of some phenomena such as clustering and freezing in hard-sphere systems, and the effect of velocity gradients on these phenomena.