We investigate experimentally the bouncing motion of solid spheres onto a solid plate in an ambient fluid which is either a gas or a liquid. In particular, we measure the coefficient of restitution e as a function of the Stokes number, St, ratio of the particle inertia to the viscous forces. The coefficient e is zero at small St, increases monotonically with St above the critical value Stc and reaches an asymptotic value at high St corresponding to the classical “dry” value emax measured in air or vacuum. This behavior is observed for a large range of materials and a master curve e/emax=f(St) is obtained. If gravity is sufficient to describe the rebound trajectory (after the collision) in a gas, this is not the case in a liquid where drag and added-mass effect are important but not sufficient: History forces are shown to be non-negligible even at large Reynolds number.

PACS. 83.70.Fn Granular solids, 68.45.Gd Wetting, 81.05.Rm Porous materials, granular materials,

In this paper, we present experimental measurements for the dynamic viscosity of macroscopic ͑non-Brownian and noncolloidal͒ suspensions of bimodal sized spheres when submitted to an oscillating plane Couette flow. The measured viscosity is what we call the dynamic viscosity at finite frequency. Concerning the viscosity of such systems, numerous experimental studies have been done under steady flow conditions, i.e., at zero frequency, but few studies concern the dynamic case. Our measurements have been performed for different values of the three relevant parameters, namely the size ratio , the fraction of small spheres to total solids, and the total solid volume fraction ⌽. Our results show a viscosity reduction upon mixing, which increases as the total solid volume fraction ⌽ is increased. We analyze our results by a model that takes into account the volume fraction ⌽ and the maximum volume fraction ⌽ m , which depends on the two parameters and. On the other hand, we compare our experimental results with recent numerical simulations performed by Chang and Powell ͓J.