Low-velocity collisions of particles with a dry or wet wall

Plastic and metal spheres were impacted at low velocities in the range 0.02–0.30 m/s with a quartz plate that was dry or covered with a thin oil layer. Collisions were performed with a specially designed device in the low-gravity environment provided by a KC-135 aircraft. A pendulum-based experimental set-up was also used to perform low-velocity collisions under normal gravity. The dry restitution coefficient (ratio of the rebound velocity and impact velocity) is found to decrease weakly with increasing approach velocity, as is the general case with materials exhibiting inelastic deformation. The wet restitution coefficient is zero below a critical velocity and then increases with the impact velocity before evening out to form a plateau. A simple model for the wet restitution coefficient, ewet=edry(1−Stc/St), was found to adequately predict the restitution coefficient, as has been reported in earlier studies at higher impact velocities, where edry is the dry restitution coefficient, St is the Stokes number and Stc is the critical Stokes number below which no rebound occurs. Surface asperities are seen to cause more scatter in the data at low velocities than at high velocities. The data from pendulum experiments coincide with those collected in low gravity, thereby affirming their applicability for performing low-velocity collisions.