Coefficient of restitution for wet particles

The influence of a wetting liquid on the coefficient of restitution (COR) is investigated experimentally by tracing freely falling particles bouncing on a wet surface. The dependence of the COR on the impact velocity and various properties of the particle and the wetting liquid is presented and discussed in terms of dimensionless numbers that characterize the interplay between inertial, viscous, and surface forces. In the Reynolds number regime where the lubrication theory does not apply, the ratio of the film thickness to the particle size is found to be a crucial parameter determining the COR.