Influence of the liquid film thickness on the coefficient of restitution for wet particles

The normal coefficient of restitution (COR) for a spherical particle bouncing on a wet plane is investigated experimentally and compared with a model characterizing the energy loss at impact. For fixed ratios of liquid film thickness δ to particle diameter D , the wet COR is always found to decay linearly with St^-1, where St, the Stokes number, measures the particle inertia with respect to the viscous force of the liquid. Such a dependence suggests a convenient way of predicting the wet COR with two fit parameters: a critical COR at infinitely large St and a critical St at zero COR. We characterize the dependence of the two parameters on δ /D and compare it with a model considering the energy loss from the inertia and the viscosity of the wetting liquid. This investigation suggests an analytical prediction of the COR for wet particles.