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 $\delta$ 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 dependency 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 dependency of the two parameters on $\delta/D$ 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. 
Thomas M\"uller and Kai Huang
cond-mat.soft