  A thorough understanding of the energy dissipation in the dynamics of wet
granular matter is essential for a continuum description of natural phenomena
such as debris flow, and the development of various industrial applications
such as the granulation process. The coefficient of restitution (COR), defined
as the ratio between the relative rebound and impact velocities of a binary
impact, is frequently used to characterize the amount of energy dissipation
associated. We measure the COR by tracing a freely falling sphere bouncing on a
wet surface with the liquid film thickness monitored optically. For fixed ratio
between the film thickness and the particle size, the dependence of the COR on
the impact velocity and various properties of the liquid film can be
characterized with the Stokes number, defined as the ratio between the inertia
of the particle and the viscosity of the liquid. Moreover, the COR for
infinitely large impact velocities derived from the scaling can be analyzed by
a model considering the energy dissipation from the inertia of the liquid film.
