The influence of a liquid film 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 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 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.

Abstract.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.Keywords: coefficient of restitution, impact, wetting, particle-laden flow, granular flow PACS: 45.50.Tn, 47.55.Kf Understanding the energy dissipation associated with particle-particle interactions is crucial for describing the collective behavior of granular matter [1], i.e., large agglomerations of macroscopic particles. The coefficient of restitution (COR), firstly introduced by Newton as the ratio between relative rebound and impact velocities [2], can be used to characterize the energy dissipation at the particle level. This number provides one of the basic ingredients of computer assisted modeling, such as molecular dynamics (MD) simulation, which has been developed into a powerful tool to describe the large scale collective behavior of granular matter in the past decades [3,4]. Besides the energy dissipation from particle-particle interactions, the dissipation arising from the interstitial air or liquid has to be considered when coping with natural phenomena such as dune migration [5] or debris flow [6], as well as with various industrial applications such as granulation process [7,8].The experience of building sand sculptures tells us that the rigidity of a granular material increases as a small amount of a wetting liquid is added. This is largely due to the cohesion arising from the formation of capillary bridges between adjacent particles [9]. The so-called wet granular matter behaves dramatically different from noncohesive dry granular matter while agitated, with emerging critical behavior, such as phase transitions [10] and pattern formations [11], being traceable to the energy or force scale of a single capillary bridge. In order to gain insights into the dynamical behavior of wet granular matter, it is essential to explore the COR and the associated energy dissipation of wet impacts. A recent investigation reveals that the dependence of the COR on various particle and liquid properties can be scaled with two dimensionless numbers:...