Oblique collisions and rebound of spheres from a wetted surface

Abstract

Plastic and metal spheres were dropped from various heights onto a quartz disk covered with a thin layer of viscous oil and inclined at various angles with the horizontal. Rebound was observed only above a critical approach velocity, similar to that observed for head-on collisions when the disk is horizontal. The tangential component of the sphere's velocity is reduced only a small amount by the collision, owing to sliding lubrication/friction forces that also impart a small rotational velocity to the sphere. In contrast, the normal component of velocity is reduced substantially by viscous losses, and so the rebound angle of the sphere relative to the surface of the disk is smaller than the impact angle. The normal component of restitution and the rebound angle increase with the normal Stokes number based on the normal component of the impact velocity.