Is there solid-on-solid contact in a sphere-wall collision in a viscous fluid?

Sumit Kumar Birwa, G. Rajalakshmi, Rama Govindarajan, Narayanan Menon

Published 2016-11-10Version 1

We study experimentally the process of normal collision between a sphere falling through a viscous fluid, and a solid plate below. As has previously been discovered, there is a well-defined threshold Stokes number above which the sphere rebounds from such a collision. Our experiment tests for direct contact between the colliding bodies, and contrary to prior expectations shows that solid-on-solid contact occurs even for Stokes numbers just above the threshold for rebounding. The details of the contact mechanics depend on the surface quality of the solids, but our experiments and a model calculation indicate that such contact is generic and will occur for any realistic surface roughness.