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

TL;DR

This study experimentally investigates sphere-wall collisions in a viscous fluid, revealing that solid-on-solid contact occurs even above the rebound threshold, challenging prior theoretical assumptions.

Contribution

It provides experimental evidence that solid contact occurs in sphere-wall collisions in viscous fluids, contrary to previous theories, and highlights the role of surface roughness.

Findings

Solid-on-solid contact occurs above the rebound threshold.

Fluid dissipation dominates energy loss.

Contact mechanics depend on surface properties.

Abstract

We study experimentally the collision between a sphere falling through a viscous fluid, and a solid plate below. It is known that 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 theoretical predictions, shows that solid-on-solid contact occurs even for Stokes numbers just above the threshold for rebounding. The dissipation is fluid-dominated, though details of the contact mechanics depend on the surface and bulk properties of the solids. Our experiments and a model calculation indicate that mechanical contact between the two colliding objects is generic and will occur for any realistic surface roughness.