  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.
