Effects of surface roughness on rebound strength in particle-wall collisions in air and liquids

Experiments to examine the effects of surface roughness on the strength of rebounds from particle-wall collisions, in liquids and in air, are presented. The target walls are glass plates and have three different values of surface roughness: less than 0.3 mu m, 10.5 mu m, and 40.1 mu m. Particles are glass beads with diameters from 1.05mm to 2.35mm. The experiments are conducted in tap water, sugar-water, and in air. Pre-impact Stokes numbers in liquids are in the range 20-170. For impacts in air, no significant effect of the surface roughness on rebound velocity is observed. In liquids, rough surfaces yield stronger rebounds than smooth surfaces. For a given ratio of surface roughness/particle's radius, the enhancement of the surface roughness on the rebound compared to the smooth one increases when the impact Stokes number decreases toward the "rebound threshold". These observations are in qualitative agreement with suggestions in the literature.