Particle-wall collisions in a viscous fluid

This paper presents experimental measurements of the approach and rebound o f a particle colliding with a wall in a viscous fluid. The particle's traje ctory was controlled by setting the initial inclination angle of a pendulum immersed in a fluid. The resulting collisions were monitored using a high- speed video camera. The diameters of the particles ranged from 3 to 12 mm, and the ratio of the particle density to fluid density varied from 1.2 to 7 .8. The experiments were performed using a thick glass or Lucite wall with different mixtures of glycerol and water. With these parameters, the Reynol ds number defined using the velocity just prior to impact ranged from 10 to approximately 3000. A coefficient of restitution was defined from the rati o of the velocity just prior to and after impact. The experiments clearly demonstrate that the rebound velocity depends on th e impact Stokes number (defined from the Reynolds number and the density ra tio) and weakly on the elastic properties of the material. Below a Stokes n umber of approximately 10, no rebound of the particle occurred. For impact Stokes number above 500 the coefficient of restitution appears to asymptote to the values for dry collisions. The coefficients of restitution were als o compared with previous experimental studies, In addition, the approach of the particle to the wall indicated that the particle slowed prior to impac ting the surface. The distance at which the particle's trajectory varied du e to the presence of the wall was dependent on the impact Stokes number. Th e particle surface roughness was found to affect the repeatability of some measurements, especially for low impact velocities.