EXPERIMENTS ON THE LOW-VELOCITY IMPACT OF MICROSPHERES WITH PLANAR SURFACES

Measurements of individual normal and oblique impacts of microspheres with planar surfaces are described and analyzed. Incident velocities from similar to 2 to 25 m/s and angles from 20 degrees to 90 degrees were controlled in the experiments for various combinations of microsphere and surface materials. For normal (90 degrees) incidence, a single-component phase doppler particle analyzer system measured the incident and rebound normal velocities, particle diameter, and measurement volume crossing time. The resulting values of the kinematic coefficient of restitution revealed the effects of adhesion at lower incident velocities. In addition, the kinematic coefficient of restitution showed a direct dependency on surface material hardness. For oblique (< 90 degrees) incidence, a pulsed laser light sheet visualization technique was used to determine the particle incident and rebound, normal and tangential velocity components. The resulting impulse ratio's variation with incidence angle helped delineate between rolling and sliding impacts. The sliding impact results in turn provided a measure of the coefficient of dynamic friction. All results were compared using the rigid body impact model of Brach and Dunn (1995), which includes the possibility of microsphere rotation. Subsidiary experiments also were performed to assess the effect of the charge transfer during individual impact events, and to search for evidence of plastic deformation resulting from impact.