Measurements of the collision properties of small spheres

An experiment to measure the properties of the collisions between two small spheres or between a small sphere and a semi-infinite flat wall are described. The apparatus releases the particles in a free-fall without initial spin. The impacts are modeled in terms of three coefficients. The first is the coefficient of normal restitution. The second represents the frictional properties of the contact surfaces. The last characterizes the restitution of the tangential components of the velocity of the contact point for impacts that do not involve sliding. The coefficients are calculated from stroboscopic photographs of the ballistics of the particles near the collision. The results establish that the collision model provides an accurate description of the dynamics of the impacts.