Experimental investigation of collisional properties of spheres

We present experimental results on the collisional properties of spheres obtained through high-speed video analysis. An apparatus is built that produces collisions of spheres of various sizes with a wide range of impact, velocities and incidence angles. Edge detection techniques are implemented to track the position of the spheres from frame to frame whereby the translational velocities may be computed. In order to determine the rotational velocities, small markers are imprinted on the surfaces of the spheres and also tracked and matched from one frame to the next.. From the pre and post collision kinematic data, three collisional properties are directly extracted: the coefficient of restitution in the normal direction of impact, the coefficient. of friction and the coefficient of restitution of the relative tangential velocity. These measurements substantiate an existing impact model predicting exclusively rolling and sliding collisions. Finally the dependence of the coefficient of restitution on the magnitude of the normal impact velocity is studied for two different, materials which both exhibit different behaviors from what available theoretical results predict. We could not observe any size dependence of the coefficient, of restitution. This is due to the limited accuracy of our measurements but also to the possible sensitivity of the coefficient of restitution to the angle of incidence. However softer materials should provide more conclusive results

Wavelength Selection in a Vibrated Granular Layer

ABSTRACTWe present a numerical study of a surface instability occurring in a bidimensional vibrated granular layer. The driving mechanism for the formation of stationary waves is closely followed. Two regimes of wavelength selection are identified : a dispersive regime and a saturation regime. For the saturation regime, a connection is established between the pattern formation and an intrinsic instability occurring spontaneously in dissipative gases. We also address the importance of the detailed dissipation laws determining the wavelength values.</jats:p