Experimental study of the collision process of a grain on a two-dimensional granular bed

We report an experimental study on the collision of a bead on a two-dimensional hexagonal granular packing. This collision process is of crucial importance in aeolian transport of grains. We have investigated the kinematic properties of the incident bead before and after the collision, and the resulting deformation of the packing. A typical collision is characterized by the rebound of the impacting bead and the ejection of a few beads of the packing. We have shown that the properties of the rebound bead depend weakly on the impact speed and that the rebound process involves only a few bead layers of the packing. On the contrary, the ejection mechanism depends strongly on the impact speed. In particular, it is found that the number of ejected grains increases with the impact speed whereas the most likely value of their energy is practically independent of the impact speed. Furthermore, we have given evidences that the ejection process involves a great number of packing layers and therefore is extremely sensitive to the height of the packing. For small packing heights, one observes additional ejected grains which can be interpreted as being produced by the reflection of the shock wave on the bottom of the pile.