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-dimensi onal hexagonal granular packing. This collision process is of crucial impor tance in aeolian transport of grains. We have investigated the kinematic pr operties of the incident bead before and after the collision, and the resul ting deformation of the packing. A typical collision is characterized by th e rebound of the impacting bead and the ejection of a few beads of the pack ing. 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 lay ers of the packing. On the contrary, the ejection mechanism depends strongl y on the impact speed. In particular, it is found that the number of ejecte d grains increases with the impact speed whereas the most likely value of t heir 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 pa cking. For small packing heights, one observes additional ejected grains wh ich can be interpreted as being produced by the reflection of the shock wav e on the bottom of the pile.