Impact craters in granular media: grains against grains.

Impact experiments in granular media are usually performed with solid projectiles that do not fragment at all. Contrastingly, we study here the morphology produced by the impact of spherical granular projectiles whose structure is utterly lost after collision. Simple and complex craters are observed, depending on the packing fraction of the balls. Their diameters D and depths z are analyzed as a function of the drop height h. We find the same power law D ∝ h(1/4) obtained with solid spheres, but a discontinuity at a certain threshold height, related to the cohesive energy of the projectiles, shows up. Counterintuitively, instead of a monotonic increase with the collisional energy, z becomes constant above this threshold.