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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 ∝ h1/4 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. [ABSTRACT FROM AUTHOR]

Interesting collective motion emerges when several heavy intruder disks fall in a loose packed, quasi-two-dimensional granular bed of extremely light grains [F. Pacheco-Vázquez and J. C. Ruiz-Suárez, Nat. Commun. 1, 123 (2010)]. In particular, when two disks impact side by side, they initially repel and then they attract each other until they finally stop. Here we perform experiments and discrete-element soft-particle simulations to determine the range of action and the origin of these attractive and repulsive flow-mediated forces. We find that (1) the drag force on the disks fluctuate with a characteristic length linked to force chains that build up and break; (2) the repulsive force is present when the separation of the intruder disks is less than 6 times the size of the grains of the granular bed, which is the size of an aperture that allows a continuous discharge flow from a container; (3) the attractive force has a range of action between 5 and 6 times the size of the intruder disks; and (4) attraction exists only when intruders move faster than 1 m/s. These results suggest that repulsion originates from jamming of grains between intruders, and it supports the idea that attraction could be due to a "granular pressure" drop in the region between intruders caused by a high flow velocity of grains: a Bernoulli-like effect. However, our results do not rule out other mechanisms of interaction, like fluctuation-induced forces. [ABSTRACT FROM AUTHOR]