Penetration of rigid rods, flexible rods, and granular jets into a low-density granular medium

We study experimentally the dynamics of rigid and flexible rods penetrating vertically in a two-dimensional granular bed composed of expanded polystyrene spheres. In contrast to the vertical penetration observed for a single spherical projectile, high-speed videos reveal that a rod rapidly deviates from its initial vertical direction due to inhomogeneities of the bed packing fraction. Then, the rod rotates due to the net torque induced by the resistance force, describing a curved trajectory until it reaches its final depth. A short rod tends to deviate faster than a longer rod due to smaller moment of inertia. Moreover, long flexible rods always lose their vertical alignment and experience buckling, whereas rigid rods of the same size penetrate deeper before being deviated. Additionally, we compared the results with experiments and molecular dynamics (MD) simulations of a vertical array of particles, which also loses its verticality and stops adopting a final horizontal configuration. The granular array penetrates considerably less than the rods of equivalent mass and length, and the stopping mechanism is based on vertical-to-horizontal momentum transfer during a collisional process of the constituent particles. Finally, a possible three-dimensional extension is outlined.