The viscoelastic-like response of a repulsive granular medium during projectile impact and penetration

We study experimentally the penetration of a projectile into a two-dimensional granular bed composed of magnetic repelling grains. The projectile can experience either repulsive, attractive or neutral interaction with the particles generating different dynamics: i) the repulsive intruder compresses the granular bed without contact and experiences rebounds before stopping; ii) the attractive intruder is first accelerated when it approaches the bed, then, some magnets attach to its surface increasing its effective diameter and the drag force acting on the intruder until it reaches the repose; and iii) the neutral projectile penetrates deeper into the granular bed than in the two previous cases because there is no magnetic interaction with the particles. In addition, we developed molecular dynamics simulations able to reproduce the experimental results and used to determine the effect of the magnetic strength and the role of friction between the particles and the container walls. The results show that the medium behaves similar to a viscoelastic micellar fluid on impact, and the projectile dynamics can be modelled by a spring-dashpot model. Our findings can be useful in the design of new magnetic granular dampers.