Cooperative motion of intruders amid smaller grains*

From the locomotion of animals in sand to the anchoring of buildings to the ground, the question of why it is so difficult to move an object within grains is ubiquitous in nature and human activities. Unlike fluids, granular systems transmit forces through interparticle contacts, with preferred directions according to the external applied force. We investigate numerically the forces and structures in a two-dimensional granular system displaced by an intruder. By varying the physical conditions of the problem, we verified that the contact networks percolate forces from the intruder towards the walls, being responsible for jammed regions (which, in some cases, can lead to the complete stop of the intruder's movement) and high values of the intruder’s drag force (Carvalho et al., Phys. Rev. E, 2022). In addition, we investigate how the motion of a set of intruders affects the system dynamics. We observed that the intruders present a cooperative behavior, with a final configuration related to their initial arrangement in space, and that there are optimal separations between intruders for minimum drag. Our results bring new insights into the cooperative dynamics of intruders, paving the way for mitigating the excessive drag suffered by these objects.