Reciprocal swimming in granular media: the role of jamming and swimmer inertia

We use particle simulations to reveal two distinct propulsion mechanisms for a scallop-like swimmer to propel itself in granular media by reciprocally flapping its wings. Based on the discrete element method, we examine the structure, kinematics and contact forces of particles near the swimmer to quantify how jamming manifests as stagnant zones near the swimmer in a frictional granular medium, which are less intense during the opening stroke than the closing. This broken symmetry is quantified by the difference in the number of strong particle contact forces formed during opening and closing, which shows a linear relation with the swimmer’s net displacement across various swimmer and medium configurations, all favouring the opening stroke. We identify a secondary propulsion mechanism in a dynamic regime with significant swimmer inertia, as the flapping period approaches the coasting time for a moving swimmer to come to rest under the medium resistance. In this case, the swimmer’s net displacement is correlated to the ratio between these two time scales, and the swimming direction favours the closing stroke due to the smaller medium resistance as the swimmer coasts with closed wings.