Understanding the Propulsion of a scallop-like swimmer in Granular Media

Locomotion in granular media is often complicated due to the soft and yielding nature of granular materials. Simple and reciprocal swimming motions, which typically result in no locomotion in Newtonian liquids, can potentially generate propulsion in granular media. This study examines the mechanisms underlying the locomotion of a swimmer with two flapping wings in granular materials. In both lab experiments and discrete element method (DEM) simulations, we found that the swimmer could generate persistent locomotion by opening and closing its wings reciprocally. For a shallow swimmer, we found that wing rotation led to the formation of heaps and valleys on the free surface, which resulted in hysteresis in the resistive force within a stroke cycle. This, combined with a geometric factor of the scallop, resulted in the generation of propulsion. For a deep swimmer away from the free surface, reversed locomotion direction was observed in DEM simulations, which could possibly be due to a second mechanism involving the jamming of particles near the rotating wings.