Propulsion of a scallop-like swimmer in viscoelastic granular hydrogels

We experimentally study a scallop-like swimmer that flaps its wings reciprocally in a nearly frictionless, cohesive granular medium consisting of hydrogel spheres. Significant locomotion is found when the swimmer's flapping frequency matches the inverse relaxation time of the material. At higher or lower frequencies, we observe no motion of the swimmer apart from a short initial transient phase. The swimmer moves in the opposite direction compared to its motion in a cohesion-free frictional granular material of hard plastic spheres, in which the propulsion mechanism is related to granular jamming. For the granular hydrogels, we use X-ray radiograms to reveal that the wing motions create low-density zones, which in turn give rise to a hysteresis in drag and propulsion forces. This time-dependent effect, combined with the swimmer's inertia, accounts for locomotion at intermediate frequencies.