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

TL;DR

This study uses particle simulations to uncover two distinct propulsion mechanisms for a reciprocally flapping swimmer in granular media, highlighting the roles of jamming effects and swimmer inertia in locomotion.

Contribution

The paper introduces a novel analysis of reciprocal swimming in granular media, identifying jamming effects and inertia-driven propulsion as key mechanisms.

Findings

Jamming effects are less intense during the opening stroke.

Net displacement correlates with the difference in contact forces during strokes.

Inertia influences swimming direction when coasting time is comparable to flapping period.

Abstract

We use particle simulations to reveal two distinct propulsion mechanisms for a scallop-like swimmer to locomote itself in granular media by reciprocally flapping its wings. Based on the discrete element method, we examine the kinematics and contact forces of particles near the swimmer to identify jamming effects induced by 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 favoring 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 favors the closing stroke due to the smaller medium resistance as the swimmer coasts with closed wings.