Swimming in Granular Media

TL;DR

This paper investigates how large intruders can swim in granular media by modeling their contraction and extension, revealing optimal conditions for fluidizing the bed to enable movement with minimal resistance.

Contribution

It introduces a simple model of periodic intruder motion in granular media and identifies conditions for effective swimming by balancing fluidization and re-solidification.

Findings

Optimal fluidization conditions for swimming in granular media

Swimmers must balance speed to avoid voids and re-solidification

Fundamental time-scale of granular packing influences swimmer motion

Abstract

We study a simple model of periodic contraction and extension of large intruders in a granular bed to understand the mechanism for swimming in an otherwise solid media. Using an event-driven simulation, we find optimal conditions that idealized swimmers must use to critically fluidize a sand bed so that it is rigid enough to support a load when needed, but fluid enough to permit motion with minimal resistance. Swimmers - or other intruders - that agitate the bed too rapidly produce large voids that prevent traction from being achieved, while swimmers that move too slowly cannot travel before the bed re-solidifies around them i.e., the swimmers locally probe the fundamental time-scale in a granular packing.