The swimming of a deforming helix

TL;DR

This paper investigates how deforming a helical shape in a viscous fluid can generate propulsion, analyzing the physics, optimization, and effects of environmental factors on such microswimmers.

Contribution

It introduces a detailed analysis of propulsion mechanisms of deforming helices and explores how environmental factors influence their motion and efficiency.

Findings

Deforming helices can produce non-reciprocal swimming strokes.

Configuration parameters significantly affect propulsion efficiency.

Walls, gravity, and defects can enhance or alter swimming behavior.

Abstract

Many microorganisms and artificial microswimmers use helical appendages in order to generate locomotion. Though often rotated so as to produce thrust, some species of bacteria such Spiroplasma, Rhodobacter sphaeroides and Spirochetes induce movement by deforming a helical-shaped body. Recently, artificial devices have been created which also generate motion by deforming their helical body in a non-reciprocal way (Mourran et al., Adv. Mater., 29, 1604825, 2017). Inspired by these systems, we investigate the transport of a deforming helix within a viscous fluid. Specifically, we consider a swimmer that maintains a helical centreline and a single handedness while changing its helix radius, pitch and wavelength uniformly across the body. We first discuss how a deforming helix can create a non-reciprocal translational and rotational swimming stroke and identify its principle direction of motion. We then determine the leading-order physics for helices with small helix radius before considering the general behaviour for different configuration parameters and how these swimmers can be optimised. Finally, we explore how the presence of walls, gravity, and defects in the centreline allow the helical device to break symmetries, increase its speed, and generate transport in directions not available to helices in bulk fluids.