Enhanced active swimming in viscoelastic fluids

TL;DR

This paper demonstrates that viscoelastic fluids can enhance microorganism swimming speeds through a fluid-structure interaction mechanism, challenging previous assumptions that such fluids hinder locomotion.

Contribution

It introduces a physical mechanism explaining non-Newtonian enhancement of swimming speed using a modified Taylor sheet model.

Findings

Viscoelasticity can increase swimming amplitude.

A transition from hindered to enhanced swimming occurs.

Fluid-structure interaction explains the enhancement.

Abstract

Swimming microorganisms often self propel in fluids with complex rheology. While past theoretical work indicates that fluid viscoelasticity should hinder their locomotion, recent experiments on waving swimmers suggest a possible non-Newtonian enhancement of locomotion. We suggest a physical mechanism, based on fluid-structure interaction, leading to swimming in a viscoelastic fluid at a higher speed than in a Newtonian one. Using Taylor's two-dimensional swimming sheet model, we solve for the shape of an active swimmer as a balance between the external fluid stresses, the internal driving moments, and the passive elastic resistance. We show that this dynamic balance leads to a generic transition from hindered rigid swimming to enhanced flexible locomotion. The results are physically interpreted as due to a viscoelastic suction increasing the swimming amplitude in a non-Newtonian fluid and overcoming viscoelastic damping.