The role of body flexibility in stroke enhancements for finite-length undulatory swimmers in viscoelastic fluids

TL;DR

This paper investigates how body and fluid elasticity influence the swimming behavior of undulatory micro-organisms in viscoelastic fluids, revealing that body flexibility can enhance speed while fluid elasticity often causes slowdown, especially at high amplitudes.

Contribution

It provides an asymptotic analysis linking body and fluid elasticity to shape changes and swimming speed, validated by numerical simulations, offering new insights into micro-organism locomotion in complex fluids.

Findings

Elastic shape changes increase swimming speed for soft swimmers.

Fluid elasticity generally causes swimmers to slow down.

High amplitude strokes in elastic fluids lead to stress accumulation and slowdown.

Abstract

The role of passive body dynamics on the kinematics of swimming micro-organisms in complex fluids is investigated. Asymptotic analysis of small amplitude motions of a finite-length undulatory swimmer in a Stokes-Oldroyd-B fluid is used to predict shape changes that result as body elasticity and fluid elasticity are varied. Results from the analysis are compared with numerical simulations, and the small amplitude analysis of shape changes is quantitatively accurate at both small and large amplitudes, even for strongly elastic flows. We compute a stroke-induced swimming speed that accounts for the shape changes, but not additional effects of fluid elasticity. Elastic induced shape changes lead to larger amplitude strokes for sufficiently soft swimmers in a viscoelastic fluid, and these stroke boosts can lead to swimming speed-ups, but we find that additional effects of fluid elasticity generically slow down swimmers. High amplitude strokes in strongly elastic flows lead to a qualitatively different regime in which highly concentrated elastic stresses accumulate near swimmer bodies and where dramatic slow-downs are seen.