Squirming through shear-thinning fluids

TL;DR

This paper investigates how shear-thinning non-Newtonian fluids affect the swimming behavior of spherical squirmers, revealing complex effects on speed and the limitations of traditional decomposition methods.

Contribution

It provides the first detailed analysis of swimming in shear-thinning fluids using asymptotic and numerical methods, highlighting nontrivial effects on swimmer speed.

Findings

Shear-thinning rheology causes both faster and slower swimming depending on surface actuation.

Traditional drag and thrust decomposition methods may not apply straightforwardly in complex fluids.

The study combines asymptotic analysis with numerical simulations to explore swimmer behavior.

Abstract

Many microorganisms find themselves immersed in fluids displaying non-Newtonian rheological properties such as viscoelasticity and shear-thinning viscosity. The effects of viscoelasticity on swimming at low Reynolds numbers have already received considerable attention, but much less is known about swimming in shear-thinning fluids. A general understanding of the fundamental question of how shear-thinning rheology influences swimming still remains elusive. To probe this question further, we study a spherical squirmer in a shear-thinning fluid using a combination of asymptotic analysis and numerical simulations. Shear-thinning rheology is found to affect a squirming swimmer in nontrivial and surprising ways; we predict and show instances of both faster and slower swimming depending on the surface actuation of the squirmer. We also illustrate that while a drag and thrust decomposition can provide insights into swimming in Newtonian fluids, extending this intuition to problems in complex media can prove problematic.