Steady Motions of Single Spherical Microswimmers in Non-Newtonian Fluids

TL;DR

This paper investigates the propulsion behaviors of spherical microswimmers in non-Newtonian fluids, revealing how shear properties affect their speed and challenging classical swimming constraints like Purcell's scallop theorem.

Contribution

It provides new insights into microswimmer dynamics in complex fluids and demonstrates that reciprocal motions can produce net movement in non-Newtonian environments.

Findings

Shear-thickening fluids slow down traction-driven microswimmers.

Shear-thinning fluids speed up traction-driven microswimmers.

Purcell's scallop theorem does not apply in non-Newtonian fluids.

Abstract

In biological systems, microswimmers often propel themselves through complex media. However, many aspects of swimming mechanisms in non-Newtonian fluids remain unclear. This study considers the propulsion of two types of single spherical microswimmers (squirmers) in shear-thickening and shear-thinning fluids. The slip-driven squirmer propels faster/slower in shear-thickening/thinning fluids than in Newtonian fluids [C. Datt et al., ''Squirming through shear-thinning fluids,'' J. Fluid Mech. 784, R1 (2015)]. In contrast, we discovered that a traction-driven squirmer exhibits the opposite trend, moving slower/faster in shear-thickening/thinning fluids than in Newtonian fluids. In addition, we have shown theoretically that Purcell's scallop theorem does not hold in non-Newtonian fluids when a squirmer with reciprocal surface motions is used. The present findings open up possibilities for the design of new types of microswimmers that can achieve translational motion from a single reciprocal motion in non-Newtonian fluids.