Two-sphere swimmers in viscoelastic fluids

TL;DR

This paper investigates how two-sphere swimmers move in viscoelastic fluids, revealing that size differences and fluid properties enable net propulsion despite reciprocal motion.

Contribution

It introduces the effect of fluid viscoelasticity on two-sphere swimmers, showing how it induces propulsion when spheres differ in size, a novel insight in low-Reynolds number swimming.

Findings

Viscoelasticity causes propulsion of unequal-sized spheres.

Reciprocal motion does not lead to net movement in Newtonian fluids.

Rigid spheres in viscoelastic fluids behave similarly to elastic spheres in Newtonian fluids.

Abstract

We examine swimmers comprising of two rigid spheres which oscillate periodically along their axis of symmetry, considering both when the oscillation is in phase and anti-phase, and study the effects of fluid viscoelasticity on their net motion. These swimmers both display reciprocal motion in a Newtonian fluid and hence no net swimming is achieved over one cycle. Conversely, we find that when the two spheres are of different sizes, the effect of viscoelasticity acts to propel the swimmers forward in the direction of the smaller sphere. Finally, we compare the motion of rigid spheres oscillating in viscoelastic fluids with elastic spheres in Newtonian fluids where we find similar results.