The mechanism of propulsion of a model microswimmer in a viscoelastic fluid next to a solid boundary

TL;DR

This study investigates how a model microswimmer's propulsion in a viscoelastic fluid near a solid boundary is affected, revealing complex interactions that can enhance swimming speed at finite amplitudes.

Contribution

The paper provides a mechanistic explanation for the decoupling of viscoelastic and boundary effects and develops a numerical method for arbitrary wave amplitudes.

Findings

Viscoelasticity tends to slow the swimmer.

Boundaries tend to speed up the swimmer.

At finite amplitudes, boundaries can increase swimming speed due to viscoelastic effects.

Abstract

In this paper we study swimming of a model organism, the so-called Taylor's swimming sheet, in a viscoelastic fluid close to a solid boundary. This situation comprises natural habitats of many swimming microorganisms, and while previous investigations have considered the effects of both swimming next to a boundary and swimming in a viscoelastic fluid, seldom have both effects been considered simultaneously. We re-visit the small wave amplitude result obtained by Elfring and Lauga (Gwynn J. Elfring and Eric Lauga, in Saverio E. Spagnolie, editor, Complex Fluids in Biological Systems, Springer New York, New York, NY, 2015), and give a mechanistic explanation to the decoupling of the effects of viscoelasticity, which tend to slow the sheet, and the presence of the boundary, which tends to speed up the sheet. We also develop a numerical spectral method capable of finding the swimming speed of a waving sheet with an arbitrary amplitude and waveform. We use it to show that the decoupling mentioned above does not hold at finite wave amplitudes and that for some parameters the presence of a boundary can cause the viscoelastic effects to increase the swimming speed of microorganisms.