Optimal swimmer can be puller, pusher, or neutral depending on the shape

TL;DR

This paper investigates how the shape of microswimmers influences their optimal propulsion strategy, revealing that nearly spherical swimmers can be pullers, pushers, or neutral depending on their shape profile, with the third shape mode being decisive.

Contribution

It introduces a theoretical framework using the minimum dissipation theorem to determine the flow fields and swimming types of nearly spherical microswimmers based on shape asymmetries.

Findings

Optimal swimmer shape depends on the shape profile.

Shape asymmetry can change swimmer from neutral to pusher or puller.

The third shape mode primarily determines swimming type.

Abstract

The ability of microswimmers to deploy optimal propulsion strategies is of paramount importance for their locomotory performance and survival at low Reynolds numbers. Although for perfectly spherical swimmers minimum dissipation requires a neutral type swimming, any departure from the spherical shape may lead the swimmer to adopt a new propulsion strategy, namely those of puller- or pusher-type swimming. In this study, by using the minimum dissipation theorem for microswimmers, we determine the flow field of an optimal nearly spherical swimmer, and show that indeed depending on the shape profile, the optimal swimmer can be a puller, pusher, or neutral. Using an asymptotic approach, we find that amongst all the modes of the shape function, only the third mode determines to leading order the swimming type of the optimal swimmer.