Effective swimming strategies in low Reynolds number flows

TL;DR

This paper explores how microscopic swimmers can effectively migrate across shear flows by combining flow-induced deformations and swimming, analyzing models that leverage elastic structures and orientation-dependent friction.

Contribution

It introduces new models for microswimmer migration that utilize external flow-induced deformations without internal energy sources, extending understanding of low Reynolds number swimming strategies.

Findings

Migration velocity depends on deformation patterns and amplitude.

Flow-induced deformation can enable propulsion without internal energy.

Thermal fluctuations may be exploited to enhance propulsion.

Abstract

The optimal strategy for a microscopic swimmer to migrate across a linear shear flow is discussed. The two cases, in which the swimmer is located at large distance, and in the proximity of a solid wall, are taken into account. It is shown that migration can be achieved by means of a combination of sailing through the flow and swimming, where the swimming strokes are induced by the external flow without need of internal energy sources or external drives. The structural dynamics required for the swimmer to move in the desired direction is discussed and two simple models, based respectively on the presence of an elastic structure, and on an orientation dependent friction, to control the deformations induced by the external flow, are analyzed. In all cases, the deformation sequence is a generalization of the tank-treading motion regimes observed in vesicles in shear flows. Analytic expressions for the migration velocity as a function of the deformation pattern and amplitude are provided. The effects of thermal fluctuations on propulsion have been discussed and the possibility that noise be exploited to overcome the limitations imposed on the microswimmer by the scallop theorem have been discussed.