Reaching for the surface: Spheroidal microswimmers in surface gravity waves

Kunlin Ma, Nimish Pujara, and Jean-Luc Thiffeault
Phys. Rev. Fluids 7, 014310 – Published 25 January 2022

Abstract

Microswimmers (planktonic microorganisms or artificial active particles) immersed in a fluid interact with the ambient flow, altering their trajectories. In surface gravity waves, a common goal for microswimmers is vertical migration (e.g., to reach the free surface or to dive to deeper depths). By modeling microswimmers as spheroidal bodies with an intrinsic swimming velocity that supplements advection and reorientation by the flow, we investigate how shape and swimming affect vertical transport of microswimmers in waves. We find that it is possible for microswimmers to be initially swimming downwards but to recover and head back to the surface and vice versa. This is because the coupling between swimming and flow-induced reorientations introduces a shape dependency in the vertical transport. From a wave-averaged analysis of microswimmer trajectories, we show that each trajectory is bounded by critical planes in the position-orientation phase space that depend only on the shape. We also give explicit solutions to these trajectories and determine the fraction of microswimmers that begin within the water column and eventually reach the surface. For microswimmers that are initially randomly oriented, the fraction that reach the surface increases monotonically as the starting depth decreases, as expected, but also varies with shape and swimming speed. In the limit of small swimming speed, the fraction of highly prolate microswimmers reaching the surface is 0.5, suggesting that these swimmers would be able to choose direction of vertical transport with small changes in swimming behavior.

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  • Received 19 October 2021
  • Accepted 10 January 2022

DOI:https://doi.org/10.1103/PhysRevFluids.7.014310

©2022 American Physical Society

Physics Subject Headings (PhySH)

Fluid Dynamics

Authors & Affiliations

Kunlin Ma1,2, Nimish Pujara1,*, and Jean-Luc Thiffeault2

  • 1Department of Civil and Environmental Engineering, University of Wisconsin—Madison, Madison, Wisconsin 53706, USA
  • 2Department of Mathematics, University of Wisconsin—Madison, Madison, Wisconsin 53706, USA

  • *npujara@wisc.edu

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Vol. 7, Iss. 1 — January 2022

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