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Modified far-field hydrodynamic flows induce versatile trajectories of confined microswimmers

Zehan Cao and Alan C. H. Tsang*

  • *Contact author: alancht@hku.hk
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Phys. Rev. Fluids 11, 044402 – Published 22 April, 2026

DOI: https://doi.org/10.1103/h2hy-crbv

Abstract

Hydrodynamic flows and swimming trajectories of microswimmers are strongly influenced by nearby surfaces and geometric confinements. The far-field hydrodynamic flow of a microswimmer in bulk can be effectively represented by a force dipole due to the swimmer's propulsion and drag forces, where the source dipole due to body-size effect is subdominant. In contrast, the source dipole becomes the dominant flow contribution when the microswimmer is confined strongly in two dimensions, irrespective of the propulsion mechanism of the swimmer. Recent experimental works demonstrated that this understanding is not completely correct. For microswimmers under slightly weaker confinement, its flow field has a strong dependence on the spatial arrangement of the swimmer's propulsion and drag forces as well as the swimmer's geometry. In this case, the modified far-field hydrodynamic flows of the swimmer should be approximated by placing Stokeslets and source dipoles at proper positions of the swimmer. Here we employ this flow field modification and investigate how the spatial arrangement of Stokeslets and source dipoles influences the dynamics of a microswimmer in a confined liquid film. We observe versatile swimming trajectories, including boundary collision, boundary sliding, centerline sliding, amplified oscillation, and damped oscillation, depending on the relative strengths and positions of the Stokeslets and source dipoles at different confinement heights. These results highlight the enriched dynamics of microswimmers due to the diversity of flow fields in confined geometries.

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Phys. Rev. Fluids 11, 044402– Published 22 April, 2026
  • Received 2 November 2025
  • Accepted 9 February 2026
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