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  • Letter
  • Open Access

Active particle in a very thin interfacial droplet

Airi N. Kato1, Kaili Xie1,2, Benjamin Gorin1, Jean-Michel Rampnoux1, and Hamid Kellay1,*

  • *Contact author: hamid.kellay@u-bordeaux.fr
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Phys. Rev. Research 7, L042023 – Published 27 October, 2025

DOI: https://doi.org/10.1103/882l-mzr2

Abstract

A single light-driven Janus particle confined in a very thin oil droplet at an air-water interface displays intriguing dynamics. While laser activation induces rapid horizontal motion (1mm/s–1cm/s) by thermal Marangoni flow, the particle exhibits unexpected periodic circular motions or intermittent irregular motions. We show that the trajectories are the result of a coupling between the self-propulsion of the particle and the spatiotemporal droplet thickness changes. We propose a simple model where the properties of the active particle trajectories are governed by capillary forces and torques due to the confinement of the particle in the thin droplet.

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Phys. Rev. Research 7, L042023– Published 27 October, 2025
  • Received 16 January 2025
  • Revised 23 May 2025
  • Accepted 25 September 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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