Active particle in a very thin interfacial droplet

TL;DR

This study investigates the complex dynamics of a light-driven Janus particle confined in a thin oil droplet, revealing that its motion results from a coupling between self-propulsion and droplet thickness variations, modeled by capillary forces.

Contribution

It introduces a simple model explaining the particle's trajectories based on capillary forces and droplet confinement effects, highlighting the interplay between activity and interface deformation.

Findings

Particle exhibits periodic and irregular motions due to droplet coupling.

Self-propulsion speed ranges from 1mm/s to 1cm/s.

Trajectories are governed by capillary forces and confinement effects.

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 periodic 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.