Active particle in a very thin interfacial droplet

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 ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mn>1</a:mn> <a:mspace width="0.16em"/> <a:mi mathvariant="normal">m</a:mi> <a:mi mathvariant="normal">m</a:mi> <a:mo>/</a:mo> <a:mi mathvariant="normal">s</a:mi> <a:mo>–</a:mo> <a:mn>1</a:mn> <a:mspace width="0.16em"/> <a:mi mathvariant="normal">c</a:mi> <a:mi mathvariant="normal">m</a:mi> <a:mo>/</a:mo> <a:mi mathvariant="normal">s</a:mi> </a:mrow> </a:math> ) 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.