Control of Janus Droplets via Optothermally Induced Marangoni Forces

Micro-scale double emulsion droplets are a versatile material platform for dynamic optical components, that generate unique color effects, form liquid lenses, or enable rapid food-borne pathogen detection. Reliable in-situ control of the droplet morphology generally requires careful design of responsive surfactants and delicate tuning of the droplet’s chemical environment. Here, we present a simple alternative for droplet actuation: Using optically-induced thermal gradients, an interfacial tension differential is generated across the internal capillary interface of Janus droplets. The interfacial tension differential causes droplet-internal Marangoni flows and a net torque, resulting in a predictable and controllable reorientation of the droplets. The effect is quantitatively described with a simple model that balances gravitational and thermal torques. These optothermally-induced Marangoni dynamics, which require only small temperature gradients, represent a promising mechanism for controlling orientation and tilt of droplet-based micro-optical components and biologically-inspired artificial microswimmers.