Sustained optothermal propulsion at the air-water interface by pulsating thermocapillary instability

Abstract

In an axisymmetric meniscus lifted from a free water surface and locally heated through near infrared radiation by an optical fiber, natural buoyancy and geometric constraints confine the hot liquid near the apex. This confinement enhances the thermal gradient along the meniscus surface and, above a critical optical power, drives the system into an unstable convection regime. By analyzing the water refractive index oscillations at the meniscus apex and the simultaneous advection of nearby floating particles, we demonstrate that the instability is characterized by periodic thermocapillary pulses with tunable frequency and directionality. By controlling these features, we modulate the hydrodynamic forces generated by the pulsed surface flow and actuate particles at the meniscus interface in sustained oscillations or stable orbital trajectories. With about 20 mW optical power and particles ranging from 0.1 to 0.5 mm, rotational velocities up to 600 rpm are achieved, the highest reported for continuous optothermal actuation. Our results establish a highly efficient propulsion mechanism at fluid interfaces with potential applications in light-powered micromotors, interfacial transport and soft microrobotics. More broadly, the optically heated meniscus configuration provides a simple and adaptable platform for generating, controlling and detecting large thermocapillary instabilities on fluid surfaces.