High-velocity optothermal whirlpool actuation at the air-water interface

Abstract

In a meniscus lifted above a free water surface by an optical fiber delivering near infrared radiation, the upper confinement of the heated buoyant liquid amplifies the surface temperature gradient, driving particularly strong thermocapillary effects. When the temperature gradient within the meniscus becomes very large, the stationary convection flow destabilizes in a periodic pattern of surface hydrothermal waves. We show that these light fueled waves can be controlled to a large extent by acting on the system parameters, and harnessed to propel buoyant macroparticles along stable closed trajectories at exceptionally high speeds. With 20 mW of input power at 1550 nm, we observe fast linear oscillations of 0.1 to 1 mm particles with peak velocities up to 2 cm/s and a whirlpool-like stable orbital motion with rotation rate up to 600 rpm, the largest reported so far for optothermal actuation. These findings establish a new efficient method for contactless actuation on liquid surfaces with direct application in microfluidics and in the realization of efficient light powered micromotors. At the same time, the proposed fiber meniscus configuration provides a simple and adaptable platform for studying optothermal fluid instabilities and for generating and controlling surface thermocapillary waves.