Motion of optically heated spheres at the water-air interface

Antoine Girot, Noémie Danne, Alois Würger, Thomas Bickel, Kuan Fang Ren, Jean-Christophe Loudet, and Bernard Pouligny

A micrometer-sized spherical particle classically equilibrates at the water-air interface in partial wetting configuration, causing about no deformation to the interface. In condition of thermal equilibrium, the particle just undergoes faint Brownian motion, well visible under a microscope. We report experimental observations when the particle is made of a light-absorbing material and is heated up by a vertical laser beam. We show that, at small laser power, the particle is trapped in on-axis configuration, similarly to 2-dimensional trapping of a transparent sphere by optical forces. Conversely, on-axis trapping becomes unstable at higher power. The particle escapes off the laser axis and starts orbiting around the axis. We show that the laser-heated particle behaves as a micro-swimmer with velocities on the order of several 100 µm/s with just a few milliWatts of laser power.

DOI