la6b00181_si_005.avi (29.39 MB)
Motion of Optically Heated Spheres at the Water–Air Interface
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posted on 2016-02-26, 00:00 authored by A. Girot, N. Danné, A. Würger, T. Bickel, F. Ren, J. C. Loudet, B. PoulignyA 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 microswimmer with velocities on
the order of several 100 μm/s with just a few milliwatts of
laser power.