Motion of Optically Heated Spheres at the Water–Air Interface

Abstract

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 microswimmer with velocities on the order of several 100 μm/s with just a few milliwatts of laser power.

Effect of surface-active contaminants on radial thermocapillary flows

“…Likewise, it has been shown that the height profile of nanometer-thin films can be controlled by light-induced temperature gradients [9]. Direct conversion of light into work has also been evidenced in the field of active matter: actuation of microfabricated rotors [10] and micron-sized spherical particles [11,12] at the liquid-air interface have been accomplished recently, confirming that thermocapillarity is a highly efficient mechanism for colloidal self-propulsion.…”

“…In the present study, we consider the hydrodynamic response of a surfactant-laden interface to a thermocapillary flow. Although many experiments are performed with radially symmetric sources [11,[16][17][18][19], little is known regarding 3D axisymmetric geometry. Also, arbitrary liquid depths have not yet received much consideration.…”

Nanoscale Seebeck effect at hot metal nanostructures

“…The main results of the present paper concern the slip velocity along metal surfaces. Though local heating of gold structures is widely used for manipulating of particles and cells [2][3][4]6] or powering microswimmers [7][8][9][10][11], there is at present no systematic study of the creep flow along a conducting surface. Evidence for thermoelectric driving of hot silica particles with a granular gold cap, was reported by one recent experiment [9]: probing the particle's self-propulsion velocity in 10 mM solutions of NaCl, LiCl and NaOH, revealed a saltspecific effect, which agrees qualitatively with the Seebeck coefficients of these electrolytes, S S S NaCl LiCl NaOH > > .…”

“…Laser-illuminated metal nanostructures provide versatile local heat engines [1], with optofluidic applications such as trapping of nanoobjects [2,3], manipulation of biological cells [4], microflows in capillaries [5], and colloidal assembly [6]. Similarly, thermally powered artificial microswimmers rely on the conversion of absorbed heat to motion; experimental realizations include metal-capped Janus particles that are driven by surface forces [7][8][9][10], and interface floaters that are advected by their self-generated Marangoni flow [11,12]. Force-free localization and steering have been achieved by temporal [13]or spatial [14] modulation of the laser power.…”

Laser-heated microswimmers: optical manipulation of active particles

“…Laser-heated metal or carbon nanostructures provide versatile local heat sources, with applications ranging from photothermal imaging [1] to thermally powered artificial microswimmers [2]. The latter rely on the conversion of heat tomotion; experimental realizations include Janus colloids driven by surface forces [3], and interfacial particles that are advected by their self-generated Marangoni flow [4,5]. Force-free localization and steering have been achieved by temporal [6] or spatial [7] modulation of the laser power.…”