Switching between positive and negative movement near an air/water interface through lateral laser illumination

Abstract

The directional movement of an aqueous solution containing gold nanoparticles under laser irradiation (532 nm) through the vicinity of an air/water interface is reported. It is shown that unidirectional flow is generated along the path of light-travel when a laser is irradiated parallel to the interface. On the contrary, liquid flow toward the light source is caused when the incident angle of the laser is tuned so as to make the total reflection at an air/water interface. Such switching of the flow direction, positive or negative to the light propagation, was applied to the movement of mm-sized floating object, revealing that repetitive back and forth motion of the solid object is generated. This directional switching of photo-induced movement is interpreted by taking into account the flow caused by the spatial gradient of the surface tension due to the thermal effect of the incident laser. A numerical model with a Navier-Stokes-type equation reproduces the essential aspects of the switching of the direction observed in the present study.