An approach of bubble generation and manipulation by using the photothermal effects of laser irradiation on light absorbing particles.

The photothermal effects have shown the possibilities for applications in optical manipulation. In this paper, an approach is demonstrated to generate and manipulate a bubble using the photothermal effects. First, a high-power laser is used to irradiate the light absorbing particles for creating a microbubble. The bubble grows up to a diameter of a few hundred micrometers in several seconds due to the diffusion of dissolved gases. The bubble does not float up and is confined at the lower boundary of the sample cell by the thermocapillary force. The force is induced by laser heating of the particles at the bubble base. Second, the bubble can be manipulated following the laser focal spot. The bubble is dragged by the horizontal component of thermocapillary force. The bubble re-grows as it moves because it absorbs the dissolved gases in its migration path. The bubble floats up finally when it grows up to the maximum size. The perpendicular component of thermocapillary force can be estimated equal to the buoyancy of the floated bubble and is about 38 nN at the laser power of 130 mW. Furthermore, we show the generation and manipulation of the bubbles in a capillary. The reason for the decrease in movement velocity in the capillaries has been studied and discussed. The approach of bubble manipulation shows a potential application in transporting the microparticles.