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.

Microbubbles have important applications in optofluidics. The generation and growth of microbubbles is a complicated process in microfluidic channels. In this paper, we use a laser to irradiate light-absorbing particles to generate microbubbles in capillary tubes and investigate the factors affecting microbubble size. The results show that the key factor is the total area of the light-absorbing particles gathered at the microbubble bottom. The larger the area of the particles at bottom, the larger the size of the microbubbles. Furthermore, the area is related to capillary tube diameter. The larger the diameter of the capillary tube, the more particles gathered at the bottom of the microbubbles. Numerical simulations show that the Marangoni convection is stronger in a capillary tube with a larger diameter, which can gather more particles than that in a capillary tube with a smaller diameter. The calculations show that the particles in contact with the microbubbles will be in a stable position due to the surface tension force.