Characteristics of the IR Laser Photothermally Induced Phase Change in Microchannels with Different Depths

The photothermal effect induced phase change is an important phenomenon in optofluidics. In this work, therefore, the characteristics of the phase change in microchannels with different depths induced by a 1550 nm infrared laser under both low and high laser powers was visually studied. It was revealed that at low laser power, the liquid body could be always advanced as a result of the induced evaporation-condensation-coalescence process regardless of the microchannel depth, which can function as a micro pump. The μ-PIV testing results further demonstrated the coalescence was a dominant mechanism in the interface advancement. Interestingly, although large depth increased the absorption length of the laser and thus improved the temperature and enhanced the evaporation, the advancing effect became weak due to the increase of both the flow resistance and liquid water content to be driven. At high laser power, for small depth microchannel, the liquid body was advanced at the beginning. Once a liquid slug along with a sealed gas slug was formed, the liquid body started to move backward, which can function as chemical separation. However, as the microchannel depth increased, despite that the evaporation was enhanced, such phenomena hardly happen because enhanced evaporation allowed large droplets to be generated. Air bubbles instead of a gas slug were easily entrapped in the liquid body during the coalescence process. These air bubbles quickly grew up due to high temperature, which could be an obstacle to the advancing movement of the liquid body or even block the laser heating. Therefore, it can be concluded that the microchannel depth plays an important role in the photothermally induced phase change process. The obtained results are helpful for the design and operation of the photothermal effect based optofluidic microdevices.