Stabilization of Vapor-Rich Bubble in Ethanol/Water Mixtures and Enhanced Flow around the Bubble

This study investigates the behavior of microbubbles generated by the local heating of an ethanol/water mixture and the surrounding flow. The mixture is photothermally heated by focusing a continuous-wave laser on a FeSi2 thin film. Although the liquid is not degassed, vapor-rich bubbles are stably generated in an ethanol concentration range of 1.5-50 wt % The vapor-rich bubbles absorb the air dissolved in the surrounding liquid and exhale it continuously as air-rich bubbles similar to 1 mu m in diameter. For the same ethanol concentration range, the solutal-Marangoni force becomes dominant relative to the thermal-Marangoni force, and the air-rich bubbles are pushed away from the high-temperature region in the fluid toward the low-temperature region. Furthermore, it was experimentally demonstrated that Marangoni forces do not significantly affect the surface of vapor-rich bubbles generated in ethanol/water mixtures, and they produce a flow from the high-temperature to the low-temperature region on the vapor-rich bubbles, which moves the exhaled air-rich bubbles away from the vapor-rich bubbles near the heat source. These effects prevent the vapor-rich and exhaled air-rich bubbles from recombining, thereby resulting in the long term stability of the former. Moreover, the flow produced by the vapor-rich bubbles in the nondegassed 0-20 wt % ethanol/water mixture was stronger than that in degassed water. The maximum flow speed is achieved for an ethanol concentration of 5 wt %, which is 6-11 times higher than that when degassed water is utilized. The ethanol/water mixture produces vapor-rich bubbles without a degassing liquid and enhances the flow speed generated by the vapor-rich bubbles. This flow is expected to apply to driving and mixing microfluids.