Effect of water/ethanol mixture concentration on flow speed around microbubbles

This study investigates how the ethanol concentration in water/ethanol mixtures affects the formation of vapor-rich bubbles and the surrounding flow speed. Bubbles were generated by local heating of FeSi₂ thin films immersed in the water/ethanol mixture using a continuous wave laser. The results showed that vapor-rich microbubbles were generated stably in an ethanol concentration of 1.5–50 wt%, even though the mixture was not degassed. We found that air-rich bubbles with a diameter of approximately 1 µm were exhaled from the generated vapor-rich bubbles and the solutal-Marangoni force acting on these air-rich bubbles contributed to keep them away from the vapor-rich bubbles. Therefore, vapor-rich bubbles could exist stably without absorbing large amounts of air molecules. Furthermore, observations on the flow speed induced by vapor-rich bubbles revealed that the flow speed was higher at the ethanol concentration of 0–20 wt% than in degassed water. The maximum flow speed was achieved for an ethanol concentration of 5 wt%, which was 6–11 times higher than that when degassed water was utilized. This flow is expected to be applied to driving and agitating fluids in micro heat exchangers.