Stabilization of vapor-rich bubble in ethanol/water mixtures and enhanced flow around the bubble

TL;DR

This research demonstrates the stable generation of vapor-rich microbubbles in ethanol/water mixtures using laser heating, revealing enhanced flow dynamics and stability mechanisms relevant for microfluidic applications.

Contribution

It introduces a novel method for stable vapor-rich bubble generation in ethanol/water mixtures without degassing, highlighting the dominant solutal-Marangoni forces and flow enhancement effects.

Findings

Vapor-rich bubbles are stably generated in 1.5-50 wt% ethanol/water mixtures.

Flow speeds are 6-11 times higher in ethanol/water mixtures compared to degassed water.

Marangoni forces influence bubble behavior but do not significantly affect bubble surfaces.

Abstract

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 FeSi$_2$ 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 $\sim$ 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. Further, 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 non-degassed 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.