Microbubble dynamics in boiling water enable precise fluid manipulation

Clock pots never boil, and as an old proverb, many of us are at least keeping our eyes on the pot, waiting for the bubbles to begin. Behind the complex physical mechanisms at work, it’s satisfying to see the rolling boil in the end.

When this occurs, the bubbles formed will continuously change in shape and size. These dynamic movements affect the flow of the surrounding fluid, thereby affecting the efficiency of heat transfer from the heat source to water.

Manipulating small amounts of liquid at high speeds and frequencies is essential for processing large numbers of samples in the medical and chemical fields, such as cell sorting. Microbubble vibrations create flow and sound waves and aid in liquid manipulation. However, the collective behavior and interaction of multiple bubbles are not well understood, and their applications are limited.

A team of researchers at Kyoto University have been motivated to better understand bubble behavior, and have developed an experimental setup that accurately adjusts the distance between microbubbles and uses laser light for photothermal degassed water. This paper has been published in Small.

“We were able to establish a new way to fundamentally change the flow of liquids by simply adjusting the arrangement of the bubbles,” says first author Xuanwei Zhang.

The team successfully generated two bubbles about 10 micrometers in diameter, vibrating spontaneously at sub-megahertz frequencies, and investigated how the vibrations affect each other. Using this device, researchers were able to accurately control the fast movement of bubbles at sub-megahertz frequencies and ambient flow.

After comparing the results with theoretical equations, the team found that the pressure generated by the vibrations of each bubble explains the interaction between the bubbles. They found that when adjacent bubbles synchronize the vibrations and changed the distance between the bubbles by just 10 micrometers, the vibrational frequency changed by more than 50%.

“We didn’t expect to observe such a clear vibrational bond between the two vibrating bubbles, but the vibrations of the bubbles produced were very stable and very reproducible over time.” These properties allowed the team to capture the vibrational changes of the two bubbles when their relative position was adjusted slightly.

The results of this study provide new liquid control tools for the medical and chemistry fields where faster analysis and data collection are essential. The researchers used degassed water, but similar effects can be achieved with water-alcohol mixtures, allowing this method to be applied to a wide range of applications.

In the future, the team plans to actively select the vibrational frequencies and modes of bubbles, control the larger array of bubbles, and analyze the sound waves and flows generated around them.