Bubble control

Kyoto, Japan -- A watched pot never boils, goes the old saying, but many of us have at least kept an eye on the pot, waiting for the bubbling to start. It's satisfying to finally see the rolling boil, behind which complex physical mechanisms are at play.

When this happens, the bubbles that form continuously change in shape and size. These dynamic movements influence the surrounding fluid flow, thereby affecting the efficiency of heat transfer from the heat source to the water.

Manipulating small amounts of liquid at high speeds and frequencies is essential for processing large numbers of samples in medical and chemical fields, such as in cell sorting. Microbubble vibrations can create flows and sound waves, aiding in liquid manipulation. However, the collective behavior and interactions of multiple bubbles is poorly understood, so their applications have been limited.

Motivated to better understand bubble behavior, a team of researchers at Kyoto University has developed an experimental setup to precisely adjust the distance between microbubbles, employing laser light to photothermally heat degassed water.

"We were able to establish a new method to fundamentally alter the liquid flow by simply adjusting the arrangement of bubbles," says first author Xuanwei Zhang.

The team successfully generated two bubbles measuring about 10 micrometers in diameter that spontaneously vibrate at sub-megahertz frequencies, investigating how their vibrations affect each other. Using this apparatus, the researchers were able to precisely control the fast movements of bubbles at sub-megahertz frequencies as well as the surrounding flow.

After comparing the results with theoretical equations, the team found that the pressure generated by each bubble's vibration accounts for the interactions between bubbles. They discovered that neighboring bubbles synchronize their vibrations, and that changing the distance between bubbles by just 10 micrometers altered their vibration frequency by more than 50%.

"We did not expect to observe such clear vibrational coupling between two oscillating bubbles, but the vibrations of the bubbles we generated were very stable over time and highly reproducible," says corresponding author Kyoko Namura. These characteristics enabled the team to capture changes in the vibrations of the two bubbles when their relative positions were even slightly adjusted.

The results of this study provide a new fluid control tool for the medical and chemical fields, where faster analysis and data collection are indispensable. Though the research team used degassed water, similar effects can be achieved with water-alcohol mixtures, rendering this method applicable to a wide range of applications.

In the future, the team plans to explore ways to actively select bubble vibration frequencies and modes, control larger arrays of bubbles, and analyze the sound waves and flows generated around them.