Using sound waves, researchers have been able to suspend, move, and manipulate objects. New research has taken this a step further, as a team from China has managed to use acoustic waves to create bubbles from soap droplets.
The approach, reported in Nature Communications, is a reverse of what usually happens in soap bubbles, which burst into droplets. The researchers trapped the droplets midair using a technique called acoustic levitation and turned them into a flat film. The levitation was then adjusted to turn the flat film into a buckled surface.
Each droplet was then forced to vibrate at a resonance frequency, a special frequency that forced it to oscillate with larger and larger amplitudes. The team discovered that at a certain point, the droplet abruptly inflates into a bubble. The specific volume at which it happens depends on the frequency. Smaller droplets can be created with higher frequencies.
“Similar phenomena have been observed before, but this is the first time the underlying mechanism was uncovered,” lead author Duyang Zang, from the Northwestern Polytechnical University, told Newsweek. “Moreover, this is the first time the bubble can be formed in a controlled manner via this mechanism. We can adjust the volume of the air cavity through external dragging to triggering the transition.”
The team also tested other ways to create bubbles. Once the flat film is created, it is possible to make a bubble by pulling the middle of the film with a needle. They also found that it's possible to create it by passing a ring around the edge of the film.
Turning films into bubbles is important to the production of a range of goods, from foods and cosmetics to pharmaceuticals and even ultra-light materials. More often than not the approach requires careful planning of how to best mix liquid and gas to get foaming and emulsification. And it needs to be done quickly to make sure the bubbles don’t burst.
Acoustic levitation could help with these two aspects plus more. It could allow scientists to create bubbles quickly (each bubble is made in milliseconds) without having to worry about the mixture. What's more, the bubbles would be long-lasting, each remaining stable for tens of minutes. Real-life applications are still far in the future but the team suggests that their approach could be used easily to make hollow-structured materials or even capsules to deliver medicines.