Physics

Sound makes droplets levitate and turn into bubbles, and we can’t stop watching

Building a bubble out of a floating drop of liquid may sound like weird sci-fi, but that’s exactly what physicists have just achieved. They manipulated sound waves to levitate a droplet in mid-air, then close it around an air pocket.

Unlike the ephemeral soap bubbles you might know from your childhood, the resulting bubble was incredibly stable, holding its shape for up to tens of minutes, the researchers said.

Apart from just being incredibly cool, this research could have important implications in both manufacturing and research of various materials.

“Bubble formation plays an important role in the preparation of foams, which have extensive applications in industries concerned with food, cosmetics, pharmaceuticals, ultra-light materials, and mineral flotation,” the researchers wrote in their paper.

“Common approaches to forming bubbles are to exert intense shear to the liquid via turbulent mixing or flow focusing techniques or to use microfluidics.”

The technique, deployed by physicists at Northwestern Polytechnical University in Xi’an, China, exploits a known technology called acoustic levitation.

It involves using powerful sound waves to counter the force of gravity and levitate a tiny object in place. It’s a well-described phenomenon, and is used in the development of pharmaceuticals to keep samples untainted by contact with a surface.

It’s not even the first time it’s been used to levitate a droplet and turn it into a bubble – this kind of result was reported in 1991 and again in 2016. But exactly how it was occurring remained unclear.

The research team had two goals: to see if they could replicate the phenomenon; and to record it happening with high-speed cameras to observe the mechanisms at play.

Using a variety of liquids, they levitated a number of droplets with ultrasound. They found that increasing the intensity of the sound waves would cause the droplet to flatten out, then buckle into a bowl-like shape, resembling a contact lens.

When the cavity inside this bowl grew big enough, the sound waves would start to bounce around inside. The resulting resonance stretched the droplet into an upside-down bowl shape, and then, finally, a hollow bubble as the bowl’s “rim” closed around the cavity.

This bubble was, much like the original drop, an ellipsoid shape, but much larger in volume. Increasing the frequency of the ultrasound resulted in a smaller bubble.

Two other techniques – using a needle and a ring to help shape the bubble – also resulted in the desired shape. This could help develop a new way to create floating, hollow materials or levitating liquid capsules.

Well, it could be used for that eventually. Right now, only one bubble can be produced at a time, so it’s far from useful for foam production. But in its current form, the technique has revealed more knowledge about the dynamics of bubbles, and the acoustic manipulation of fluids.

It also provides fundamental science to pave the way for those future discoveries.

“The most important finding of this study is that the cavity encapsulated by the acoustically levitated buckled liquid film can be regarded as an acoustic resonator that is independent of the liquid properties,” the researchers wrote in their paper.

“Our results establish a unique bubble formation method and create an excellent platform for studying bubble physics, such as oscillation, drainage and evaporation.”

The paper has been published in the journal Nature Communications.

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