Sound waves can make bubbles in levitated drops of liquid

Save your breath: A new way to make bubbles requires only sound waves.

Scientists made the bubbles in levitated drops of liquid, held aloft with sound waves. Tweaking the sound waves caused the hovering drop to balloon into a bubble.

The team formed the bubbles using a variety of liquids, including water. Increasing the intensity of the sound made the liquid first buckle into a concave shape. Then the sound waves resonated inside the droplet’s newly formed cavity, causing a rapid expansion of the liquid film until it closed in on itself into a hollow bubble, researchers report September 11 in Nature Communications.

Despite their reputation as a child’s plaything, bubbles are serious business. They’re important in manufacturing processes used for food, pharmaceuticals, cosmetics and ultralight materials. So a new technique for bubble creation might find uses in industry.

The scientists suspended the liquid drops using a well-established technique called acoustic levitation. In the method, pressure from intense sound waves is used to hold up small objects and even move them around (SN: 8/24/13, p. 10).

The floating bubbles last a surprisingly long time — tens of minutes. As kids know, a soap bubble on a wand sticks around only a short while before the soapy solution drains to the bubble’s bottom and it pops (SN: 1/21/17, p. 32). But the levitation slows down the liquid’s drainage, putting off the bubble’s burst.