Although bubbles normally burst into droplets, a method to achieve the reverse phenomenon, that is, forming bubbles from droplets, is described in Nature Communications this week. The findings help identify strategies for the manipulation of fluid-fluid interfaces with potential applications in the fabrication of soft materials.

Acoustic levitation is a common technique used to study droplet dynamics by which a droplet can be made to hover in the air under the effect of sound waves. Acoustic pressure has previously been exploited to deform liquid drops by flattening them into a thin film and by inducing buckling.

Duyang Zang and colleagues combine these previously observed effects to achieve controlled bubble formation. They show that a droplet is first deformed into a thin film by acoustic radiation force. Next, the ultrasonic field causes the film to buckle into a bowl shape, which encloses a resonant cavity. The resonance causes the cavity to grow and the liquid interface to curve around it, creating a closed bubble, the authors find.

The observed process might offer an alternative route to bubble formation, which could have applications in foam preparations in the food, cosmetics and pharmaceutical industries.