Inducing drop to bubble transformation via resonance in ultrasound

Bubble formation plays an important role in industries concerned with mineral flotation, food, cosmetics, and materials, which requires additional energy to produce the liquid–gas interfaces. A naturally observed fact is, owing to the effect of surface tension, a bubble film tends to retract to reduce its surface area. Here we show a “reverse” phenomenon whereby a drop is transformed into a bubble using acoustic levitation via acoustic resonance. Once the volume of the cavity encapsulated by the buckled film reaches a critical value V(*), resonance occurs and an abrupt inflation is triggered, leading to the formation of a closed bubble. Experiments and simulations both reveal that V(*) decreases with increasing acoustic frequency, which agrees well with acoustic resonance theory. The results afford enlightening insights into acoustic resonance and highlight its role in manipulating buckled fluid–fluid interfaces, providing a reference for fabricating unique core–shell-like materials.