Listening to soap bubbles pop reveals the physics behind the bursts

A soap bubble’s swan song is a quiet “pfttt.”

Put your ear next to a soap bubble, and you might hear a high-pitched sound as it bursts. Now, scientists have characterized that sound using an array of microphones and analyzed the physics behind the sound of popping bubbles, scientists report in the Feb. 28 Physical Review Letters.

A bubble’s burst begins with a rupture in its soapy film (SN: 1/12/17). The rupture grows as the film retracts, altering the forces from the film pushing on the air within the bubble, physicist Adrien Bussonnière and colleagues report. The shifting forces cause pressure changes — sound — that microphones can pick up. Additionally, as the film retreats, soap molecules get packed together near the edge of the film, changing surface tension in the area, which also alters the forces on the air and affects the sound.

To study blink-and-you’ll-miss-it events like bubble bursts, scientists typically turn to high-speed video. But the new technique illustrates how acoustics can reveal the changing forces that produce certain sounds, including potentially the rumble from within a volcano or the buzzing of a bee, says Bussonnière, of Université de Rennes 1 in France. “Images cannot tell the whole story.”