Soap bubbles’ ‘pop’ reveals the physics of the bursts

A soap bubble’s final act is a quiet “pfttt.”

Put your ear next to a bubble and you might hear a high-pitched sound as it bursts. Scientists have now recorded that sound with an array of microphones. These reveal the underlying physics of that sound.

The team shared its findings February 28 in Physical Review Letters.

A bubble’s soapy film pushes on the air inside it. When that bubble bursts, it begins with a break, or rupture, in the soapy film. As the rupture enlarges, the soap film retracts and shrinks. That shift in the size of the film changes the force pushing on the air within the bubble, says Adrien Bussonnière. He is a physicist in France. He works at Université de Rennes 1.

He and colleagues recorded the sounds of bursting bubbles. These showed that the changing forces in the ruptured bubble cause changes in the bubble’s internal air pressure. The change in pressure is what the microphones record.

The researchers also found that as the soapy film retreats, soap molecules pack together more tightly. They become more dense near the edge of the film. This increased density now changes how much the molecules in the film are attracted to one another. That’s called surface tension. The change in surface tension changes the forces on the air, which change over time — and affect the sound.

The bubble burst is fast. It’s a blink-and-you’ll-miss-it event. So to see it, scientists typically turn to high-speed video.

In this new study, the team didn’t focus solely on watching the disappearing act. They listened to it, too. These researchers wanted to understand the properties of the sound as the bubble burst. This area of physics is known as acoustics.

Their recordings demonstrate how acoustics can reveal the changing forces that produce certain sounds. These could include everything from a bubble burst to the rumble from within a volcano to the buzzing of a bee, says Bussonnière. “Images,” he emphasizes, “cannot tell the whole story.”