Everyone loves the satisfying pop when a bubble bursts, but what exactly produces the sound? Adrien Bussonni猫re at Sorbonne University in Paris, France, and his colleagues studied audio of bubbles popping to find out 鈥� and it involves an imbalance of forces.
On a fundamental level, sound is a pressure wave. The air inside a bubble is under more pressure than the air outside it, as it is compressed by the surface tension of the bubble film 鈥� that is, until it bursts. 鈥淭he forces applied by the liquid soap film are not balanced anymore,鈥� says Bussonni猫re, and the imbalance of forces produces a sound.
The researchers used a syringe to inflate soap bubbles and either allowed them to pop by themselves or pricked them with a pin once the bubbles were around 1 centimetre in diameter. They used a bright light to illuminate the bubbles as they popped, creating iridescent interference patterns, the colours of which revealed the thickness of the bubble film.
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Bussonni猫re and his聽team found that as the bubble pops, the film nearest to the initial hole becomes slightly thicker, and this thickness propagates as a shockwave through the bubble. The film behind the shockwave from the expanding hole is left thinner, with less surface tension, allowing the air inside to expand and create a pressure wave. As the film retracts further, the inside air is no longer held back from the outside and snaps outwards, creating a second source of sound.
The exact nature of the shockwave and inside air pressure is determined by the chemical composition of the soap film. Since it is easier to listen to a pop than measure the shockwave, the researchers are now hoping that they can reverse the relationship to learn about the chemicals that make up a bubble from the sound it makes when it bursts.
Physical Review Letters
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