Physicists have listened to burst the bubble

Physicists have measured the sound pressure of bursting soap bubbles by using microphone array (plurality of receivers of a sound, working in concert) and analyzed the results using the decomposition of the acoustic field in spherical waves. Based on the theoretical foundations of aeroacoustics, physics has proven that the sound of bursting bubble — a direct consequence of the compression of air by the surface tension of the soap film. Comparing the experimental data with the simulation results of the rupture of the soap film, they established the dependence between the acoustic signal profile and quantitative parameters of these models. The work of scientists published in the journal Physical Review Letters.

Soap bubbles are used not only on the playgrounds. In physical laboratories of interest to them, no less: physicists learn to properly inflate the bubbles to do their record-large and understand their freezing. But, like every kid, physics know that the most interesting — not to inflate the bubble and burst it.

The existence of the bubble is possible through the equality of the forces exerted on its shell. The air pressure inside of it higher than the surrounding atmospheric pressure, so the internal air volume is trying to expand and break a soap film, in which it is enclosed. It counteracts the force of surface tension: it holds the air inside the bubble the same way as the elastic forces of the rubber to resist the expansion of the compressed air inside the balloon. In smooth air, all the forces balance each other, and when the bubble attains a spherical shape, its surface area is minimal.

As a rule, this harmonious balance did not last long. Even if external influences do not have time to damage the bubble, it’s still pretty fast bursts. Under the action of gravity the film layers flow down from top to bottom, causing the top of the bubble becomes thinner over time. When its thickness reaches a critical value, the film ruptures, releasing the air inside. Not compensated by the force of surface tension continues to compress the film, further accelerating the air. This process can be seen on the frame frequency of the shooting or hear: when the bubble bursts, it produces a characteristic high frequency cotton, caused by the pressure drop.

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