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. This characteristic sound conveys the underlying physics at play during the bursting.
At first, before the bursting, bubble’s soapy film pushes on the air inside it as it tends to minimize its area, this is known as Laplace pressure. Each portions of the bubble push inward and balance each other.
When a bubble bursts, it begins with the appearance of a hole, usually on top of the bubble where the film is thinner. Due the surface tension pulling on the edge, the hole enlarges really fast (around 10 m/s). The initially balanced forces pushing on the inner air are no longer balanced, the forces present on the surface taken by the hole disappears, resulting into a net force applied on the air and directed toward the top.
If the bursting was slow such force would result into an air jet through the hole and the bubble would deflate as a balloon. However, as the liquid film retracts faster than the time it takes for the air to move, this force emits an acoustic wave with a peculiar symmetry. Indeed, this force compress the air on the top of the bubble, dilatate the one below, and doesn’t modify the pressure at the equator. This is known as a dipolar source.
To fully understand this signature the final step is to model the force applied by the soap film on the air. If the film remains at rest this force simply the surface tension times the remaining film area projected onto the axis of the bursting. However, soap molecules, which stabilized the interface, pack together more tightly resulting into a local decrease of the surface tension which modify the force and in turn the soap bubble “pop”.
To summarize, the pop of a soap bubble arises from the force applied by the soap film on the air during the bursting. This sound is directional, you can only hear it if you are in the axis of the bursting. Moreover, the sound depends on the soap used to make the bubbles through the fast compaction of the molecules. So this seemly quiet “pftt” actually carries a profusion of information’s.
