How a bubble holds its shape
Scroll through the short life of a soap bubble, from first breath to the pop, and the geometry that holds it together.
A soap bubble is one of the thinnest things you’ll ever see with your own eyes. Scroll slowly.
A breath of air pushes into a soap film. Surface tension pulls the film as tight as it can, and the tightest shape around a volume of air is a sphere.
The colours aren't pigment. Light bounces off both the outer and inner surface of the film, and the two reflections interfere. As gravity drains the film, it thins, and the colours drift.
When two bubbles touch, they share a wall. Equal bubbles settle with their centres exactly one radius apart, because that's where the three films meet at 120°. They always meet at 120°.
Eventually the thinnest spot gives way. A hole opens and its rim races around the film at metres per second, breaking it into droplets. The shape was only ever a balance.
What you just watched
Why a sphere. Surface tension acts like a stretched sheet that wants to shrink. For a fixed amount of air inside, a sphere is the shape with the least surface, so that’s where the film settles.
Why the colours move. A soap film is a few hundred nanometres thick, about the same as a wavelength of visible light. Some colours reinforce after the two reflections, others cancel, depending on the thickness at each spot. Gravity pulls liquid downward, so the top thins first. The colours you see are a live map of thickness.
Why 120°. In the 1870s, the Belgian physicist Joseph Plateau found rules that every foam obeys: films meet three at a time, always at equal angles of 120°. It’s the same balance of forces as a tug of war with three equal teams.
Why it pops. Once any spot becomes too thin, the film can’t hold it. The rim of the hole collects liquid as it goes and moves too fast to follow by eye.