A soap bubble is an object of ethereal beauty - a shiny bauble, floating on air, reflecting the world on a backdrop of swirling, translucent colours. It hovers and falls slowly to the floor - gradually the skin of the bubble turns to lace, the colours fade and suddenly it is gone.
As with most things, there's a lot of physics happening in there while you are just admiring the bubble's shape and shimmer.
- The bubble is a pocket of air, held in place by a microscopically thin layer of soapy water. The water provides surface tension and the soap molecules form a lipid bilayer, trapping water molecules within and preventing rapid evaporation - water alone evaporates far too quickly to allow a bubble to last. A bubble can last a very long time if evaporation is prevented, for example by increasing the humidity of the surrounding air.
- The surface tension of the water mixture means that the bubbles will always form with the smallest possible surface area to volume ratio - hence single bubbles are spherical, and combinations of bubbles form more complex geometric shapes depending on how many bubbles come together.
- The colouration of a bubble is directly related to the thickness of its skin. White light contains all the colours of the rainbow, each colour being a different wave length. When white light hits the bubble, some of it is reflected from the outer surface of the film, and some from the inner surface. The reflected waves interfere and sometimes cancel each other out, so that only the complimentary colours come back to the eye. Thicker membranes appear blue due to red wavelengths being cancelled out, then as the bubble thins the bubble appears blue, then green, then yellow . Finally, when the bubble is at its thinnest, it appears lacey because all the light waves interfere and cancel out, leaving black. A fraction of a second later, the bubble pops out of existance.
- The thickness of the skin of the bubble is not uniform. The bubble is usually spinning slowly because of the way it has been blown and released. The action of gravity on the molecules of the membrane counteracts the surface tension, causing the skin to be in constant motion. The result is the familiar pattern of random swirls and bands of colour which move across the surface of the bubble before it finally disappears.