soap bubbles

Small, plastic bottles of soapy water are sold as toys intended for small children. Typically, some pastel coloring is added to the solution, and the screw-capped bottle also contains a plastic "wand" which is molded into a circular loop about two centimeters in diameter at each end. The loop at the handle end of the wand is as thin as the straight segment of plastic it's connected to, but the other end's loop is considerably thicker and ridged on its outside.

The bubble-making end is dipped into the solution, which forms a thin film across the open loop. Blowing across this loop will then produce bubbles.

It takes a bit of control to do this properly. Blow with too much force, and the soap film will simply break and you won't get any bubbles. But if you blow too softly, a single bubble will just keep growing, eventually becoming unstable and popping since it isn't being pushed hard enough to break free from the wand. Blowing with the proper intensity should result in an aesthetically pleasing stream of midsized bubbles. It's not hard to do, of course; it was designed for little kids.

This activity tends to be much more interesting when performed outdoors. Indoor confinement and lack of air currents just result in a bunch of bubbles falling to the floor. Outside, bubbles will drift every which way, even in apparently calm air, as they're light enough that the slightest vorticity will quickly disperse them. On a larger scale, they tend to follow the prevailing winds and updrafts around buildings. Blowing soap bubbles is an excellent way to see for yourself that the atmosphere is indeed a chaotic system.

Evaporation gives soap bubbles a lifespan on the order of seconds, though this seems to be a function of humidity. Collisions with solid objects generally result in the bubbles' destruction; however, a freshly-made soap bubble often has sufficient elasticity to bounce off a smooth, hard surface. While thinner bubbles containing less soap solution will evaporate sooner, they don't necessarily last longer than heavier ones, which become unstable as the earth's gravitational field disrupts the balanced distribution of the bubble's mass.

Though intended as inexpensive "fun" for children, soap bubbles can also be an effective relaxation device for the nominally adult, particularly those whose scientific or artistic bent gives them a greater appreciation for intricate detail. The swirling rainbow interference bands on a bubble's surface are fascinating, and fine-tuning bubble sizes is an art in itself. They're also an excellent way to entertain (or infuriate, I'm not really sure which) a dog or cat, which will chase down bubbles with great vigor until becoming bored once it realizes it can't catch them.

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.

http://www.exploratorium.edu/ronh/bubbles/bubbles.html

Soap Bubbles. The blowing of soap bubbles is of great antiquity, and is to be seen depicted on an Etruscan vase in the Louvre. The beautiful play of colors familiar to all is due to the excessive but variable thinness of the soap films. The spherical form of the ordinary soap bubble is a direct result of the action of surface tension, the geometrical condition being that with given volume the surface must have minimum area.

Entry from Everybody's Cyclopedia, 1912.