An Excerpt from What Einstein Didn’t Know: Scientific Answers to Everyday Questions
Why are soap bubbles round?
Let’s put it this way: You’d be pretty surprised if they were square, wouldn’t you? That’s because all of our experience since we were babies tells us that Mother Nature prefers smoothness. There just aren’t many natural objects that have sharp points or jangling edges. The major exception is certain mineral crystals, which occur in beautifully sharp geometric shapes. That may be why some people believe that crystals and pyramids are endowed with supernatural powers.
But that is metaphysics, not science. Bubbles are round — spherical — because there is an attractive force called surface tension that pulls molecules of water into the tightest possible groupings. And the tightest possible grouping that any collection of particles can achieve is to pack together into a sphere. Of all possible shapes — cubes, pyramids, irregular chunks — a sphere has the smallest amount of outside area.
As soon as you release a bubble from your bubble pipe or from one of those more modern gadgets, surface tension makes the thin film of soapy water assume the smallest surface area that it can. It becomes a sphere. If you hadn’t deliberately trapped some air within it, the soapy water would continue to shrink down to a solid spherical droplet, as rain drops do.
But the air on the inside is pushing outward against the water film. All gases exert a pressure on their captors because they consist of freely flying molecules that are banging up against anything in their way. In a bubble, the inward surface-tension forces of the water film are exactly balanced by the outward-pushing pressure of the air inside. If they were any different, the bubble would either shrink or expand until they were equal.
Try to blow more air in to make a bigger bubble? That makes more air pressure inside. All that the water film can do to counterbalance the increased outward pressure is to expand its surface, making more inward-directed surface-tension forces. So it very cooperatively grows in size. But it must get thinner in the process, because there is only so much water to go around. If you keep blowing more air in, the film eventually won’t have enough reserve water to spread out into a bigger surface, and the ultimate catastrophe occurs: Your bubble bursts.
Exactly the same thing happens with bubble gum, except that instead of surface tension as the inward, contracting force, it’s the elasticity of the rubber in the gum. (Yes, rubber.) Elasticity, like surface tension, “means let’s always try to assume the smallest possible shape.”