Soap Bubbles: Part 1

love soap bubbles. They're beautiful, delicate, and though they live only briefly, it's a glorious moment. They float on the air, brilliant colors wobbling over their surface, and when they pop into nothingness, it's just an opportunity to make more bubbles! This issue and next I'll talk about the chemistry, physics, and computer graphics of soap bubbles. Here I'll focus on the physics of soap films, which are, after all, what bubbles are made of. We'll see what happens when soap dissolves in water and discuss some surprising properties of soap films. With this grounding in how films work, in the next issue I'll discuss how they give rise to the brilliant colors and beautiful 3D clusters that we associate with soap bubbles. An important idea in soap films of all sorts is surface tension. For our purposes, we can think of surface tension as a force that lies in a very thin plane right at the surface of a liquid-air interface. It's a contractive force, trying to pull the surface into the smallest shape possible. Figure 1 shows a schematic view of how this works. Molecules in the liquid pull on each other uniformly, so that the forces upon them are basically neutralized. But for molecules near the surface, all the pull is in the plane of the surface or back into the liquid. We can make bubbles with soapy water much more easily than with regular water. Why? Soap decreases the surface tension. Plain water has a surface tension of about 72.25 dynes per square centimeter at 20 degrees Celsius. When you add soap, this surface tension drops by about 65 percent. It's this diminished surface tension that lets soap bubbles form and last. The higher tension in plain water causes it to pull against itself too tightly to form big, stable bubbles. That's why raindrops and dew-drops are little blobs of liquid (usually as small and spherical as the circumstances allow) rather than little bubbles or bits of froth. Soap films have an interesting chemical structure. Soap molecules are the metal salts of long-chain fatty acid molecules. When dissolved in water, the soap molecules break apart and ionize. Let's look at this more closely. A common soap, sodium stearate, has the chemical symbol C17H35COO − Na +. When added to water, this molecule breaks into two parts. The metal component splits off by itself and floats around …