Surface Tension and Soap Bubbles

By Tracy V. Wilson, HowStuffWorks.com

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In spite of its childlike lightheartedness, blowing soap bubbles is a pastime that has a key science principle at its core. Surface tension defines almost everything about a bubble, from its mere existence to its shape.

The molecules in a liquid -- for example, water -- attract each other. If they didn't, they would fly apart, and the result would be more like a gas than a liquid. In the middle of a glass of water, the molecules all attract one another equally. But the molecules on the surface don't have upstairs neighbors to hold onto. They stick the ones next door instead, and the result is surface tension. In a zero-gravity vacuum, this attraction among molecules would cause the water to form a self-contained sphere.

If you've ever dipped a bubble wand into a plain glass of water and tried to blow a bubble, you probably haven't had much luck. Water's surface tension is too high -- it sticks together too well -- to make a bubble. Add a surfactant, like liquid dish detergent, and the problem disappears: You can blow as many bubbles as you want. A few things happen at once when you add the detergent. The surfactant, whose molecules do their own attraction-repulsion dance, spread out on the surface of the water. What's more, one end of each surfactant molecule is attracted to water, but the other repels it. Because of this, the surfactant lowers the surface tension of the water.

If you dip a bubble wand into this mixture, things get interesting:

When you remove the wand, extra water drips from the bottom, leaving a soapy film across the wand's opening.

If you blow into the opening, the film starts to bulge -- there's enough surface tension to hold it together, but the tension is low enough to allow it to be flexible.

If you're careful, the bubble will separate from the wand and close itself off. The air pressure inside the bubble is greater than the pressure outside, so it forms a sphere and maintains its shape.

Surface tension holds the bubble together until wind, unequal pressure or contact with an object breaks the tension. When this happens, the molecules let go of each other one after another, and the bubble falls apart. Imagine a ring of children holding hands, each one letting go of the next, one at a time. That's what the collapsing bubble looks like