Why does soap form bubbles?

by Stephanie Chasteen on May 2, 2009

I got a good question on my Adopt a Physicist forum last week, from an 8th-grader named, for privacy purposes, “S.F.”  I asked them to look around for interesting things around them and ask me about the physics of them.  He/she wrote:

Actually today I did notice some strange things.  I was washing my hands, and I noticed that once the water hit the soap, it turned into bubbles . . . why is that?  Thanks again for responding.

First off, why does the soap bubble form at all?  This is a bit of chemistry.  Soap molecules have two ends — one end likes to stick to water, and the other end is repelled by water.  The bubbles you see when you wash your hands are caused by this property of the soap molecules.  The soap molecules “surround” the water molecules, with the “water sticky” bits pointed towards the water, and the “water repellent” bits pointed away from the water.  This is what the surface of a soap bubble is — a thin layer of water sandwiched between the soap molecules.

So, the soap has a tendency to separate the water from itself, out into these thin sheets.

Why are the soap bubbles round?  Ever notice how if you blow a bubble from a wand that is some weird shape, it still turns into a spherical bubble?  This is a nice bit of geometry.  It turns out that if you want to enclose some volume (say, of air), then the shape that does that with the least surface area is a sphere.  In other words, if there’s the same amount of air inside a football and a soccer ball, the soccer ball takes less material to make than the football does.

So the bubbles form spheres because this uses the least amount of soap (and thus energy) to form the bubble.

Wikipedia has a really nice entry on soap bubbles with some links to some good pictures.

Golden Gate Bridge in a soap bubble:  Mila Zinkova

Golden Gate Bridge in a soap bubble: Mila Zinkova

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