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
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{ 10 comments… read them below or add one }
I’m still not entirely clear on this … the bubble is filled with air, not water. So the surface of the bubble is a sandwich of strata of soap, water, and more soap?
Also, soccer balls *are* footballs
The bubble is filled with air, yes, but then the surface of the bubble itself is a sandwich of water-soap-water. So, from the inside then, it’s lots of air, then water, then soap, then water (then the outside air). I edited the post to make it a bit clearer.
And hey, buddy, this is America. Soccer balls not equivalent to footballs.
Sorry, I’m still confused… “a thin layer of water sandwiched between the soap molecules” sounds like the opposite of what you just said.
Soccer balls may not be equivalent to footballs, but they are a subset of footballs.
That’s because it is. That’s what I get for trying to quickly answer physics questions while distracted! My comment should have read:
“The surface of the bubble is a sandwich of soap-water-soap. So, from the inside then, it’s lots of air, then soap, then water, then soap (then the outside air).
Thoughtful post and well written. Please write more on this if you have time.
Whats….like….the summary for it. Cuz Im a lil confused. Thanks!
Hi Soccergurl,
Can you explain which part you’re confused about, and I’ll try to make it clearer?
The images at http://en.wikipedia.org/wiki/Lipid_bilayer might be helpful (or not) to envision the surface of the bubble — which is a layer of water sandwiched between the soap molecules, for the reasons described in the post.
Happy to give more detail!
how does detergent form bubbles when a water flow on it …….?
I’ve been searching all over the web for a good explanation of how (soap) bubbles form and still haven’t found one although yours comes close. I think a more basic problem is how a soap-water film can form on a closed edge (like the ring of wire) without breaking. What actually happens when the wire is drawn out of the solution? If soap films have lower surface tension/energy why do they seem more resistant to rupturing? Do the larger molecules in soap help stabilize the film (water molecules are of course very short)? I’m a researcher and looking for ways to reduce foaming in polymer solutions.
It sounds like the Marangoni Effect is what stabilizes soap bubbles.