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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{ 19 comments… read them below or add one }

avatar Bill Ward May 5, 2009 at 12:51 am

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 :-)

avatar sciencegeekgirl May 5, 2009 at 5:33 am

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. :-P

avatar Bill Ward May 5, 2009 at 10:49 pm

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.

avatar sciencegeekgirl May 5, 2009 at 10:52 pm

Sorry, I’m still confused… “a thin layer of water sandwiched between the soap molecules” sounds like the opposite of what you just said.

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).

avatar Mary July 3, 2009 at 4:55 pm

Thoughtful post and well written. Please write more on this if you have time.

avatar soccergurl12 March 4, 2011 at 3:19 am

Whats….like….the summary for it. Cuz Im a lil confused. Thanks! :D

avatar sciencegeekgirl March 6, 2011 at 11:19 pm

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!

avatar patrick yater^^ June 9, 2011 at 8:31 am

how does detergent form bubbles when a water flow on it …….?

avatar Albert December 8, 2011 at 11:13 pm

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.

avatar Albert December 9, 2011 at 7:44 pm

It sounds like the Marangoni Effect is what stabilizes soap bubbles.

avatar Lauren February 25, 2012 at 11:56 pm

This is a very good website in fact the first one i clicked on when i typed in my precise question to google.

Sciencegeekgirl could you please tell me clearer what happens when you lift the wire out of the solution and what happens when you blow through the wire to make the bubble.

Also do you have any interesting ways to make bubbles or any suggestions for a variable to investigate for a yr 8 assesment?

Thanks for your post :)

avatar Lauren February 26, 2012 at 12:13 am

Could you also go into more detail about why a bubble forms in soapy water?

avatar Lauren February 26, 2012 at 2:16 am

Don’t worry about the detail in why a bubble forms in soapy water i understand but still any suggestions for variables and what happens with the wire in the solution?

avatar Stephanie Chasteen February 27, 2012 at 4:46 pm

Hi Lauren,

I’m not quite clear on your question but will try to address. When you ask what happens to the wire as you lift out of solution, do you mean why it forms a flat disk on the wire ring? That would be because of the energy minimization described above; it uses the least energy for the soap film to form a flat disk.

If you mean, why does it stick to the wire, that would be because of adhesion of the water to the metal. Wikipedia actually has a nice entry on adhesion.

If you mean, why does the soap film bulge out when you blow on it, that’s because the wind is putting energy into the soap film. Again, it tries to minimize its energy, so the surface that is formed is the surface that uses the least energy. You can imagine that a film that is hugely distorted, like a big amoeba, would have huge stresses and strains on it, pulling it back into a less complicated shape. That’s what I mean by minimizing energy. The shape that has the least stresses and strains. Once the bubble pinches off, this shape is a sphere.

For variables, there are a lot of things that are added to bubble solution to make it last longer (like glycerin). You can try with/without glycerin. You can try differnet types of detergent. Does a detergent meant for washing clothes work as well as a detergent meant for washing hands? I bet not; those for washing hands are aimed at really getting off the grease, so would be more likely to surround those water molecules. This is getting into chemistry more than my background allows, though. I know there are some good sites suggesting such activities: Take a look at http://howtosmile.org/, which has suggestions for after-school science activities. Or googling “soap bubble school activity” might get you somewhere, since I haven’t thought about this a lot myself.

avatar Sarah November 30, 2012 at 5:42 am

This really helped me with my Science project, but I have to turn in citations… what would be the proper citation in MLA form? because I can’t find enough info. to make my citation…

Thanks:)

avatar Stephanie Chasteen December 1, 2012 at 4:19 am

Use the MLA citation format for websites and refer to my website as you would any other website. Though, I must say, this website isn’t the best thing to cite for a science project. While I think that what I write is good stuff, it’s still just the output of me, one person. It’s better to use this site as a starting point, and then refer to other sites for reference-worthy information. Things ending in .edu or .org are typically good resources.

avatar karl dandyl isler June 23, 2013 at 12:45 am

Thank you for this .. I use it in my experiment .. Thank you .. Im a Filipino..:)

avatar karl dandyl isler June 23, 2013 at 12:46 am

Thank you for this.. I used it in my experiment about soap.. Thank you .. :)

avatar RP October 2, 2013 at 12:31 am

Hi so i am working on a bio lab report and i have a question that i just cant seem to answer…maybe you could help me. My question is:
lets just say you have 1o ml of water and 3ml of soap combined, you make a bubble and measure it then you add another 3ml of soap to the already 1o ml of water and 3ml of soap and you blow another bubble and it is bigger. How come when you add more soap to water the bubbles become bigger?
thank you this will help alot,
RP

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