How does soap make bubbles?
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From the brains behind Brains On, this is the Moment of Um. Moment of Um comes to you from APM Studios. I'm Mark Sanchez. Been pulling weeds for hours on the go. I smell like sweaty socks now it's time to take a bath. Got the water nice and hot but I feel like a dope.

I squeeze the bottle hard and put on way too much soap. I got the bubble trouble, bubble trouble blues. You know I do. Ah, this is nothing to sing about. The bubbles keep piling higher and higher. What is going on? My name is Isabella, and I'm from Carmel, Indiana. My question for you is, how does soap make bubbles?

The first important point is that it's not just soap. It's soap and water. And in fact, it's mostly water, really a lot of water with a little bit of soap in it.

I'm Frank Bates, and I'm a Regents Professor at the University of Minnesota. I teach classes in chemical engineering and also in material science and engineering. And that's all about how to make materials and how to process them into items you're all familiar with. I work in particular on plastics.

Soap bubbles are actually very thin film of water. It's not a lot of water. There's air on the inside and air on the outside. And so the big question is, how does that film of water stay that way? If you take a little plastic loop like you'd get with a bubble maker, so you have a bottle of soapy water and a plastic loop that you pull out some of the soapy water,

And you get a film, a very thin film. It might have some colors that you see in it. That film is stable. But if you tried to do that with just pure water, it wouldn't work. You would pull the plastic loop out and it would break the film and it would go into droplets of water. And once you have the film, if you blow on it, you can sort of make it bulge out. And when it bulges out, it pinches off and it makes the soap bubble.

I'm going to ask you to imagine a big crowd of people who come crowding into a soccer stadium or a football stadium. And when they go out and on the field, they'll start going in all directions and disperse and you have no control over them.

Well, what happens if we add into that crowd maybe one out of every 300 people is a person who doesn't like being in the middle of a crowd?

That person would rather go to the edge and they like people, but they like to be at the edge of the people. They want to not be in the middle of the crowd. So if I have a 10,000 people and one in 300 doesn't like to be in the middle and they quickly go to the edge, they're going to create a boundary.

And if they all hold hands, they're going to keep the crowd of people from actually spreading out. They're going to keep them constrained inside that boundary that they form. So if you think about the 300 people as being water molecules, so molecules are the sort of fundamental microscopic form of water. If each person is a water molecule and the one in 300 is

are soap molecules. When I pull a film of water, those soap molecules don't want to be in the middle of the water. They want to go to the boundary where the air is and form a surface layer. And that's like the people in the crowd that don't want to be in the crowd. They go to the edges. And so the soap

spontaneously, quickly goes to the boundary and it holds the water and keeps it from forming a droplet. It keeps it as a thin film. You can think about it constraining the water in that thin film form. So if I put more soap in the water, I'm going to end up getting a thinner and thinner film. If I put less soap, I'll get a thicker and thicker film.

And it's that combination of the soap, which wants to go to the boundary, the surface of the water, along with the water itself that gives you this fluid-like liquid film that forms the droplet. And that's how a soap bubble is made. What a satisfying answer. I guess I'll just slip back into my bath.

These bubbles are great. I'm sitting in my tub. I got my rubber ducky and it's time for a scrub. No more trouble. No bubble trouble. If you like this episode, take a second to subscribe wherever you listen to podcasts.

And if you feel like sniffing out some science, check out the Brains On podcast, where we have a whole episode about dogs and their amazing sense of smell. If you have a moment of um question, we'd love to help you answer it. Drop us a line by going to brainson.org slash contact. See you tomorrow and the next day and every weekday. Until then, um...