Storm in a Teacup: The Physics of Everyday Life(25)
Agnes had done something very simple and very clever. She had suspended a small metal disk (something about the size of a button) on the end of a string and let it sit on the surface of the water. Then she had measured how much force it took to pull it away from the surface. The mystery was that the water held on to the disk; you had to pull harder to get it off the water surface than you would have had to pull to pick it up off the table. That pull from the water is what we call surface tension; so in measuring the pull Agnes was measuring surface tension. She could then investigate the surface of the water, even though the thin layer of molecules responsible for the pull was far too small for her to see directly. We’ll see how in a minute; but first, back to the bath.
A bath full of pure water is a jostling swarm of water molecules playing a very crowded game of bumper cars. But one of the things that makes water such a special liquid is that all those molecules are very strongly attracted to all the other water molecules around them. Each one has a larger oxygen atom and two small hydrogen atoms (that’s the two Hs and an O in H2O). The oxygen sits in the middle with the two hydrogens stuck to it on either side, making a shallow V-shape. But although the oxygen is very strongly attracted to and bonded to its own two hydrogen atoms, it’s also flirting with any others that happen to go by. So it’s constantly tugging on the hydrogen from other water molecules. This is what holds water together. It’s called hydrogen bonding, and it’s very strong. In the bath, water molecules are constantly pulling on the other water molecules around them, tugging the whole mass of water together.
The water molecules on the surface are a bit left out. They are being pulled by the water molecules underneath them, but there’s nothing above them to pull back. So they’re being pulled down and sideways but not up: and the effect of this is to make the surface behave like an elastic sheet, pulled tight over all the water molecules below the top layer, and pulling itself inward so that it is as small as possible. This is surface tension.
As you run the tap, air gets carried downward into the bath, making bubbles. But when these bubbles float up to the surface, they can’t last. The round dome of the bubble is stretching the surface and the surface tension isn’t strong enough to haul it back. So the bubbles burst.
One of the things that Agnes did was to set up her button so that it was being pulled upward, but not quite hard enough to pull it off the surface. And then she touched the surface of the water nearby with a drop of something like detergent. After a second or so, the button would pop off the surface. The detergent had spread across the water, and it had reduced the surface tension. All it takes to reduce the surface tension is to provide a thin top layer so that the water molecules don’t have to be the ones right at the surface.
When it’s finally time to add the bubble bath, it’s time to say goodbye to a clean, flat, minimal surface. That dollop of scented gloop gets carried down into the water and immediately does its best to hide at the edges. Each molecule has one end that loves water and one end that hates water. If the end that hates water can find some air, it will stay with it, but the water-loving end isn’t giving in either. So anyplace where water touches air, a thin layer of bubble bath sits right at that surface. It’s just one molecule thick, and each molecule is the same way around so that the water-loving ends are all still submerged in water, and the water-hating ends are all still in air. With this thin coating, a large surface isn’t a problem. The bubble bath doesn’t provide the strong pull that water does, so the elastic sheet effect becomes really weak. It’s time for a surface party, and that’s what the foam is. By reducing the surface tension, bubble bath makes it easier for bubbles to last because their large surface is much more stable.
It’s probably worth noting here that we associate the white foam with things getting cleaner, but in modern detergents the best stuff for sticking to the water surface and making the foam is not the best stuff for pulling dirt and grease off clothes and plates. You can make a very good cleaning detergent which hardly makes foam at all, and in fact the foam often gets in the way. But the purveyors of cleaning products did such a good job of convincing people that beautiful white foam was your guarantee of a thorough cleaning that they’ve now backed themselves into a corner. Foaming agents are now added to make sure there are bubbles, because otherwise consumers complain.
Like viscosity, surface tension is something that we’re aware of up here at our size scales, although it’s usually less important than gravity and inertia. As you get smaller, surface tension pushes its way up the hierarchy of forces. It explains why goggles fog up and how towels work. And the real beauty of the world of the small is that you can contain many tiny processes inside one giant object, and their effects add up. For example, it turns out that surface tension, which only dominates in the tiniest of situations, also makes possible the largest living things on our planet. But to get there, we need to look at another aspect of surface tension. What happens when the surface separating a gas and a liquid bumps up against a solid?
My first open-water swim turned out not to be for the faint-hearted. Fortunately, I didn’t know that beforehand so I couldn’t worry about it. When I was working at the Scripps Institution of Oceanography in San Diego, the big annual event for my swim team was from La Jolla beach to Scripps pier and back, 2.8 miles across a fairly deep marine canyon. I had only ever swum properly in swimming pools, but I’m always up for trying something new and I’d been swimming a lot, so I turned up and hoped I didn’t look like too much of a rookie. The mass entry to the water was a bit of a scrum, but it got better after that. The first part of the swim was across the top of a stunning kelp forest, and it was almost like flying. The sun glinted through the huge stalks of bull kelp just like it does in forests on land, and then the kelp disappeared downward into the murky depths, reminding me that there was quite a lot swimming about down there that I couldn’t see. Once we were past the kelp, the water got choppy, and I had to pay much more attention to where we were going. And that was getting harder. The pier was fuzzy on the horizon, and I couldn’t see anything at all down below. After slightly too long, I realized that the reason everything had disappeared was that my goggles had fogged up. Oh.