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.
Inside my plastic goggles, sweat had evaporated from the warm skin around my eyes. The harder I worked, the more evaporated. The air trapped between me and my goggles was now a mini-sauna, full of hot, humid air. But the ocean around me was nice and cool, and so my goggles were being cooled from the outside. When water molecules in the air bumped into the nice cool plastic, they gave up their heat and condensed, becoming liquid again. But that wasn’t the problem. The problem was that as all those water molecules found each other on the inside of my goggles, they stuck together, far more attracted to each other than they were to the plastic. Surface tension was pulling them inward, forcing them to collect in tiny droplets so that there was as little surface as possible. Each droplet was tiny—perhaps 10–50 microns across. So gravity was insignificant compared with the surface forces sticking them to the plastic and there was no point in waiting around in the hope of them falling off.
Each little droplet acted like a lens, bending and reflecting the light that hit it. When I lifted my head to look for the pier, the light that had been traveling straight to my eyes was messed up by the droplets. Like a tiny house of mirrors, they had scrambled the image so that I was just looking at vague gray fuzz. I stopped briefly to rinse out my goggles, and for a while I had a crystal clear view of the pier again. But the fog came back. Rinse. Fog. Rinse. Eventually I just stuck next to my swimming partner because she had a bright red swimming cap, and the red made it through the silly little water droplets.
When we reached the pier, we paused to check that everyone was OK. With a bit of time to think, I finally remembered something I’d been taught just a week or so before by a scuba diver. Spit in your goggles, and rub it over the inside of the plastic. At the time, I’d made a face, but now I didn’t want to go all the way back across the canyon blind, so I spat. And the swim back was a completely different experience. That was partly because my swim partner had decided that she was bored and wanted it all done with, and I had to struggle to keep up. But it was mostly because I could see—swimmers, kelp, the beach we were aiming for, the occasional curious fish. Human saliva acts a bit like detergent: It reduces surface tension. My goggles were still a mini-sauna and the water was still condensing, but surface tension wasn’t strong enough to bunch it up in droplets. So it was just spread out in a thin film covering the entire surface. Since there were no watery lumps and bumps and boundaries, light could travel through in a straight line, and I could see clearly. Back at the beach, I stumbled out of the water euphoric, partly with relief for having finished the swim, and partly with a new appreciation of what the underwater world had to offer.
This is one way to stop things fogging up: to spread a thin layer of surfactant on the surface. Lots of things will do that job—saliva, shampoo, shaving cream, or expensive commercial anti-fog spray. If the surfactant is ready and waiting, any water that condenses will immediately be coated in it. By providing that coating, you are weakening the surface tension, and swaying the battle of forces in each fog droplet so that the water covers the plastic evenly. The water can stick to the whole surface of the goggles, as long as there are no stronger forces to pull it away. Surface tension is the only other force that stands a chance of competing, so when you weaken that, the problem vanishes.??
So one solution is to reduce the surface tension. But there is another solution: increasing the attractiveness of the goggles. A droplet on its own will suck itself up into a ball. If you put it on plastic or glass, it will sit up high and barely touch since the water molecules will shuffle about until as few as possible touch the plastic. But if you put the droplet on a solid surface that attracts water molecules nearly as strongly as other water molecules do, the water will snuggle up to that surface. Instead of a perky near-spherical droplet, you get a flattened drip that feels the pull of the surface as much as it feels the pull of its neighbors. These days, I buy goggles that have a coating on the inside that attracts water—it’s called hydrophilic. Water still condenses, but it spreads out along that surface, attracted to the coating. Condensation in goggles is here to stay, but fogging up is a thing of the past.??