Storm in a Teacup: The Physics of Everyday Life(26)

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.??

Weakening surface tension has its uses. But that pull between individual water molecules is really strong. And the smaller the volume of water you’re interested in, the more it matters. So what surface tension is really useful for is plumbing on the tiniest scales. Down there, you don’t need pumps and siphons and huge amounts of energy to shunt water around; you just need to make things small enough for gravity to be irrelevant and let surface tension get on with the hard work. Mopping up is boring, but the world would be very different without it.

I’m a messy cook, reasonably competent, but far more interested in the cooking process itself than the trail of devastation that I tend to leave behind me. This makes me nervous when using other people’s kitchens. Years ago in Poland, I set out to make apple pie for the international group of volunteers I was working with at a school.§§ It didn’t start well. The tall, fierce school cook bellowed “NO!” with some enthusiasm when I asked whether I could use the kitchen, and it took me a few puzzled seconds to remember that we’d been talking in Polish and “no” is their word for “yeah.” My Polish wasn’t very good, and I didn’t follow all of the details that came next, but I took away the very strong message that the kitchen was to be left clean. Very clean. No spilling anything. Definitely immaculate. So later that evening, after she’d gone home and I’d assembled all the ingredients, of course the first thing I did was to knock over a large and newly opened carton of milk.