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.
My first reaction was just to will the milk to vanish, so that the stern cook would never know it had existed. Milk is slippery and sticky, can’t be picked up or swept away, and this particular batch of it was advancing along the kitchen floor at an alarming rate. But there is a tool for gathering a liquid together, for putting it all back in one place. It’s called a towel.
As soon as the towel touched the milk, the liquid had a new set of forces bossing it about. Towels are made of cotton, and cotton attracts water. Down there on a tiny scale, water molecules were attaching themselves to cotton fibers, and slowly creeping over the surfaces of each fiber. And water molecules are so strongly attracted to each other that the first one to touch the towel can’t crawl upward by itself. It can only move up if it brings the next water molecule with it. And that one has to bring the following one. So water creeps up the cotton fibers, bringing everything else in the milk with it. The forces sticking the water to the towel fibers are so strong that the measly downward pull of gravity becomes totally irrelevant. What went down happily goes back up.
But this is only half the story. The real genius of a towel is its fluffiness. If a towel could only coat each of its fibers in a thin layer of water, it wouldn’t be able to collect together much liquid at all. But the fluff gives the towel lots of air pockets and narrow channels. Once water finds its way into a narrow channel, it’s being pulled upward on all sides, and the water in the middle just gets dragged along as well. The narrower the channel is, the more surface there is for each drip of water in the middle. Fluffy towels have loads of surface, and very narrow gaps in between, so they can suck up a lot of water.
As I watched the puddle of milk disappear into the towel, tiny water molecules were crowded together, jostling inside the fluff. The ones at the bottom were just going along with the crowd, sticking to the other water molecules next to them. The ones touching the cotton were clinging on to both the cotton and the water molecules on the other side, holding their position. The ones touching dry towel were latching on to the new dry cotton and, once they were attached, pulling others up behind them, filling the gaps in the structure. The ones at the surface were tugging on the water molecules directly below them, trying to surround themselves with as many other water molecules as possible, and pulling water upward in the process. This is capillary action. Gravity was pulling down on all of that milk, wherever it was in the fluff. But gravity couldn’t compete with the forces holding the entire thing up, the ones at the top, where milk just touched dry cotton inside millions of tiny air pockets. As I turned the towel over and moved it around, different regions of the towel filled up, storing water in the pockets.
Water will keep creeping upward through the gaps, bringing other water with it, until the sum of those tiny forces from a multitude of pockets is finally balanced by the pull of the planet. This is why when you dip the edge of a towel in water, the liquid will spread quickly upward for a couple of inches and then stop. At that point, the weight of the water is exactly balanced by the upward pull of the surface tension. The narrower the channels in the fluff, the more surface there is to contribute surface tension, and so the higher the waterline will be. Scale really matters here—if you made fluff that had the same shape but was a hundred times bigger, it wouldn’t be absorbent at all. But when you shrink the shape, you shift the hierarchy of forces and up the water goes.
The best bit of the whole thing is that if you leave the towel out to dry, water will evaporate from those pockets and disappear into thin air. As a means of getting rid of a problem, that’s hard to beat; the towel collects and holds on to the liquid until it floats off of its own accord.??
The spill vanquished, I finished the apple pie and left the kitchen in a suitably immaculate state. But I had one final problem stored up that no amount of surface science could have helped me with. The whipped cream that I served with the apple pie was thoroughly unpleasant, as the faces of the pie consumers made pretty clear. It wasn’t the best way to learn the Polish word for “sour,” the one that preceded the word “cream” on the pot. Still, you live and learn, and I won’t make that mistake again.
The reason why towels are made of cotton is that cotton is mostly cellulose, long chains of sugars that water molecules stick to very easily. Cotton wool, kitchen towel, cheap paper: All of these are absorbent because they have a fluffy structure on a tiny scale, made out of water-loving cellulose. The question is: What are the limits of this size-dependent physics? If you make the channels as small as physically possible, what can you do with them? It’s not just towels that suck water up tiny channels made of cellulose. Nature got there long before us. The mightiest example of what the physics of the small is capable of is also the largest living organism on our planet: the giant redwood.