The mighty thorny dragon. Credit: wikimedia

There once was a dragon, who lived in the desert and loved to eat ants. He liked it so much that he gave up the ability to drink for that: he made his mouth perfect for eating, but useless when it came to sipping.

Instead, he learned something way cooler: how to conjure water from sand itself, summoning a force even stronger than gravity.

It’s actually a true story: the thorny dragon is a majestic, 20cm long Australian lizard. A recent study revealed that, to drink, it uses a complicated network of narrow channels that start from its feet, suck water from the ground. Then, snaking between its scales, they deliver the water directly into the lizard’s throat.

We bottled that force into wondrous technologies such as… er… paper towels.

Seriously, though, paper towels do suck water against gravity. Since nothing is pushing the water up, that’s more or less sorcery. However, scientists prefer to call it capillarity, and it’s fascinating.

You probably noticed that water sticks to stuff. It’s scientifically called—I kid you not—wetting. But water also sticks to itself, and these two things combined make capillarity work.

In a way it’s like keeping up a hammock. The net is made to stay together, it’s suspended from the poles, and it sags in between under its own weight. In a tube with water it’s much the same: the tube walls act as many tiny poles around the water surface hammock.

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Water in a capillary tube is like a hammock, but the poles around it grow a little on their own. credit: Stefanie Laubscher/pixabay

Water has an important difference from the hammock net: it self-sticks to the tube walls. That means it can climb (a little) up the wall. That’s why capillarity works so well in narrow channels, with walls close to each other. Basically, the distance between the “hammock poles” is small, so there’s less space for the net to sag.

A paper towel and its maze of narrow tubes under the microscope. credit: Mattheyses Lab/Emory University

Though it seems a little more mundane, paper towels work the same way. Inside their mesh hides an intricate network of tiny tubes through which water can climb. A little dragon magic in our kitchens.

If you want more
  • Water sticks to stuff because of van der Waals forces. They’re very cool, but they deserve their own thing.
  • Now that we know how to get stuff wet, can we do the opposite? Well, yes… stay tuned!!

Cover photo: CC0 Tom Mathews/pixabay

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