We've seen water seem to travel uphill in gravity hills. That was an illusion, but physics has found a way to make water zip up some tiny stairs like it's Rocky in a training montage, using the much-beloved Leidenfrost Effect.
The Leidenfrost Effect is something that can be demonstrated in the kitchen at home, or with liquid nitrogen in a lab. Either way, it's fun to watch, and relatively straightforward to explain. When a liquid hits a certain temperature it turns into steam. The steam, when it's hotter than the surrounding air, moves upwards very fast. The force of that steam has been used to power turbines and move trains, but the Leidenfrost Effect doesn't require anything so grand. When a liquid such as water, hits a sufficiently hot pan, the edge of the liquid that makes contact vaporizes. It turns to steam so quickly that the water on top of it doesn't even get a chance to touch the surface. The steam moves upwards and lifts the water, keeping it off the hot surface of the pan. As the steam escapes around the water, more parts of the water droplet hit the pan and vaporize in turn, keeping the effect going. The water beads up and skitters around on a cushion of steam, until it finally shrinks down to nothing.
The liquid doesn't have to be water, and the surface doesn't have to be as hot as a pan on a gas fire. All that matters is the temperature disparity. You'll often see people "taking a sip" of liquid nitrogen to demonstrate the Leidenfrost Effect. The person doesn't drink the liquid nitrogen. If they did, it would turn to steam in their stomach, expand, and could quite possibly cause internal damage. They just hold it in their mouths. The heat of their mouths is so high in comparison to the liquid nitrogen that the very edge of the liquid nitrogen turns to steam, lifting the rest of the liquid nitrogen away from their mouths and protecting the living cells from the cold of the liquid nitrogen. They blow out a column of nitrogen steam as the nitrogen is slowly vaporized.
Both of these demonstrations work because they don't require the liquid to go in any particular direction. As long as the liquid stays up and off the hot surface, the experiment is a success. People had trouble, for some time, putting the Leidenfrost Effect to any kind of useful work because the chaotic movement of the liquid and the steam made it difficult to direct.
Physcists at the University of Bath have found ways to direct the water. In particular, they found ways to make the water climb uphill. Any drops of water placed on a smooth, hot surface that's on a slight incline tumble downhill quickly. Gravity pulls them along. But if the surface isn't smooth, if it has tiny serrated teeth on it, like a saw blade, the drops will climb the tiny stairs, going uphill under their own power. In the video above, the drops roll downhill in the foreground, while in the background, they climb the sandpaper-like surface of a hill. Bigger teeth on the saw allow the droplets to climb steeper and steeper hills.
What's more, substructures inside the teeth can guide the water droplet one direction or the other. The shape of the structures matter, of course, but the exact motion of the water varied most with the temperature of the surface itself. Although the exact physics of the guided Leidenfrost droplets isn't worked-out, it seems that the drops are guided by the most efficient heat transfer, which varies with both the shape and temperature of the surface.
It's possible that water droplets that will just propel themselves uphill over a heated surface might find use in energy output systems or in computer cooling systems. For now, it's just cool to watch water zip up a hill like it has somewhere to be.
Image: Andrew Magill