Want to know how to make a drop of water "walk" in a certain direction? Want to know how to make it walk a little bit, and then pause, as if it were wondering whether it left its front door unlocked? We'll tell you how.
This is another manifestation of the Leidenfrost effect, which is best known for letting people walk across hot coals. This is what happens when a liquid makes contact with a surface much hotter than the liquid itself is. It can happen when room-temperature water hits a very hot surface — or when liquid nitrogen hits a room-temperature surface. What matters is the difference between two temperatures, not the temperatures themselves.
When the drop hits the surface, the very bottom of the drop vaporizes instantly. This vapor makes a little cushion, on which the rest of the drop sits. Over time, more and more of the drop evaporates until it's all gone, but while the cushion lasts, the droplet moves smoothly over the heated surface.
The Leidenfrost effect doesn't just lead to random motion. If the surface is textured, it can make the drops "walk" in a certain direction, or even walk uphill. The video below gives us a clue about why.
This droplet is walking steadily, to the right, over a serrated surface. We can see that the serrations have a sharp slope on the right and a gentle one on the left. As steam comes off the droplet, it will have a hard time hopping the barrier on the right, but can climb the slope going left. The steam goes left, the droplet gets pushed to the right, and the water walks.
To make the drop come to a standstill, all we have to do is put it on the slope-less grooves, in the video at the top. The steam isn't channeled in one direction, and so the drop loses momentum and stops.
[Source: Annual Reviews Extra]