(electronic music) - [Darcy] Have you ever seen a drop of water climb stairs or navigate a maze?

Yeah, it's possible.

Thanks to something called the Leidenfrost effect.

It's one of the weird tricks water has up its sleeve.

And it's this simple trick grandmothers use to tell if the griddle is ready to cook pancakes.

(jaunty music) (logo whooshing) If you've wondered how to tell if your pan is ready, sprinkle some water onto the skillet.

We like to use iron.

If it hangs out and boils away in place, it's not hot enough.

If the droplets dance around, the skillet is so hot that the batter will sear nicely, allowing you to flip your pancake rather than get a goopy, sticky mess of a would-be meal.

So the Leidenfrost effect happens when water hits a very hot surface, but let's take a step back.

You all know that water boils at 100 degrees Celsius at sea level.

Temperature is really just a measure of how fast the atoms or molecules in something are moving.

When water is a liquid, the molecules have enough energy to wander around somewhat freely, but not enough that they wander off altogether.

But above the boiling point, the moving molecules have enough energy to break away from their compatriots and fly freely into the expansive gaseous state.

When you drop liquid water onto a hot surface it quickly boils into steam, but heat that plate up even more and the droplets skitter and slide across the plate without appearing to boil at all.

(air whooshing) What is going on?

Congratulations, you've achieved Leidenfrost.

(air whooshing) No, you are not a druid.

Well, probably not.

When the droplet hits the plate, the underside touching the plate instantly boils.

That creates a tiny layer of steam under the droplet.

It actually insulates the rest of the droplet from the heat of the plate, keeping it from boiling.

Water vapor conducts heat 30 times less well than liquid water because steam has fewer molecules per volume.

The water vapor under the droplet works a little like a hovercraft.

As the droplet slides around the plate without a hovercraft skirt, the vapor will leak away, but any new liquid contact of the droplet will flash vaporize, so the cushion can persist overall.

Given enough time, so long as the droplet stays on the plate, it will eventually boil away, but it takes far longer than a droplet that doesn't achieve Leidenfrost.

The exact temperature of the onset of Leidenfrost can be complicated.

It can vary depending on what the surface is like.

Is it smooth or rough or what impurities are in the water?

Different scientists have reported Leidenfrost happening everywhere from about 150 degrees Celsius to over 500 degrees Celsius.

While scooting around like Luke's land speeder is cool, it actually gets better.

Back in 2006, some researchers realized you can control which way the droplets go.

If you mill a metal surface into a series of ratchet-like ridges, then Leidenfrost droplets will crawl across the plate, in the direction of those ridges.

The drops can even climb a gentle slope, scaling the ridges like they're tiny stairs.

Some other scientists realized you could arrange a series of these milled blocks, so water droplets can find their way through a maze.

Science for the win!

(air whooshing) But is the Leidenfrost effect more than a party trick?

Yes, yes, it is.

One way nuclear power plants and other industrial sites get rid of excess heat is by boiling water.

If they let their heat exchangers get too hot, the Leidenfrost effect will make them less efficient, potentially causing enough heat to build up that it damages the equipment.

Better controlling the Leidenfrost effect in nuclear reactors might also prevent cataclysmic explosions, like what led to the disasters at Chernobyl and Fukushima.

The Leidenfrost effect could even be used to generate electricity.

Water isn't the only thing that experiences the effect.

Liquid nitrogen does it.

A block of dry ice will, too, thanks to sublimation.

Remember how you can get drops to go in one direction?

If you make those ridges in a ring, you can get them to go in a circle.

Place a column of dry ice on that surface, and it slowly spins.

Now, add some magnets and copper wire, and you've got yourself an electric generator.

That's so cool.

I might even call it Leidenfrosty!

(crowd booing) Let us know any other weird states of matter you'd like to learn about.

We might just make a video out of them.

Be sure to hit subscribe and thumbs up on the way out, and hey, thanks for watching.