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 the simple trick grandmothers use to tell if the griddle is ready to cook pancakes.
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, its 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.
What is going onnnn?
Congratulations, you’ve achieved Leidenfrost.
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 plate, keeping it from boiling.
Water vapor conducts heat 30 times less well than liquid water because steam has fewer
molecules per volume.
That water vapor under the droplet works a little like a hovercraft - a slick mode of
travel where the ship speeds around on a cushion of forced air.
As the droplet slides around the plate without a hovercraft’s 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 landspeeder 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, the 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!
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.
Let us know any other weird states of matter you’d like to learn about.
We might just make a video out of them!
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And hey, thanks for watching…