It’s nice to know, in this ever-changing world, that there are some things we can depend
on.
The laws of thermodynamics, for instance.
Solids, liquids, and gases always change phases at predictable temperatures and pressures…
right?
Well, brace yourselves, because there are some phenomena that test even these seemingly
basic rules of nature!
Consider for a moment: the Leidenfrost effect.
The Leidenfrost effect is what you see when a liquid comes in contact with an object that
is a lot hotter than the liquid’s boiling point.
When this happens, the liquid doesn’t boil away, as you’d expect. Instead, it hovers
over the superheated surface, suspended on a tiny cloud of its own gases.
This rather strange effect is named after Johann Gottlob Leidenfrost, a German physician
who published a scientific paper on the phenomenon back in 1756.
You can witness it for yourself, if you sprinkle water onto a really hot frying pan.
The droplets will zip around on the pan’s surface for a little while, before they start
to boil and evaporate.
If you see the droplets skittering around like that, it means the pan has reached what’s
called the Leidenfrost point, which for water is about 220 degrees Celsius.
Past the Leidenfrost point, the bottom surface of the water droplet turns to vapor so quickly
that it creates a little insulating pocket under the drop.
And pressure from the vapor keeps the droplet aloft, like a tiny little hovercraft.
As it scoots around on the toasty surface, molecules of water keep turning into vapor
from the underside of the droplet, in a process known as film boiling.
But the part of the Leidenfrost effect that’s the most interesting to scientists is how
the droplets move.
With so little friction between the liquid and the surface, even slight disturbances
in the vapor pockets -- caused by escaping molecules of gas -- are enough to send the
droplets ricocheting all over the place.
And the bigger the droplet, the less stable its vapor cushion is, so the more it moves
around.
And these droplets basically propel themselves, travelling up to a meter under lab conditions
-- sometimes even further.
Plus, it turns out that if a droplet encounters an incline of finely milled grooves, like
a miniature set of stairs, the grooves will create just the right disruptions in the vapor
to jostle the droplet UP THE STEPS.
It didn’t take long for scientists to realize that if they could only control the way the
droplets moved, they’d have a great new technology on their hands.
Controlling the dynamics of tiny amounts of liquid has applications for everything from
pharmaceuticals to physics, in technologies like micro-cooling electronics and ink-jet
printing.
So in 2012, scientists in the UK first managed to create a maze just for Leidenfrost droplets.
Guided entirely by the orientation of the grooved, heated surfaces in the maze, the
droplets darted through like tiny, self-motivated pinballs.
The only problem was, scientists could only control the droplets when they were above
the Leidenfrost temperature. And 220 degrees is …. hot! So it just wasn’t practical
enough to be all that useful.
But then, in 2014, another group of scientists -- this time in France -- decided to push
their Leidenfrost droplets even further. They wanted to try to get them to appear at less
scalding temperatures.
They found that by coating the grooves of a surface with hydrophobic, or water-repelling,
chemicals -- like the stuff you spray on boots to keep them dry -- they could get the droplets
to appear way below the Leidenfrost point.
At temperatures below even a hundred degrees, the droplets were behaving just like their
hotter counterparts, scaling the grooves like pros.
So if you happen to see water droplets zooming around a hot pan, you can take comfort in
the knowledge that there’s nothing wrong with the laws of thermodynamics.
The droplets just happen to have a very comfortable cushion.
Thanks for watching this episode of SciShow. If you want to keep getting smarter with us,
just go to youtube.com/scishow and subscribe.