The Inverse Leidenfrost Effect

Now you've probably heard of the Leidenfrost effect. That's when a volatile droplet like water levitates over a hot surface because it's floating on a little cushion of its own vapor. Here I'm gonna try to create the inverse Leidenfrost effect where we levitate a droplet on a bath of liquid nitrogen. It's inverse because the droplet is not actually creating the vapor; it's the bath beneath. It's the liquid nitrogen that's creating the vapor.

There have been a couple of recent papers about this phenomenon, so I called up one of the scientists to ask how can I do this and how does it work?

"Hello?"

"Hi!"

"Hi, how are you?"

"Good, and you?"

"I am doing very well. So, I was gonna ask you, do you think I would be able to replicate this without too much difficulty?"

"If you have liquid nitrogen."

"Yes. It's really simple. It's really basic. What you need is a polystyrene box like 20 by 20 centimeters. A bit thick so that it insulates, and then I had two beakers: one like that size and another one that is smaller in which the experiments... ah yeah. I will put him away.

Um, so two beakers..."

"Okay, so when you have a look at this setup, I have a piece of styrofoam that has a cylindrical cutout in it, into which I have poured some liquid nitrogen, and the purpose of that is to get this outer space as cold as possible and insulated from the rest of the air.

Then I have this large beaker that is full of liquid nitrogen. As you can see, it is boiling, and then I have the innermost beaker which is not boiling. So I have still liquid nitrogen in the middle that is not boiling, and that is what I want because that is where we can actually conduct the experiment and try to get the inverse Leidenfrost effect to work.

Okay, I have 100 microliters of silicone oil, and I'm gonna attempt to drop it onto this bath of liquid nitrogen. Whoa! Oh my God, look at that!"

"What?!"

"There were a whole bunch of droplets, and they were all levitating on the surface. Now that they are in there, now the liquid nitrogen is boiling, so this is not good..."

"Hey, future Derek here. Sorry my hair looks stupid, I'm gonna go get it cut, but that last experiment didn't go very well because the liquid nitrogen in the middle beaker was boiling, and there was nothing I could do about it.

So I tested a few droplets on there, but you couldn't really see the effect nicely because of that boiling bath. So I'm gonna try again today: clean beakers, new liquid nitrogen. Let's give it a shot!

There it is! Got a droplet of silicone oil, and it is bouncing around back and forth. That is pretty cool! What's amazing about this effect is that it can continue almost indefinitely. It's been observed to last for tens of minutes. Unlike the Leidenfrost effect where the droplet is used up making the vapor cushion that supports it, here the supporting vapor comes from the bath, so it can continue indefinitely even after the drop has frozen.

The heat required to evaporate the bath comes not only from the droplet but also from the warm atmosphere around the experiment.

Although on my second attempt I was able to prevent the small beaker of liquid nitrogen from boiling, my setup was a bit unstable. So the boiling in the outer beaker shook the inner beaker, interfering with the droplets' motion. But in Anaïs's professional setup, you can see how the drop always moves in straight lines.

But why should the droplet be moving at all? And what keeps it moving? A lot of people have been putting drops on a bath and have observed the movements, but nobody has tried to explain it like that yet.

"So how did you explain it?"

"There is something that happens at the interface, so you have a floating drop with a thin vapor layer above the bath, and then what seems to happen is that this vapor layer is not uniformly thick. But then at some point, there is a tiny instability like a capillary wave that grows under it."

"But why is there a wave?"

"Just for example because you'd never deposit the drop perfectly, nicely, and smoothly. So you create tiny waves that come back below the drop. I think you can see that here as I add a droplet to the bath. Notice the waves generated in the middle of the beaker?

These little waves lead to an asymmetry underneath the droplet where one side is higher than the other. More nitrogen gas escapes out this side. Now you might think this would drive the droplet in the opposite direction, but it doesn't. The gas actually drags the droplet along with it, kind of like how wind over your windshield pushes raindrops along.

What is great here is that this instability instantly reappears each time the drop comes close to a wall. So when you have a wall in the bath, you have a small liquid nitrogen meniscus. So the drop starts to climb it, and then the propelling force reverses and then pushes it back.

And this creates a nice star pattern, so it's like self-propelled forever and repelled from the walls, which is very cool!"

"So do you think this research has applications, and what sort of applications might those be?"

"If you imagine an embryo of an animal, like a mouse or something. A very, very young embryo is like 10 cells, so it's incredibly small. So what they do is like they put it into liquid that allows cryopreservation and then freeze it.

And what is great about this is what they do is they take the drop and put it into liquid nitrogen and then it freezes. What... and if the freezing is fast enough, you don't grow ice crystals, so you preserve the embryo.

And what is great about this is like if you imagine instead of just putting an oil drop like what I did, you put an embryo with the cryopreservance on the liquid bath, then you can make it into a canal like this.

Then you can generate them like in a random way and move them around because they are self-propelled, so they would go in the direction that you want them to move. So you could imagine making something where you could at the same time freeze your cells or chemicals and then move them along without generating any contaminations because you're basically not touching them.

That's an interesting potential application of this."

"Hey, this episode of Veritasium was supported by viewers like you on Patreon and by Audible. There's never been a better time to start listening on Audible. You know, today driving down to get my haircut, I was listening to this book: "Bad Blood: Secrets and Lies in a Silicon Valley Startup" by John Carreyrou.

It's the inside story of the company Theranos and its founder, Elizabeth Holmes. She was this Stanford dropout who became the head of a medical technology company valued at around 10 billion dollars.

But was her technology legit or a massive fraud? If that sounds interesting to you, you can download this book when you start a 30-day trial. Just go to audible.com slash veritasium or text veritasium to five hundred five hundred, or you can pick any other book you like from Audible's unmatched selection of audio content.

Now, I like listening in the car because I find it makes LA traffic much more bearable, but you can listen at home or at the gym or just on-the-go. Listen on any device, anytime, anywhere.

Every month as an Audible member, you get one credit good for any audiobook you choose, plus two Audible originals from a changing selection that you can't find anywhere else. If you don't use a credit one month, it rolls over to the next month. If you don't like a book that you got, you can exchange it no questions asked, and your library is yours to keep to listen to whenever you like even if you cancel your membership.

Now Audible members also get access to audio fitness and health workouts to begin your year on the right foot, so there's no better place to listen than Audible.

Get started with a 30-day trial when you go to audible.com slash veritasium or text veritasium to five hundred five hundred.

Audible: the most inspiring minds, the most compelling stories, the best place to listen. Now I want to thank Audible for supporting me, and I want to thank you for watching."