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

nitroge. 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 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 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 like 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 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 it's like self-propelled forever and

repelled from the walls which is 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

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 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 is like you put an embryo with the cryopreservance on

the liquid bath then you can you can make it into a canal like this and 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

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