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
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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