Why Don't Liquids Splash In a Vacuum? - en - Twincloud's Youtube Subtitle Extractor

This is what it looks like for a drop of liquid to hit a hard surface in our

normal atmosphere. But if you remove the air, something unexpected happens.

When a drop of the same liquid falls from the same height on the same

surface, suddenly the splash completely disappears.

Today, we're going to be exploring how the absence of air makes splashes

disappear. One of the most distinctive features of liquids is that when they

hit a hard surface, they splash. But what if I told you that splashing

doesn't just happen because of the liquid, but it happens because of the

air. In 2005, physicists from the University of Chicago were trying to

measure the energy of liquid droplets flying away from splashes. But they were

curious what would happen at lower pressures. Would the little droplets

that shoot out during the splash get ejected at higher speeds? So, they

dropped some alcohol drops in a vacuum and then went to check the high-speed

footage. When they went to look at the high-speed footage, there was no splash.

One of the authors of the paper, Sydney Nel, said that they fell onto the floor

laughing because they were so surprised by the result. So, today I'm going to be

seeing if this is really true. It doesn't seem like it would be true. It

seems like without air, it would splash even more without anything slowing down

the tiny drops. In preparing to do this, it turned out to be more difficult than

I originally thought. First, with each drop, it needs to fall onto a non-wet

surface so that the liquid that's already there doesn't interfere with the

drop. So, that either means that A, I get one shot with one single drop of

alcohol, or B, I need to figure out a way to get a new dry surface under my

drops of liquid in the vacuum chamber. Well, I know for a fact that I won't be

able to get a good shot of the drop falling with only one try. So, I need to

figure out how to get a new clean, dry surface under every drop. First, I

thought about using my magnets to push a new glass slide under every drop. But

then, I only get like two tries until my glass slides run out. So, that's not

going to work. I need to figure out a way to dry off any drops that have

already fallen. So, how can I clean off a surface without being able to access

the surface? Okay, so what I have here is just a turntable that I attached a

round mirror to the top. So, when I turn it on.

So, let's see if this works. We got some liquid on it.

Bye-bye. And it's gone. Okay. So, I think that's

going to work great. So, we have to be able to turn it on

while our lid is closed. So, what I have here is some copper tape going under the

seal here. These are connected to this switch here. So, all I do

outside the chamber, turn it on, and it turns on my centrifuge here.

Now, how do I feed the drops in the chamber? Well, I originally was trying

to use a pump, but I couldn't find a pump that was slow enough to do a single

drop of liquid at a time. It probably does exist, but I don't have one. So,

the drops are going to have to be manually fed in. Now, I do have a way to

get a tube into my chamber through this port here. And then once I'm at low

pressure in the chamber, the atmosphere is going to push the liquid into the

chamber really fast. So, I'll just start the vacuum for a second so that it can

get some negative pressure in there a little bit so it can suck in my drops of

alcohol. [Music]

So, I have it pinched here. And you can see if I just put the bottom in the

liquid. Now, I slowly open it. I'm slowly unpinching it. And you can

see the alcohol going slowly into the chamber.

I can control it actually really well how much goes in. Okay. So, let's see

what this drop looks like when it falls. For the regular pressure drop, you can

clearly see a splash with little drops flying everywhere.

Now, let's try the one at low pressure. Okay, now let's keep going.

Okay, I think we're at low enough pressure now. 685 mibar. Okay, let's see

if we can do this. Try to get one single drop.

I didn't see a splash. [Music]

There's no splash whatsoever. Even though I read the paper, it's still

so surprising. I would have never guessed that this would happen in a

vacuum chamber. Look how cool that is. So, what's the mechanism for this? Well,

normally when a drop hits the surface, it's a lot more violent than you would

expect. As a drop hits the surface, the liquid actually spreads faster than the

speed of sound in air. The air can't get out of the way fast enough. So, the

spreading edge of the liquid compresses the air ahead of it. Then, the

compressed air gets trapped and squeezed beneath the droplet spreading edge. The

air pressure pushes back against the thin sheet of liquid spreading outward

and sends droplets flying in all directions making a big splash. But if

you take away the air, then you get no shock wave and no compressed air that

flattens the splash out above the surface to throw the drops. So without

air, you just get the drop flattening out like a pancake. The rim of the

droplet spreads smoothly and evenly outward. So splashing is not just a

property of the liquid, it's a liquid gas interaction. This is so cool and

counterintuitive because usually things get thrown much easier in a vacuum. But

in this case, it's the air that's making the splash, not just the momentum of the

drop. Without air, no matter how fast or forcefully a drop falls, it won't

splash. In the author's later experiments, they drop the drops from up

to 3 m high in a vacuum, and they still didn't splash. What's crazy is that

there's almost a transition point to when there's little enough air for no

splashing to occur. Under 30 kilopascals, they saw the splashing

disappear. Above that, there was splashing. So, the higher the pressure,

the more splashing there is when it hits a surface.

And thanks for watching another episode of the Action Lab. I hope you enjoyed

it. If you haven't subscribed to my channel yet, remember to subscribe and

we'll see you next time. [Music]