- Hey, I'm Dianna, and you're watching "Physics Girl."
There's a really interesting party trick,
where you hit the top of a glass bottle with water in it.
I'm gonna go ahead and put on a glove,
and use a mallet. (giggles)
(taps bottle)
Why can't I keep my eyes open?
(taps bottle)
(groans)
(taps bottle)
(Dianna and Cameraman laugh)
And if you hit it hard enough with your muscles,
the bottom of the glass bottle breaks out sometimes.
(taps bottle) Oh!
And it's really confusing why it would even be possible
to hit the top of the bottle, and the bottom breaks out.
We got to film this with a high-speed camera,
which, on its own, was really cool.
But the other reason I wanted to make this video
is that the phenomenon is related
to how a certain type of shark
uses a shockwave to kill its prey,
it's related to a method for making honey flow faster,
and it's related to why head injuries
are actually maybe even more traumatic than we ever knew.
I started looking into this phenomenon,
thinking about it from a physics perspective,
and I found this guy demonstrating it on YouTube,
and he included an explanation.
- [Alan] Striking your palm down around the opening rapidly
compresses the air,
exerting an instantaneous pressure on the water.
- And that's where I had to pause
because, ding, ding, ding, ding,
my science BS meter was going off.
So we're gonna fact-check this viral video,
which is maybe a terrible idea, but here we go.
The real explanation all starts with a little mystery,
a little story in 1893.
There was this warship called the HMS Daring,
designed to go super, super fast.
One of the first ever torpedo boat destroyers.
It was meant to be one of the fastest warships in the world,
reaching up to 27 knots, which, I guess, is super fast
for a 260-ton member of the 19th century British Royal Navy.
But when they first tested the Daring,
it was nowhere near that speed,
and they couldn't figure out why.
For all of its 4200 horsepower, it was a complete failure.
No one knew what was wrong
until they looked at the propellers.
And they found, not cracks, not damage, but bubbles.
These little bubbles and little clouds of bubbles
were forming on the propellers,
and slowing down the warship.
But the propellers were completely submerged underwater.
There was no source of air.
So if it wasn't air bubbles,
then they didn't know what was going on,
until two engineers, Barnaby and Parsons,
discovered the bubbles were a phenomenon called cavitation,
which was actually theorized over 100 years earlier
by a famous mathematician, Leonhard Euler,
who came up with this idea
that something moving fast through water
would, quote, "leave a void," like a bubble of nothingness.
But cavitation had never been observed before,
until they were testing this warship.
Cavitation happens when fast objects travel through fluids,
like this bullet.
As the bullet travels through the water,
this massive bubble forms, and it immediately collapses.
It's easy to see, though, in this video
of a bullet going through ballistics gel.
When something moves fast through water,
it creates an area of low pressure behind it,
just like, you know how a big truck moving on a highway
will suck you forward behind it,
or how bikers draft behind each other?
That's because right behind the vehicle or the bike,
there is this area of low pressure
that I learned is called a slipstream,
which is why you feel sucked forward,
because of the low pressure.
It's the same idea with cavitation.
A bullet moving through water quickly
will create this area of low pressure behind it.
So low, that something happens to the water.
Actually, I'm gonna show you what happens
with a little experiment.
You can do this at home, with adult supervision.
Get some water close to boiling,
and then pour it into a syringe.
Close the syringe off airtight, and then pull it,
and it'll start to boil, it's vaporizing.
The water is turning into a gas
because when you decrease the pressure
by pulling the syringe, or you could decrease the pressure
by taking the water up to a higher altitude
where the air is at a lower pressure,
and then the phase diagram
from your high school chemistry textbook tells you
that water will eventually change phase
when you change the pressure, it'll vaporize.
In fact, if you went up to 60,000 feet,
and you spit outside, your spit would start to boil,
assuming that your saliva is at body temperature.
So if your bullet moves fast enough in the water,
it'll leave behind an area of low pressure,
because it's pushing into water ahead of it,
so that's creating an area of high pressure in the front,
and then leaving behind an area of low pressure.
Although, in some cases, it creates low pressure in front.
That's a really fun little aside.
But typically, you get higher pressure in front,
and lower pressure behind,
creating the vapor bubble behind the moving object,
which is exactly what the engineers
working on the HMS Daring found
when they looked at the propellers.
When the pressure behind the propeller
dropped below 0.46 atmospheres, the water vaporized.
But as soon as the propeller slowed down,
or the bubbles moved away from the propeller,
the bubbles would collapse, but super violently.
The collapse takes less than five milliseconds.
It's so violent,
that they damage metal propellers little by little.
The bubbles will even sometimes collapse
with so much power, they release a flash of light
in a process called sonoluminescence.
You can see that in the bullet footage. Check it out.
Right here, you'll notice these subtle flashes of light
as the cavitation bubbles collapse back down.
People don't know exactly why the light is produced.
There are some theories that involve virtual particles,
or that it's a process similar to Hawking radiation,
which is the radiation produced
at the event horizon of black holes.
But don't leave this video
and say, "Black holes are formed behind boat propellers,"
because that's not what I'm saying.
The takeaway here is that we don't fully understand.
We don't know yet how sonoluminescence works.
It's an open area of research in physics,
if you're interested.
So cavitation bubbles were slowing down the propellers.
As the engineers noted, quote,
"The greater portion of the power of the engine
was going into formation and maintenance of these cavities
rather than propulsion of the vessel."
So, did they fix it? Yes, they did.
They were able to get the big old ship up to 32 knots,
the quote, "fastest boat ever," for the late 1800s.
How did they do it?
Well, they increased the surface area of the propeller,
which decreased the efficiency,
but they added more propellers.
And these days, cavitation is quite a well-known phenomenon
with boat propellers.
You see old ship propellers with tons of damage,
all these little indentations,
because that bubble collapse is so violent.
Yeah, it's so violent, it can maybe break glass.
My obsession with this trick started
when I had access to a very fancy slow-motion camera,
and I was thinking of what to film
at insanely high speeds for you guys.
My friend, AJ suggested
that we do the bottle-breaking trick.
(taps bottle)
(dramatic music)
I didn't know a whole lot about it,
so I found that video about the guy in his backyard
telling how to do the trick,
and going into this explanation
with some diagrams and fancy jargon.
And he talks about how you push your hand down,
and you compress the air,
which pushes on the incompressible water,
which is, so far, not wrong, but he claims
that the compression is strong enough to break the bottle.
To figure out if that's true,
you can look up the standard beer bottle burst pressure,
which is 50 to 70 PSI, which is pretty high.
So to get that,
you need to compress a sizeable volume of air
into the bottle.
In fact, you would need to compress
about three liters of air into the bottle
to get it to burst,
which is certainly not what you're doing.
If his theory were right,
you would be able to break the bottle without any water,
and you can't.
And this trick even works
if you put a cap back on before you try it.
So, that explanation is totally wrong.
The reality is actually so much weirder and cooler.
It turns out, it's cavitation
across the bottom of the bottle, blasting out the base.
But how do we know?
We can't see it. It happens so fast.
We have to slow it down nearly 1,000 times normal speed.
Check it out.
What happens if you smack the bottle down really fast is,
the bottle moves as fast as the hammer,
and basically leaves the water behind.
It's like if you pull the tablecloth
out from underneath some stuff,
and the stuff stays behind on the table.
The bottle is pulling away from the water,
and the water stays behind because of inertia.
It's left behind,
and it creates this low-pressure area at the bottom.
And you can't see that with your naked eyes.
It happens so fast.
But with the high-speed camera,
we filmed at 18,000 frames per second.
You can see the cavitation bubbles forming,
and then you can see them collapsing.
And as soon as they collapse, all of the energy
at the interface between the vapor bubble and the water
gets concentrated into nearly a single tiny point,
causing an enormous spike in pressure and temperature,
and releasing a powerful shockwave
that shatters out the bottom of the glass.
And the other cool thing that I'd never seen before,
we tried this with carbonated water,
but that's been shown, generally, not to work,
but we noticed this strange moment.
We did the hit with the carbonated water,
cavitation bubbles formed, and they collapsed a little bit,
and that immediately created all these bubbles
throughout the entire bottle.
I had initially wondered why there would be bubbles
in the rest of the bottle.
Well, it turns out, cavitation bubbles in carbonated water
end up creating nucleation sites for the carbonation,
for the dissolved CO2 to come out of the water.
And then they collapse,
but the shockwave isn't as strong in carbonated water,
but we can still see the shockwave here,
traveling through the entire bottle,
and creating more nucleation sites,
where the CO2 bubbles out
through the rest of the bottle, pretty cool.
That is my stunt double. Do you not see the resemblance?
(taps bottle)
So, we figured out what's going on with the bottle.
So all there is left to do
is tell you some cool facts about cavitation,
and try to blow your mind.
Some animals can make cavitation bubbles.
When scientists were studying the force
of a mantis shrimp claw hitting a shell,
they saw two comparable force peaks,
and they didn't know what the second one was,
but it turns out, it's a cavitation bubble collapsing.
Then there's the thresher shark.
They have these long, whip-like tails,
and they strike at schools of sardines
by flinging their bodies forward over their heads.
So they do a, kind of like a somersault in the water,
pushing the tips of their tails
to speeds of over 30 miles an hour.
And at the peak of the tail slap,
researchers think, they're not quite sure,
but they think that they cause the water to cavitate,
and that stuns or kills the prey.
That's pretty cool.
Cavitation is also a super, super active area
of research in science, for cancer treatments,
cleaning, making honey easier to flow.
And I'll go through all these really quickly.
For cancer treatments, you can use ultrasonic cavitation
to target and destroy cancerous tissues.
So you can imagine creating cavitation bubbles
that collapse and destroy the tissues
by using really, really high-frequency waves,
above the limit of human hearing.
With the honey, you use cavitation
to break up sugar crystals so that it can flow more easily.
And the last interesting instance of cavitation,
and I think most relevant, showing up in the world
is related to the bottle demo,
but it's actually research into head traumas.
The idea is that if your head gets hit,
the liquid inside your skull stays where it was,
just like the water in the bottle stayed where it was,
and cavitation bubbles will form inside your skull,
and then collapse, potentially increasing the damage
in your brain during a head trauma.
So scientists are currently studying that phenomenon,
for good reason.
Well, that's it. That's all I've got.
I hope you enjoyed this video.
I really enjoyed learning about cavitation.
There's so much more. There's even more.
Go Google cavitation.
See what other kinds of research are going on.
You can't say you didn't learn anything
on the internet today.
Be careful cutting your hair by yourself,
and happy physicsing.
(uplifting music)