- [Diana] All right.
Nice. Oh yeah!
Vortex rings.
They're swirling masses of fluid.
Smoke rings, bubble rings,
sometimes made by dolphins,
sometimes tied in knots.
We'll get to those later.
But sometimes, they're made by crazy Youtubers.
Hey, I'm Diana and you're watching Physics Girl.
I want to make a vortex ring in the pool.
So, my most popular video ever
is of a half ring vortex we made in the pool.
So I recently wondered,
could we make a full vortex ring?
I know it can be done
because we've already done it.
But even before we tried it,
I thought it could be done,
because we'd already made a half ring vortex in the pool.
Remember, with a plate.
You couldn't see the vortex at first,
but you could see the shadows
made by the surface of the water,
where the vortex met the air
and caused the water to dip down.
We won't be able to use that effect
to help us see the ring if it's fully submerged under water,
but we could try the food coloring thing,
which we did with the first half ring vortex
by dropping food coloring into the ends
and watching it travel down to illuminate a full half ring.
But for the full ring, hmm, there are no ends.
So we're gonna have to try to get
the food coloring in there before it forms,
or as it's forming.
I have an idea. I had an idea.
So I'm gonna show you what we did.
And then I'm gonna talk about
these vortices in real research,
like knotted vortices, tied in knots.
But first, here's what we did in the pool.
Ah! It's really cold.
(Derek giggling)
And Derek, who's being so kind
and lending me his pool and also helping me.
Thanks for your pool!
- No problem.
- We have a vortex cannon,
which is usually used for air vortices
and for shooting people
(coughing)
but we're using it in the water.
And my idea is that
I'm gonna squeeze some of the food coloring
just on the outside, in the front,
right where the vortex is coming out
and hopefully it'll get sucked in
and make a full ring vortex.
- [Derek] More food coloring?
- [Diana] A bit more food coloring.
- [Derek] Okay try that.
- [Diana] Ooh! Aw, I shot too far upwards.
Naw.
- [Derek] What is happening?
- [Diana] I don't know!
- [Derek] All right.
(upbeat music)
- [Derek] Look at it go!
- [Diana] That's the one!
(gurgling water)
Nice!
(Diana laughing)
- [Derek] I feel like I see an air bubble in there?
- [Diana] Yes! Yes!
- [Derek] Air bubble in the ring?
I just don't get it.
(upbeat music)
- [Diana] Oh it's so cold in here!
Success!
(chuckling)
So yeah, it worked.
That was a lot of fun.
Now, how could we make vortex rings even cooler?
A vortex ring is just a vortex,
like any other swirling, massive fluid.
Tornadoes, water spouts, dust devils, whirlpools.
But with a vortex ring, you take the vortex line,
which is the actual scientific term
for the line that all of the fluids swirls about,
and you wrap it in a circle.
But you could wrap it in all different crazy shapes.
The only necessary condition
is that the vortex line cannot end in a fluid,
and in any medium.
So obviously, our half ring vortex line ended,
but it ended at the surface between the water and the air.
You wouldn't be able to make
that half ring fully underwater.
It would either close,
or it would just dissipate, breakup.
But interestingly, vortex lines
can end in fluids when they're forming or dissipating.
For example, a vortex line created by an airplane engine
can sometimes bend and stretch
until it reaches the ground where physics
will then allow it to end and stay there.
And you get weird-looking phenomena like this.
So as long as the vortex line doesn't end in the fluid,
you can make really crazy shapes with the vortex.
And for a long time,
scientists wondered how to make vortices tied in knots.
Which was hypothetically possible,
but practically a big challenge.
Until William Irvine and his team came along.
I just have to ask,
are all the bottles on the shelf back there
because you've had so many celebrations?
- That's successful graduations.
- [Diana] That's great.
We're scientists for a while
trying to make knotted vortices?
- So the history goes back a long way actually.
It goes back all the way to Lord Kelvin.
Lord Kelvin was in fact,
I think basically responsible for introducing
the idea of topology in physics and knots in physics.
So there are some accounts of Lord Kelvin himself trying,
by colliding smoke rings basically.
All the naive approaches seemed to fail.
And by naive approaches,
I mean sort of smashing vertices into each other,
hoping they will knot up.
But the solution that we found
would have been very, very hard to do in practice
before recent times.
3D printing allows you to have a sort of
rapid turnaround time and prototype
and go from idea to experiment in a day, basically.
So we ended up piggybacking
on a known fluid dynamics effect,
which is called a starting vortex.
Airplanes have these trading vortices behind them.
That's a sort of very, very well-known effect.
But what's less well-known
is that the moment in which the airplane accelerates,
it actually emits a vortex that gets left behind.
And that vortex actually connects up
with the trading vortices and closes on the wings.
Our idea was that we could use that effect
in order to engineer vorticity in the lab,
essentially by 3D printing wings,
making them have the shape that we wanted.
So we made a vortex knot, we made links rings.
Here's an example of the wing that makes the link rings.
Accelerating them, impulsively,
hoping that then they would leave behind the vortex
of the shape of the wing.
- {Diana] That's really cool.
I think you should definitely try the vortices in the pool.
I don't have a pool either. I had to borrow one.
So next time you are at a pool,
give the vortex rings a try.
Thanks so much for watching this one though.
And, costume change!
In my last video, I promised you guys something.
I showed you this phenomenon,
where if you sing a particular note into a tube,
you can't sing the note.
(off tune, muffled noise)
(chuckling)
I asked you guys to try this for yourselves
and to send me your best dying animal noises.
And you did not disappoint.
- What we can't sing is F sharp.
That's where I our gap is.
(noise muffled through the tube)
- No, I want to actually try my hardest to sing that note.
(noise muffled through the tube)
That's so weird.
(noise muffled through the tube)
- Bye bye
- What participation!
I love you guys.
I know I don't tell you often enough.
By the way, right now I'm gonna tease another PBS show.
So if you hate monsters,
then you can go watch my video on square vortex rings.
It's the least monstery video I have.
But if you like monsters, PBS Digital Studios,
which is the network that I am part of,
has a new show called Monstrum.
All about monsters and the lore behind monsters.
And now this mention of monsters
makes a little more sense in my video.
I'll put a link to Monstrum in the description
or at the end screen during the credits,
which I'm gonna cut to right now
and you should stay for
'cause there's gonna be a physics joke.
Thanks for staying all the way to the...
Here we go. Here's the credits.
(upbeat music)