[SQUEAKING]
[POP]
Check this out.
I'm just gonna drag this plate through the water in the pool.
That's it.
But look what forms-- perfect black circles
on the bottom of the pool.
And it's so weird.
They don't die.
They just keep going right next to each other.
I sped this footage up because the circles
took 3 minutes to cross the pool and didn't
seem to be dying out.
These got sharper as they went and made it all the way across.
So cool.
And look closer.
You can see what's causing them.
There are two almost unnoticeable dips
in the surface of the water.
Why don't they die out?
And how do these dark spots form?
And why do the dips move together?
So many questions.
Let's first look at what they are.
When I dragged the plate through the water,
you could see it forming little vortices,
like you get when you move a paddle through the water.
A vortex is a gas or a liquid moving
in circles with what we call a vortex line going up
through the circles.
They are everywhere-- tornadoes, water flowing down
the drain, hurricanes, or the movement
of air over an airplane wing.
Vortices often form when there's a difference in fluid speed,
like fast wind moving over slow wind.
As you move the plate forward, the water
right next to the boundary moves with the plate
because of friction.
The water further out is not moving.
It's that difference in velocity that causes a shear force
and starts the water spinning.
This is accentuated by the water in front of the plate being
forced around the sides and the water
behind the plate being sucked forward
as the plate moves along.
In this coherent vortex, our dips
can travel much further across the pool than a ripple
without dissipating.
Why?
The vortex wants to keep its angular momentum.
So it continues spinning as it moves through the water
and only loses some energy through friction
to the surrounding stationary water.
So what we appear to have here is two vortices
moving across the pool.
But it seems weird that they stay right together.
Well, when we create this vortex,
we're forcing water to move around the whole plate,
not just on the surface of the water.
Our vortex line is curved.
We're creating a half vortex ring.
You know, like a smoke ring, bubble ring, or even the plume
rising up from an explosion, but half of that.
These two dips on the water's surface
are just the ends of the half ring.
They're connected.
And that's why they stay together.
We can see this if we put a little food coloring
into the ends.
The food coloring travels from one end to the other.
And you can see the entire half ring.
So cool.
We just made a bubble ring without the bubble.
This also tells you something interesting.
The half ring is not a wave.
A wave is a disturbance moving through a medium that
transfers energy but doesn't transfer matter.
When we added food coloring, we saw clearly
that the vortex is transferring matter,
just like a smoke ring does.
So sadly, it's not a wave.
It's cooler than a wave.
Now back to this half-ring vortex.
Isn't it weird?
You've probably never seen half of a smoke ring before.
Why?
Well, according to Helmholtz's law,
a vortex line cannot start or end in a fluid.
So a vortex line either wants to close into a loop
or end at a boundary.
If you tried to cut a smoke ring in half,
it would either break up or, even cooler,
it would break up into two smaller smoke rings.
I don't know if that happens.
It would be awesome.
Since the line can't end in the fluid, it ends at the surface.
You almost never see these half vortices
though because they have to occur
on the boundary of two media.
If you can think of another way to make the entire half
ring visible, like a smoke ring or a bubble ring, let me know.
So now, why does our vortex create those cool dark spots?
Well, the balance between gravity, centripetal forces,
and surface tension creates a funnel shape
which acts like a lens.
Lenses often work by focusing light to a point,
like this Fresnel lens, which can melt pennies in the sun.
But I digress.
Our vortex lenses focus the sunlight away from the center,
like so.
And you have a bright ring surrounding a dark circle.
The ring highlights the dark circle further.
And you get these incredibly clear black spots.
So cool.
Now I highly recommend trying this at home,
especially if it's too cold to swim in the pool.
Here are some tips.
The water has to be very still.
Even a small breeze makes it harder to create these.
The sun has to be out, or it won't work.
And you can try some other really cool things.
Send your vortices at a wall.
You'll see the dark spots spread out.
Or try colliding two of them.
Happy physics-ing.
[THEME MUSIC]