[MUSIC PLAYING]
DIANNA COWERN: Thanks to Curiosity Stream for supporting
PBS Digital Studios.
Hey, yo.
It's Dianna.
And you're watching "Physics Girl," and--
[ROOSTER CROWING]
--I'm back home on Kawaii at my childhood home,
visiting my parents for the holidays--
which is why I'm filming in this bush--
and these guys are everywhere.
Anyways, you're watching Physics Girl.
And I want to get right to the money shot for you.
Because I think that this is one of the coolest things
we've ever filmed.
[SHRIEKING]
This is really cool.
You're gonna see ferrofluid droplets fall onto magnets
and find out how that's related to the epic death when
you fall into a black hole.
I showed this footage to a bunch of friends
and no one knew what was going on.
So I have to set this up for you.
I visited the YouTube space in LA to film
with my friend William Osman, who I did the recent sand
fluidization video with.
It was so fun.
William had gotten some time on the high-speed camera.
And so given some time with the Phantom, what else would
you film besides ferofluid?
For the uninitiated, and-- ugh-- how could there still
be any of you left?
Ferrofluid is an oily substance that sticks very strongly
to magnets.
I warned the guys very sternly to keep the ferrofluid far
from the magnets until we were ready to use it
because it's so easy to accidentally get
it stuck on the magnet.
[GASPS]
[LAUGHTER]
MAN: Oh.
The unthinkable happened.
MAN: Well, it could be better.
DIANNA COWERN: [GROANING]
MAN: I got an idea.
What if I try to soak up stuff on the other s-- oh, no.
DIANNA COWERN: Yeah.
It'll just take a lot of paper towels.
But it's got some really fun properties.
MAN: Oh, it's going right through the paper towel.
DIANNA COWERN: Yeah.
MAN: Oh, wow.
MAN: Whoa.
It's, like, slamming it into the--
DIANNA COWERN: Yeah.
And you could make it sing.
[BUZZING]
MAN: You can just-- they're, like-- they're,
like, flying off.
DIANNA COWERN: Yeah.
But most importantly, it forms these unbelievably crazy spikes
in the presence of a magnetic field.
And so I wanted to drop the ferrofluid
to watch the splashing dynamics with and without the magnet.
OK.
Time for the footage.
But wait.
It's a little confusing what you're
looking at in these shots.
So let me explain the setup real quick.
We dropped the ferrofluid onto a Styrofoam plate.
But in a few cases, there was a strong neodymium magnet
underneath the plate.
And in a few there wasn't.
I'll make it clear which is which.
Now the footage.
But as you watch, something strange is going to happen.
So I want to see if you can catch it.
So I'm not going to tell you what to look for.
OK.
Here you go.
[MUSIC PLAYING]
[SPLASHING]
The beauty of physics.
Did you see it?
Tell me what you saw-- (WHISPERING) down there
in the comments.
Maybe you saw something that I didn't notice.
We saw this.
[SHRIEKING]
MAN: Oh, wait-- are you saying, like,
the drop is getting stretched out?
DIANNA COWERN: Yeah, it gets elongated.
Whoa.
DIANNA COWERN: Oh my god.
OK.
That's really weird.
MAN: [INAUDIBLE]
I was wondering--
I was--
The droplet becomes elongated like a football--
the American kind-- or like a grain of rice--
the international kind.
Isn't that weird?
[MOANING]
The magnet is pulling on the entire droplet.
So you could imagine that the droplet would get compressed.
But, as you know, the closer you are to a magnet,
the stronger the attraction.
The magnet is pulling on the bottom of the droplet
harder than on the top of the droplet.
So the bottom is actually falling faster than the top.
I wish that we had tested a water droplet
next to the ferrofluid droplet so
that you could see that the ferrofluid is not in freefall.
It's actually going faster than freefall.
And it would zoom right past a normal water droplet.
[GROANING]
Next time.
OK.
So the top of the droplet can't keep up
with the bottom of the droplet because it's not
accelerating as fast, which means the entire droplet gets
stretched out.
OK.
Now let's use our imagination.
If you swap out the magnet for the moon
and the droplet for Earth, you get
a situation that describes exactly why we have tides.
So if you think about it, the earth
is essentially falling towards the moon because of gravity.
But they don't ever actually collide,
because the moon is moving so fast sideways
that it's in orbit.
So the moon's gravity pulls on different regions
of earth differently and pulls harder
on the oceans closer to the moon than on the earth itself,
which makes this happen, right?
Not quite, right?
Because there are two tides in a day.
This is not intuitive.
OK.
Let's do this.
Let's replace earth with me and two unsuspecting members
of my family.
Gravitational forces, like magnetic forces,
depend on distance.
The further away from the moon, the weaker the force will be.
So the force on Dad is the weakest, the force on me
is, meh, and the force on Mom is the strongest.
As time passes, Mom will reach the moon much faster.
Let's see that now from Earth's--
or our-- perspective.
The distance increases between me and my parents.
So the high tides are just like high school graduation.
They are the result of one tide being pulled away
from the Earth, which is being pulled away
from the other tide.
So Earth's water gets pushed down from the poles
onto the side of the Earth.
So if you imagine the oceans as a ferrofluid droplet encasing
the earth, that droplet gets elongated.
And where it's elongated is where the high tides happen.
One more crazy thing about tidal forces.
That's the name that we give these gravitational forces that
are a differential strength across a body and cause
differential pulling.
The tidal forces from a black hole are strong.
They're so strong that the difference between the forces
on your head and your feet would be enough to pull you apart.
This unfortunate circumstance has
been termed spaghettification.
And we're seeing spaghettification in action
with the ferrofluid.
How crazy is that?
Epic black hole death modeled right there
at the YouTube space.
OK.
Enough about my love for playing with ferrofluid.
People have gotten real creative with the actual uses
of ferrofluid in industry.
For example, it could be used as a construction
material inside your eyeball.
The idea is that you would guide the ferrofluid using
a magnet inside your body to patch a hole in the retina
or something.
Again, this is a proposed use.
This is not something happening at your local doctor's office.
I've been obsessed with fluid dynamics lately.
I actually just went to a fluid dynamics research conference.
And a couple of you guys came and said hi.
Thank you if you said hi to me.
But the reaction was mostly like, what are you doing here?
I just wanted to see the research, OK?
It was just interesting stuff.
But what I realized at this conference
is that there's so much more research
than you would expect that goes into droplets and splashing
dynamics.
OK.
That's all I've got for splashes and ferrofluid.
Or is it?
I couldn't help myself.
I set up my high-speed little camera
and dropped a bunch of things to watch them splash.
So please, listen to these fun announcements
while watching crazy footage of droplets.
"Physics Girl" has been accepting subtitle submissions.
Thank you so, so, so, so much to those of you
who have reached out and submitted subtitles already.
If you speak another language, you
can submit translations for subtitles using
the link in the description.
Thanks to John for filming at the YouTube space
and to Cameron Esacoff for operating the Phantom camera.
And William also filmed a video while we were there.
They dropped their year-old trailer
park gingerbread creation and we filmed that in slow motion.
Check it out.
I'll put a link to his channel and that video
in the description.
Thank you for making physics a part of your day.
And happy physicsing.
I'm trying out different slogans.
I don't know.
I think it's time for a new one.
Happy physicsing just makes no sense.
OK.
That's all.
Except, thank you to Curiosity Stream for supporting
PBS Digital Studios.
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