DIANA: Thanks to Curiosity Stream for supporting
PBS Digital Studios.
Hey, I'm Diana.
You're watching "Physics Girl," and a video that I really
like to make is a bunch of different science experiments
that you can try at home.
So we bought all the materials for like six
different experiments, which is a lot of milk in my fridge
that says do not drink.
But we didn't even get past the first experiment,
because it was so cool.
So this video is going to be a listicle of one.
Check it out.
When you put a thin rod, like a glass stirring stick,
into a fluid that has specific properties,
and then spin the rod, it does something fluids don't usually
do.
The fluid travels up the rod.
Not all fluids would do this, of course.
I know you know that purple stuff has special properties.
It's the most viral fluid on the Internet.
This is slime.
And what we're seeing here is called the Weissenberg Effect.
It's not normal behavior for a fluid.
If you've ever beaten brownie mix,
you know that the fluid tends to fly off in all directions.
And it certainly doesn't climb in and up the mixer.
It climbs into my belly though.
So the Weissenberg Effect happens
with non-newtonian fluids.
And I'll talk about what those are.
But it's so cool.
We wanted to be able to visualize
how the fluid is moving.
So we made a few different colors of slime.
And then put them together in a container
so we could draw the liquid up the rod from the point
where all the colors meet.
This is just an excuse for me to make pretty colors.
I love colors.
So this is what happened with the colors.
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Why does this happen?
What is the Weissenberg Effect, besides just another effect
named after some dude?
I talked to my friend, Dan, who's a plasma physicist,
and also happens to be the friend who
helped me with the colors, about what's going on.
So this is a-- they call it slime.
It flows like a fluid.
So if I hold it like this, it will flow as if it's a fluid.
But if I put it suddenly, it even snaps in half.
The fluid moving around and getting
dragged along this pen, that's going to happen in any fluid.
DIANA: Even water.
Even water, yeah.
But with this fluid, this material
also has a unique property called viscoelasticity.
Not only is this fluid viscous, but it will bounce back
to its original position.
That's not the case with all fluids.
They elasticity of this fluid makes
it want to come back to its original position, right?
So I'm going to use this string as a model for that.
So as this turns--
That's the sound of science.
DAN WALSH: it wraps up this string or the fluid,
and it's tightening as it goes, that creates a tension.
When you have a tension that's pulling around
it actually creates an inward force
that causes it to collapse.
So think about what's going to happen
when this cylinder of fluid starts to squeeze in.
It's going to push the inner fluid up.
And that's what's driving the Weissenberg
Effect to send this up higher and higher up the rod.
So the upward motion of the Weissenberg Effect
is essentially the fluid squeezing itself up for rod.
A nice self hug.
It's also kind of like spaghetti when you twirl it
around a fork, except spaghetti is not a liquid,
until it's in my belly.
I'm hungry.
I mentioned that this works with non-newtonian fluids,
and I didn't tell you what those are.
So a newtonian fluid is a fluid that acts normally.
It's like water, if you suck water up a straw,
the harder you suck, the more water will come.
But with non-newtonian fluids, if you
put something called shear strain on them, which
is kind of this sliding kind of motion,
there's some non-newtonian fluids, like this kind,
like slime, that get thicker.
They're called shear thickening fluids.
And it's the elasticity plus the shear thickening
of the slime that causes it to tighten around itself
as you're stretching it out with the twists--
fluid dynamics.
This experiment is a demo you must try at home.
And if you want to make your own slime and do the Weissenberg,
here's the recipe that we used.
One half a cup of water, mixed with half a cup of PVA
glue, or that washable multipurpose white glue.
It's Elmer's, people.
And then separately, mix 1/2 a cup of warm water
and a quarter teaspoon of Borax.
Then mix all that together.
Then profit.
You can also add food coloring to the water,
or add glitter, or mashed bananas, or whatever you want.
And the key to seeing the Weissenberg effect
is to get a rod that you can make spin slowly.
You can even do it by hand with some types
of non-newtonian fluids.
And if you want to turn this into an experiment, rather than
just a demo, try controlling the amount of Borax
versus the amount of glue that you
put in and see if that affects how far you
can get it to climb up the rod.
Or try something else.
But whatever you do, if you try this at home,
you better tweet it at me.
Use the hashtag science slime--
slime for science.
Thanks for watching this video.
I've got another video that's a bunch of fun demos
you can do with water.
So check that one out.
Subscribe if you want more physics and happy physicsing.
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