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- One of my subscribers was doing some plumbing work
on his house one day, joining two PVC pipes together
using PVC cement, when a dollop of the PVC cement
dropped into a puddle by his feet.
He noticed something very strange started to happen,
so he spent the rest of the day
adding droplets of PVC cement to the puddle.
He even sent me this video.
Isn't that crazy?
See how those dollops of PVC cement
are spinning around and moving in strange ways.
I managed to recreate the effect
under lab conditions here in my studio.
It's surprisingly difficult to film
a transparent thing floating in a transparent thing.
The key is to use extreme lighting angles
to take advantage of the differences in refractive index.
But anyway, here it is.
It's surprising how jerky the motion is, actually.
It seems to spin in fits and starts,
and then it will jerk around the bowl.
It's odd.
I think I have an explanation for this behavior,
but first, PVC cement is actually
really interesting in itself.
It's not glue, it's cement.
That's an important difference.
PVC cement is made of PVC,
but it's PVC that's been dissolved in a solvent,
so when you add it to your pipes,
that solvent actually dissolves the PVC
in the pipes a little bit,
and all the while, this solvent is evaporating.
Once it's all gone, all you're left with is PVC,
and that's the difference.
If you glue two things together,
what you end up with is two separate things
with glue in between, but when you join
two PVC pipes together with PVC cement,
what you end up with is one continuous piece of PVC.
It's essentially welding.
By the way, this isn't how you join PVC pipes together,
this is just how I've done it for illustration purposes.
If you're gonna do some plumbing,
you'll need some kind of joint.
The PVC cement goes between the pipe and the joint.
I tried to make my own PVC cement
by looking at the ingredients online.
Basically I added some PVC pipe
to the main solvent that I found online,
methyl ethyl ketone.
I had mixed results.
I don't have a paste so much
as a squishy crumbly lump of PVC.
So why does PVC cement behave in this weird way?
Well, it reminds me of a couple of phenomena
that I've seen before, the first is the effect
of dish soap on water.
Look, you can create a tiny little cardboard boat
and propel that boat across the water using dish soap.
You add the dish soap to the cavity and it zooms off.
That's driven by a difference in surface tension
between water and soapy water.
So water has this high surface tension.
All that means is that the molecules are holding
on to their neighboring molecules really strongly,
whereas soapy water, the molecules aren't holding
onto their neighbors really strongly.
It's a much lower surface tension.
And when you've got this gradient of surface tensions,
low surface tension here, high surface tension here,
well where the molecules are holding
onto each other strongly, they retract away
from the low surface tension area,
so that low surface tension area expands.
That's easier to see with this classic demonstration,
where you add pepper to the surface of water,
and then you dip your finger in the middle,
and you've got soap on your finger.
The pepper suddenly rushes out to the sides.
It's this surface tension gradient
that propels the boat forward.
So perhaps that's what's happening with the PVC cement.
Maybe the solvent is reducing
the surface tension of the water.
We can test that in the same way,
grind some pepper onto the water
and add a dollop of the PVC cement.
As you can see nothing happens.
So that's a bit of an explanatory dead-end, but nevermind.
The other phenomenon that it reminds me
of is actually from an exhibit that I saw years ago
at the Science Museum here in London
of dry ice floating on water.
Actually this footage isn't from the Science Museum,
it's from a Science Center in Switzerland called Technorama.
This piece was created by Shawn Lani,
so thank you to Shawn for creating
such a beautiful exhibit.
And also huge thanks to Martin Fischer,
who allowed me to use this footage of the exhibit.
Actually, dry ice is more dense than water,
so it will sink if you give it a chance,
though, depending on how you manufacture it,
you can make it quite fluffy, so it will float.
And actually, if you make the pieces small enough
it'll float anyway, because of the surface tension of water,
and because you have this cushion of gas
that the dry ice is constantly creating.
That's because dry ice is solid carbon dioxide,
and it turns from a solid directly into a gas.
It skips the liquid phase, that's called sublimation,
and it's sublimates about minus 78 degrees Celsius.
So when you add dry ice to room temperature water,
it's sublimates incredibly quickly.
You get this huge volume of gas
coming from a very small space,
in other words, you get these strong jets.
And because these bits of dry ice aren't symmetrical,
these jets of gas most likely won't line up
exactly with the center of mass of the dry ice,
so you end up giving it some angular momentum.
These blobs of dry ice end up spinning.
They also end up zooming around
on the surface of the water because, well,
there's asymmetry in the sizes of the jets
on each side as well.
So perhaps something similar is happening
with the PVC cement.
Perhaps the solvent is evaporating so quickly
that it's creating jets of gas, or perhaps it's leeching
into the water incredibly quickly,
creating jets within the water.
We can test both of those hypotheses
by adding markers to the air and the water.
So for the water, we can add styrofoam balls,
and see how they move in the presence of PVC cement,
and we can add fog from a fog machine
to the air to see how the air moves around the PVC cement.
You actually don't see much with the styrofoam balls.
I think that was a poor choice of marker.
Turns out though that the pepper
I was using earlier works really well.
You can definitely see the water moving
like jets away from those blobs of PVC cement.
This one is stuck against the side of the bowl
and you can really see how random the jets appear to be,
firing off at unpredictable intervals.
As for the fog from the fog machine,
I can't really see any movement at all there,
so gas jets probably don't have an effect.
So the motion of PVC cement in water
is most likely driven by the expulsion
of solvent into the water.
I spoke to a chemist friend of mine,
Andrea Sella, to try and get a bit more insight.
He actually feels that more research is needed,
like it's not obvious what's going on,
on the chemical level.
His suspicion is that the main solvent,
methyl ethyl ketones are hydrophilic at one end,
and a hydrophobic at the other end, a bit like soap.
So when it's expelled to the surface of the water,
the hydrophilic end will point downwards,
the hydrophobic end pointing upwards,
assembling into a kind of raft that pushes outwards.
So that's an interesting idea,
but still, that's not known for sure.
Another couple of mysteries.
The first one is, if you add the PVC cement
to the water immediately, straight from the pot,
without it being exposed to air for very long,
then these fins burst out from the blob,
sometimes forming wings on either side, really strange.
The other mystery is the jerky motion of the blobs.
Okay, this is just pure speculation,
but one possibility is that,
as the solvent evaporates on the surface,
it creates this slightly harder crust of more solid PVC.
But then that crust will shrink,
as more of the solvent leaves,
so you end up with cracks appearing,
and every time a crack appears,
then jets of solvent leave from the cracks,
so you end up with this chaotic jerky motion.
It's pure speculation, but there you go.
From at least as young as six months old,
children are able to distinguish
between animate things and inanimate things,
and they can do it purely based on the way the thing moves.
Like if you show a child a dot on a screen
that moves through a parabolic arc,
then the child will recognize that as an inanimate object
that is following the laws of gravity.
But if that dot starts to move in ways that defy gravity,
the child will ascribe intention to that dot.
It's an animate object.
In other words, we have these two different models
in our brains for analyzing the motion
of things that we see in the world.
So while we can rationalize that these blobs
of PVC cement are inanimate,
the way they move squarely falls into that model
in our brain that tells us, no, no, this thing has intent.
This is an animate object.
And I think it's this conflict
between these two different parts of our brain
that make it so fascinating to watch.
A huge thank you to the two Martins,
the Martin who sent me the dry ice footage,
and Martin who sent me his footage of PVC cement
acting weirdly in a puddle,
that inspired this video you're watching now.
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