- If you blow the ink out of a ballpoint pen onto a leaf
and then put it on water, this happens.
It's cool, isn't it?
I saw a few posts on Reddit,
and I thought it'd be cool to find out what's going on.
I originally thought it would be a good idea
to do the experiment on milk
because then the ink would stand out better.
And I thought, actually,
that could get gross really quickly.
So this is actually titanium dioxide
swirling around in water, and it works really well
with the ink from a red ballpoint pen.
For comparison, this is black ballpoint pen ink.
What's really cool though about this layer of ink
ejected from the back of the leaf is that it's really shiny.
So by using a black container instead
and making sure that the white ceiling in my studio
is well illuminated, the shiny ink trail really stands out.
The floating patches of ink left behind
are really interesting, like they're quite brittle.
If I move my finger up through the ink,
you can see it breaks into lots of little shards.
It doesn't only work with leaves, of course.
Here's a piece of paper being propelled around by ink,
and here's a droplet of ink spreading round on its own.
All the ink I've shown you so far
is from Biro branded pens.
I also bought some Tesco branded pens,
and here's a side-by-side comparison
of the different colors.
It's interesting to see how the shapes vary between the two,
presumably because the chemistry is a little bit different.
This is really reminiscent of at least two phenomena
that I've seen before,
but I think the mechanism is different to those two.
In this case, the first thing you might think of
is dish soap powered propulsion.
Look here, this boat is propelled forward
when you add a drop of dish soap to the cavity,
this happens because the dish soap
reduces the surface tension of water.
So you have low surface tension here
and high surface tension here.
It's a bit like popping a balloon.
This high tension area recedes under that tension.
And so the area of low surface tension expands.
The boat is at the edge of that region
and so is carried away as it grows,
but I don't think that's what's happening with the ink.
You may also remember the spinning glue video that I made.
I'm not 100% sure what powers the spinning glue,
but what I think I worked out during that video
is that solvent is ejected from the blob into the water.
And that's what powers the movement of the blobs.
Again, I don't think that's what's happening with the ink.
To try and figure it out,
it's worth looking at the chemistry of a ballpoint pen.
Well, the pigment in an ink actually starts off as a powder
like carbon black for blank or this oisine for red,
for the dye to be able to flow onto a page,
it needs to be dissolved in a solvent.
In the case of a felt tip pen, that might be an alcohol.
And when a felt tip pen mark dries on the page,
it's because the alcohol is evaporating
leaving the dye behind,
but ballpoint pen ink is much thicker than felt tip pen.
That's because the dye in a ballpoint pen
is dissolved in an oily solvent, like phenoxy ethanol.
Do you know what?
It smells just like a ballpoint pen.
You know that, it's like a perfume you smell.
I mean, it's obvious, I suppose, but that's cool.
Phenoxy ethanol doesn't evaporate
like the solvent in a felt tip pen,
or at least not in a timely manner.
Instead of when a ballpoint pen stroke is drying
it's because the phenoxy alcohol is bonding
to the fibers of the page and also oxidizing in the air.
That's all very interesting, but from our point of view,
we want to know how it behaves on water.
When a drop of phenoxy ethanol first hits the water,
it spreads out, and then over a few seconds it disappears.
I believe that's because it's dissolving into the water.
Phenoxy ethanol isn't very soluble in water.
And in fact, if you keep adding it,
eventually droplets will sink to the bottom
instead of dissolving.
By dropping phenoxy ethanol onto water
that already had a crusty layer of ink on it,
I managed to capture this really interesting behavior.
Isn't that cool?
And I honestly have no idea why it's doing that.
The spreading out bit is important.
Presumably if there was dye
dissolved in the phenoxy ethanol,
that dye would be carried with it as it spread out.
Why would one liquid spread out so quickly
on the surface of another liquid?
Well, I think it comes down to
how well the molecules in the liquid bond to each other
versus how well they bond to water.
Like if a molecule of phenoxy ethanol
bonds more energetically to a molecule of water
than to another molecule of phenoxy ethanol,
then it will spread out across the water to minimize energy.
Here's some phenoxy ethanol with the dye dissolved in it.
And yes, as it spreads out,
it carries some of the dye with it,
but it certainly doesn't look the same
as when we use regular pen ink.
Like the ink isn't forming a crust,
it's just swirling around there dissolving into the water.
So there must be something else going on.
Another candidate is a oleic acid which is an ingredient
used as a lubricant in ballpoint pens.
I actually used a oleic acid in a video
on Matt Parker's channel to calculate Pi for Half Tau Day.
The reason you can use a oleic acid to calculate Pi
is because it will spread out into a circle,
one molecule thick on the surface of water
if you do it right, and using Avogadro's number,
you can work backwards and calculate Pi.
Yes, it's a ridiculous idea.
What's interesting though,
is if I just drop a blob over oleic acid on water,
it doesn't spread out into one molecule thick layer.
It bunches up like this.
I suspect that it's because in the Avogadro's number video
on Matt's channel, I was using a solvent,
and the concentration of a oleic acid
was really, really low,
but I don't fully understand that behavior.
But anyway, I don't think oleic acid is the thing
working to spread the encounter in this case.
Another possible candidate is a surfactant.
Surfactants are added to ballpoint pen inks
to help them bind to the paper,
and just like dish soap,
a surfactant lowers the surface tension of water.
So it might actually be that we have something similar
to that dish soap propulsion, but I'm not convinced.
Like if it was a surfactant lowering the surface tension,
then surely these ink rafts would be constantly breaking up
and crumbling and spreading out
as the surface tension was being pulled outwards.
Actually the obvious test to do
is to put the phenoxy ethanol on the tip of a leaf
on its own and see what happens.
Well, to me, that's pretty good evidence that the process
is at least partially driven by phenoxy ethanol.
But the particular way that the ink trail forms
is a bit harder to pin down.
I suspect that in combination
with some of the other ingredients,
it behaves in just the right way
so that it carries the right amount of ink with it.
The layer becomes thin fast enough,
and it dissolves into the water fast enough
that you just get this perfect raft of ink.
It's weird that the dye doesn't dissolve in the water
like it did when we were using phenoxy ethanol
on its own with the dye.
Perhaps one of the other ingredients acts as a barrier
between the diet and the water.
Maybe the oil or something reacts with the water
and becomes a hard surface that traps the dye.
I have played around
with different combinations of ingredients.
For example, here is the dye
with phenoxy ethanol and oleic acid.
And here's the dye mixed with phenoxy ethanol,
oleic acid and a surfactant.
I tried these different ingredients
in different concentrations.
I didn't get anything much different
than what I've shown you already.
So there's still a mystery here.
You know, it's been fun finding out
about the chemistry of ballpoint pens.
And I feel like we're closer
to a description of what's going on.
If you have any insights, let me know in the comments.
There'll also be a discussion happening
on the Steve Mould's sub-Reddit.
I'll leave a link to that in the description as well.
One of the best things about this job is that I get to work
through mysteries for myself until I understand them.
Like there's not that much information online
about how a ballpoint pen works,
but I was able to find the ingredients,
order them online and play with them,
see how they behave and work it out.
And they say that the best way to learn
is by doing it for yourself.
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