I wanted to make a video about this, crystal clear ice.

It's not perfectly crystal clear 'cause this is my first time doing this, but it's close enough.

So I created this whole elaborate setup to get a beautiful macro shot of a droplet of water freezing.

Now you'll notice that this droplet is vaguely pink, and that's 'cause there is a tiny, tiny amount of red food coloring in it.

What I thought would happen is that the water would freeze completely clear at the bottom, push the red food coloring upward until it had nowhere else to go and then freeze in a tiny, tiny red micro drop at the top.

But instead, these ice crystals exploded into the drop.

So I thought, okay, maybe the food coloring is doing something weird.

So I did the experiment again with just water, no food coloring, and it happened again.

So I did the experiment again, and it happened again.

And then I did it one more time after that and it happened yet again.

So now, instead of this video being about clear ice, it is gonna be about how water freezes.

And it turns out that this thing that we do in our freezers every single day is gonna take us right to the edge of scientific understanding.

Which means I spent a lot of money on equipment and did not get the results I wanted, otherwise known as science.

Look at this, this section right here, this is liquid water.

As you can see, the molecules are all over the place.

They're constantly moving.

This on the other hand is solid water, also known as ice.

Now, as we've mentioned many, many times on this channel before, ice can exist in many forms.

But this structure right here, this hexagonal structure, this is by far the most common structure at atmospheric pressure, meaning that most of the ice around us looks like this on the molecular level.

You can see it's extremely ordered.

The molecules are still vibrating, yes, but the overall structure has a regular repeating pattern, what's called a crystal lattice.

Now look at this animation a little more closely.

Do you see how each water molecule kind of clicks into place into the hexagonal pattern and the pattern keeps growing and growing until the whole thing is frozen?

Well, that's exactly what I was expecting to see in the macro footage.

I mean, obviously not the individual molecules, it's not that macro, but I was expecting a solid line of ice that just progresses steadily upward through the drop.

And that's why the fractal forest that explodes into the droplet caught me so off guard.

But it turns out that that fractal forest is just the first step of water freezing.

The second stage of water freezing happens immediately after the fractal forest and it is exactly what I was expecting.

Like look, you can see it happening right here in a droplet of pure distilled water.

The ice freezes linearly and constantly from the bottom where it's in contact with my cold plate to the very top of the droplet, and the whole thing is smooth.

It happens over the course of about a minute.

Okay, so now let's go back to our pink droplet.

This is the one with the very, very small amount of red food coloring in it.

Here is the first stage, this is the fractal forest, and here is the second stage.

The water is freezing in a nice even line from bottom to top.

And check this out.

It even pushes the food coloring upwards.

Like look, you can really see it.

It's there's very clear demarcation, this line between the crystal clear ice at the bottom and the still dissolved food coloring at the top.

When you are done watching this video, I want you to go over to your freezer and just grab an ice cube.

You might actually be able to see this exact same effect.

The clear part is water, and the cloudy part is all the stuff that was dissolved in water that got pushed downwards as the cube froze downwards.

Now by the way, when I say all the stuff that was dissolved in water, I don't mean bad things, I just mean salts, minerals, gasses, all the stuff that we would expect to be dissolved in tap water.

If your ice cubes are partly cloudy like mine are, it does not mean that your water isn't clean.

Now, the really interesting thing, and the thing this video was originally gonna be about is that this effect where water freezes pure and pushes impurities away only happens under certain conditions.

One of the main ones being if the water freezes slowly enough.

So the question is, why does this happen?

This is a really small section of an ice lattice.

This is a water molecule, liquid water just floating around, and this is a food dye molecule.

Now, this is not actually what a food dye molecule would look like, they would tend to be much bigger.

Now, let's say that all these molecules are cold, but not super cold, maybe -1 Celsius.

Lots of things could happen.

A water molecule could bump into the lattice at exactly the right orientation to form a new hydrogen bond, the lattice grows.

Or the reverse thing could happen.

This bond could break, the molecule could click out of the lattice and the lattice would shrink.

Same exact thing can happen with a food dye.

You can see there's a hydroxyl group here.

That group can form a hydrogen bond with the lattice or that same bond could break and the food dye could go back to being liquid.

Now, the important thing to note is that because this is not super cold, there's plenty of time for all these interactions to happen.

And because the water molecules tend to fit better within the lattice structure, the overall effect is that water molecules are more likely to click into the lattice and less likely to click out than the food dye molecules.

Now, overall, this means that as the lattice grows, it'll just push the food dye further and further away and never actually incorporate the food dye into the lattice.

You end up with pure ice and really concentrated food dye, and there's a clear line between them.

You could literally see this happening, like look right here in the bottom left corner, you can see this very clear demarcation between the crystal clear ice at the bottom and the still pink water on top.

So one of the coolest things I notice while doing this is that the droplets all start out clear, meaning no bubbles.

But then when you freeze them, bubbles just appear out of nowhere.

Now, why is that happening?

Well, it is the same principle as the food coloring, except now instead of food dye molecules that are getting pushed outta the lattice, it's gas molecules, dissolved gasses in water, carbon dioxide, oxygen, nitrogen.

They don't fit very well into the ice lattice, so they get pushed out until they aggregate and they get bigger and bigger and bigger and eventually a bubble forms.

And the coolest thing, oh, you can see it happening here–right there, look at that–is that sometimes the ice forms faster than the bubble can escape.

And so the bubble gets entombed in ice.

And I imagine that this is what dying in quicksand would feel like.

So everything we just talked about, that happens when water freezes slowly.

What about when it freezes quickly?

Well, in that case, when the lattice meets a molecule that isn't water, it doesn't just slowly push it outward as it builds itself from water molecules.

Instead, it builds itself so quickly that it goes around the molecule that encases it.

And so you end up with what's called an inclusion.

And inclusion is a cavity within the lattice that basically holds something that isn't water.

Could be a food dye, it could be gas, could be anything.

Now the size and the distribution of these inclusions depends a lot on how fast you freeze the water, what's dissolved in it, all kinds of other stuff.

But the overall effect is that you end up with a lattice that is studded with all kinds of stuff that isn't water, and these inclusions disperse light, and that makes the resulting ice cloudy.

Now, whether I was freezing the drops quickly or slowly, the first step was always this incredible fractal forest that would explode into the droplet, and I really wanted to capture the perfect shot of that.

But it turns out it was a lot harder than I thought, and mostly because I kept screwing up.

So the first time I just bumped the table at exactly the wrong moment.

The second time I happened to be taking the temperature of the drop with my laser thermometer also at the wrong moment.

And then I just couldn't get the focus right.

And by the way, the lens I'm using here has a depth of field of only 200 micrometers, that is 0.2 millimeters, which is one fifth, the depth of the actual drop itself.

So it is really, really hard to pull focus.

In fact, I even tried to take two plastic plates and put them very close together and then fill the gap with water so that I could be sure of having fractals forming in a very, very thin environment.

But turns out, that did not work.

As you can see here, it's just freezing linearly.

I tried.

So after freezing about a thousand of these droplets, I noticed two really cool things.

Thing one is that the pattern of the fractal forest was always similar, but never exactly the same, even when I froze and thought and refroze and thought and refroze the same droplet over and over again.

So I popped into the scientific literature and I discovered that these things are called dendrites and that science has known about them since at least 1611.

So I am very late to the party on this, and that's nobody's fault but mine.

Anyway, I said at the beginning of this video that these dendrites would take us to the very limit of scientific understanding.

And to reach the limit, you have to ask a super basic question, which is just why.

Why do these dendrites happen?

Well, let's look at some footage again, except this time, let's play it backwards and in slow motion.

You can see that there are multiple dendrites in a droplet and that every dendrite has a trunk that originates from a single specific point.

And so the question is, what is happening at that point at the moment when the dendrite is first forming?

And the somewhat surprising and sort of mysterious answer is that if the water is touching something, which this water is, we don't really understand what is happening on the molecular level at that point and time when the dendrite is first forming.

But if the water is really, really pure, not touching anything and super cold like - 38 ish Celsius, we understand initial ice formation really well.

We can predict it, we can model it, we can simulate it.

It looks like this simulation that you're looking at right now.

But again, those conditions are not very likely to happen in everyday water.

But if the water does have something dissolved in it or if it is touching something, and by the way, those categories encompass almost all of the water on earth, we don't have a great understanding of how this initial ice cluster gets formed.

We do know that impurities or surfaces help ice form at higher temperatures than they otherwise would, but we don't fundamentally understand the first stages of ice formation almost anywhere on earth.

And so here we are.

We are at the limit of scientific knowledge.

Welcome.

I’m glad you’re here.