Let's talk about ice!

Why is it so extremely slippery!

That's actually not a trivial question and physicists have been arguing about it for more than 100 years.

You probably already know that molecules behav differently in different states.

Like, in liquid, molecules form weak bonds, but they still have enough energy to slip and slide past each other to fit the shape of their container.

In solids, molecules are packed close together, often in a regular pattern, which helps solids keep a distinct shape.

Because these molecules can often nestle closer together than they can in liquids, the solid form of a material is usually denser than the liquid form.

But, ice acts a little differently.

Ice is the solid form of water.

Oh, my gosh.

That's why ice is slippery.

But ice acts a little differently.

Ice is the solid form of liquid water, but depending on the temperature and pressure conditions, ice can freeze in a lot of different ways.

In fact, there are more than a dozen variations of ice crystals.

And scientists are still finding more.

The most common ice is hexagonal ice.

It's called ice 1h.

“One” because it was the first kind discovered, and “h because its the hexagonal form.

In this form, the water molecules form layers of honeycombs that stack on top of each other to form the whole crystal.

These crystals take up more space than the same number of liquid water molecules.

That means ice is les dense than water, so it floats.

Another important type is Ice Ic.

Here the “c” stands for cubic because in this form, the water molecules create a cubic-centered crystal structure.

This kind of ice form at colder temperatures than Ice Ih, and can sometime be found in Earths atmosphere.

And as it turns out, these crystal structures are key to understanding why ice is so slippery.

But scientists in the 1800 didn't know anything about them when they started studying the problem.

So what did they think was going on?

Some of the first people to try to solve the mystery were big names like Lord Kelvin (famous for the temperature uni Kelvin) and Michael Faraday (Mr.

Electrodynamics).

Back around 1850 Kelvin and his brother studied the relationship between pressur and the melting point of water.

For example, the idea was tha if you stand on an icy sidewalk, the weight of your body coul make the ice melt a little bit, creating a liquid layer on the surface of the ice.

That layer would reduc friction, make the ice slippery, and suddenly youre not standing anymore.

Farada also thought that a thin layer of liquid on the surface of ice seemed to be the key to making it slippery.

And in 1859, he proposed that liquid wate always coats the surface of ice, even at temperatures far below freezing.

This idea didnt catch on right away, and Kelvins pressure melting theory was the most widely accepted explanation for a long time.

Then, in 1939 a pair of researchers proposed it was actually friction doin the melting, not just pressure.

Their thinking was that when a material, like an ice skate, is dragged across ice, the heat generated by friction could melt the ice enough to form a thin layer of lubricating water.

As it turns out, while pressur melting and frictional melting can play a role they dont tell the whole story.

The arguments start to fall apart when you get to temperatures of about -20°C, where it would take wa more pressure than an ice skate pushing down on the ice to melt it.

Plus, in an experiment with friction in the 1960s, friction no longer produced enough heat to melt ice at -35°C but the ice was still slippery.

With modern imaging tools, weve finally been able to take a closer loo at the atomic structure of ice.

And weve found that ice isnt such a perfect crystal, after all.

In a study published in 2024 scientists found that at around -150°C, the surface of ice actually has a mixture of Ih and Ic crystals.

And as the temperature increases, molecules near the surface and between the hexagonal an cubic regions become disordered, and the ice loses a lot of its rigid, crystalline structure.

The disordered layers are still somewhat rooted in place because theyre bonded to the bulk of the solid, bu they also have some wiggle room.

At the surface of the ice, in particular, these disordered molecules have dangling bonds that reach outward, like one of those blow up wav arm guys from car dealerships.

Those wavy arms have a lot of extra mobility, but they also have some elasticity, so they can spring back to their original position.

In ice terms, that means the wiggly molecules for a super thin quasi-liquid layer thats way more viscous than liquid water, almost more like oil.

And just like oil that layer acts as a lubricant, which is why ice feels so slippery.

In the end, Faraday wasnt so far off in 1859.

Sure, friction and pressure can play a role, but ice is slippery because well, thats just the way it is.

Its a consequence of imperfections in the ice crystal itself.

And knowing tha is actually kind of a big deal.

A lot of the worlds energy is lost to friction, so new ideas about slipperiness could also mean new ideas for lubrication and manufacturing thats more fuel, cost, and energy efficient.

Or new ways to make winte sports and winter driving safer.

After a hundred years, theres a whole world of ne possibilities for what we can do with ice.