It is not, because ice has a smooth surface and that probably the surface layer melts a bit because your shoes are warm?

That would be my reasoning, but it likely seems to be wrong, well why?

The usual hypothesis is that a microscopic layer of the surface melts, not because your shoes are warm, but because of the pressure. But the pressure is not enough to do that.

That said, as anyone who has been in icy conditions can attest, it is clear that ice at ~0° C is vastly more slippery than at, say, -20°C.

The pressure from well distributed static load divided by area is not, but on the tiny scale where the difference between grippy and less grippy surfaces happens there isn't so much "well distributed" going on. And once you add lateral force to the mix all bets are off: there will always be a point getting better grip (is surface interference the correct term?) than others, see a local force concentration and when that causes a tiny spot of phase transition there will very soon be a new point of least local slippyness getting all the attention of lateral force.

My understanding is that, at the borders, master is constantly changing between its states. A boiling pot has water molecules turning to steam, as well as vice versa. It just has more of the first probabilistically.

Maybe the same happens with ice and water.

That's more or less it, as far as I understand. There's the hexagonal lattice of solid water ice, where hydrogen bonds form a 3D tetrahedral structure which is very stable. At the boundary, there are no molecules "above" the solid latice, so those water molecules are held less tightly. The exact dynamics are challenging to predict because we can't exactly see what's going on (most simulation based).

", it is clear that ice at ~0° C is vastly more slippery than at, say, -20°C."

There is nothing more slippery in my experience, than rain on the street, that just turned frozen. But ice at -20°C is usually older and has not such a smooth surface anymore, so hard to compare in non lab settings.

If that were the case, then cold things wouldn't slip on ice, but they do (for a demonstration, pick up a rock that's been sitting out in the cold and note that it still slides on ice)

Wouldn't that just be due to the low coefficient of friction?

> Wouldn't that just be due to the low coefficient of friction?

You mean, its slipperiness?

I'm not sure a cold rock sliding across very cold ice will slide any better than if the ice was another very large smooth rock. Ice is hard and smooth. Hard smooth things have low friction.

You're talking about the "2D gas vs liquid" discussion?

Link to the research?

I had to look that up. Here's research[1][2] from 2019 about "why ice is so slippery", including some of the apparent surprises. I dunno. I'm not a physicist so some of the surprises seem like very minor things to me.

Spoiler: According to this research, ice is slippery because of a thin layer of water. (As we expected, no?) But the water layer is 1) thinner than expected, 2) more viscous than expected, and 3) contains bits of ice which help make it extra slippery.

> "Usual explanation is completely incorrect."

Only for an unusual definition of "completely" IMO. Again, IANAP.

[1] https://phys.org/news/2019-11-ice-slippery.html

[2] https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.041...

I remember listening to joe rogan when he interviewed neil degrasse tyson and he had some fascinating insights into the behavior of water with relation to pressure and temperature.

I can imagine ice on an iceskate wanting to be solid, but because that takes up too much space it becomes a liquid instead.

> contains bits of ice which help make it extra slippery

Wait, why would bits of ice make it extra slippery if it's the water that makes ice slippery?

I assume like walking on marbles or ball bearings.

Maybe the ice molecules move through the water with less resistance, since the ice molecules are already 'tied up' in a lattice structure and therefore do not form as strong of hydrogen bonds with the neighboring water molecules.

> Only for an unusual definition of "completely" IMO.

Agreed. While the "usual explanation" may lack the details that actual testing provided, deduction and logic in this case were pretty much harmonious with the findings. The water layer being thinner or more viscous than expected doesn't invalidate the basic assumption, and that's where all science starts, right? Some basic, yet to be disproved* assumption.

Exactly, this it’s the new research. Standard (incorrect) explanation is because ice melts because of the pressure from the object put on it. Robert Wood showed it to be untrue at the beginning of the twentieth century, but “popular scientists” didn’t get the memo.

Ice isn't slippery, water is. If anything I'd say ice is actually pretty sticky. A very cold ice cube will easily stick to your skin.

That's just because your skin has water molecules that freeze on contact with the ice cube. If you use tongs to put an ice cube on your desk, it doesn't stick at all, right? And the friction is very low so it's easy to kick away -- things like a leather wallet stick much better to your desk.

I've not tried it but intuitively I'd expect it to slide about as well as other very hard things of the same shape.

I've dropped ice cubes on the floor before and they definitely slide a lot more than most objects of the same shape.