What’s Happening On The Slippery Surface Of Ice?

IRA FLATOW: Hi, I’m Ira Flatow, and this is Science Friday. We’re sliding into your feed to tackle a slippery wintertime question: Just why is ice slippery? Now, ice is classified by scientists as a mineral, as long as it’s naturally occurring. So ice in a snowbank or a glacier, it’s a mineral. It’s a rock.

So you may have wondered as you strapped on your ice skates or struggled down a frozen sidewalk, if ice is a rock, just why is ice slippery? There’s been a common answer to that question that feels a little bit slippery now. Why?

Well, here to help with this is Dr. Robert Carpick. He studies tribology– that’s the science of things like friction, wear, lubrication– and the John Henry Towne Professor in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania in Philadelphia. Welcome back to Science Friday.

ROBERT CARPICK: Thank you so much, Ira. Great to be here.

IRA FLATOW: Nice to have you back. First, I got to ask you, what is the definition of tribology? Where does that originate from?

ROBERT CARPICK: Well, it’s a great question. For quite some amount of time, researchers in the area just called themselves friction engineers, and friction and lubrication engineers, and adhesion scientists. And it was all a bit too many words. And a scientist named Peter Jost in the United Kingdom decided in 1960s we need a name for this field. And he proposed tribology. It comes from the Greek word tribos, which means rubbing or sliding.

And he did what any good Englishman would do. He asked the Oxford English Dictionary to give it their blessing, and they did. And the word was born then. And that’s how the field’s been known, although it’s still not so widely known a term. So I appreciate you giving me a chance to promote it a bit.

IRA FLATOW: Yeah, we’re giving it a little bit of attention here. Seguing into the next topic with tribology, we have been told ever since I can remember that ice itself is not really slippery. It’s a thin layer of water on top that makes you slip.

ROBERT CARPICK: That’s the idea. And it makes so much sense because we know water is so much more slippery than just about anything that’s solid. But there has been a great debate going back for centuries, even Michael Faraday thought about this.

The debate has been, well, there’s probably something water or water like at the surface, but is it always there by itself? Does it appear because you’re pressing on it? They call it pressure-induced melting at the surface. Is it because you’re actually sliding? Frictional heating causes the ice to melt. Or is it something else?

That’s been the great debate. And it’s not settled, but there has been some interesting new research very recently about this that I think has shed some new light on the question.

IRA FLATOW: Well, please, please tell us what that is.

ROBERT CARPICK: Well, sure thing. There’s a researcher at Saarland University in Germany, Martin Muser is his name and his colleagues. They did some very powerful computer simulations. And thanks to the stronger computer power and better algorithms we have now for simulating materials on a computer, their simulations are, I think, pretty reliable, a lot more so than what people did their best to come up with in years past.

So it’s a theoretical computational investigation. But in their simulations, they show a very unusual effect. They find that when you bring two ice surfaces together or just some other surface in contact with ice, the water molecules themselves, they respond to that. They can sense that this other surface has come close to them.

And the reason they can sense it is that water molecules have a very strong electric dipole. When you look at a water molecule, the oxygen atom has more negative charge on it. The hydrogens, they have more positive charge. So you’ve got this separation of charges, and that’s called a dipole.

So if you bring two ice surfaces together or just some other surface that might have some dipoles, the water molecules rearrange. Its electrostatic forces that cause them to rearrange, and they go from being very ordered in a crystalline form, like you have in a solid ice crystal, to becoming very disordered and messy.

And it’s this disordered or, we call it, an amorphous layer on the surface that Muser and his colleagues claim forms, and that this layer is very soft and very slippery. That’s what they say the key is. It’s not pressure. It’s not friction. It’s sensing something in contact that causes this ordered set of molecules to become disordered.

IRA FLATOW: So that would say to me that it really doesn’t matter how cold the ice is. We always say ice could be colder, it’s harder, but there’s no temperature involved here. It looks like.

ROBERT CARPICK: Well, that’s right. And it’s been seen that skiing works perfectly well at very cold temperatures, minus 20 degrees Celsius, for example, you can still ski. So that was one of the mysteries. What these researchers show is, yep, you get this disordering happening even down to very low temperatures.

But they do point something out. The colder you go, the less slippery that disordered layer becomes. You still get it. It’s still going to reduce the friction, but it’s not as good. But it’s really slippery as you get closer and closer to the melting temperature of ice.

IRA FLATOW: OK, so this question is settled now, right?

ROBERT CARPICK: Oh, I doubt it is.

IRA FLATOW: I’m sure it’s– I doubt it. Tell me why it’s not.

ROBERT CARPICK: Oh, well, for one thing, scientists, we love to debate. And I think the researchers themselves say they need to do some experimental testing, more validation. It’s a hypothesis with support for the hypothesis from the computer simulations. But more work needs to be done to really validate it.

IRA FLATOW: Let me put your friction hat on. And being an expert in friction, I know, for example, in skiing you might want to wax your skis. How much of the slipperiness comes from the ice versus the surface of the thing in contact with? And why is the wax so important? What’s there about the interaction at the surfaces of both those things?

ROBERT CARPICK: Yeah, again, I may sound like a little bit of a broken record, but there is still a debate about this. But we know a few things.

IRA FLATOW: Uh-huh.

ROBERT CARPICK: One is wax, as we know, is water repellent. So when you put it on top of the snow, and, well, according to this new theory, you would get this disordering of the water molecules and the ice molecules right at the surface. Well, those water molecules, they do not want to be close to that wax. They don’t like it. They like each other better.

They don’t like the wax. That helps reduce the energy of interaction between the underside wax surface of the ski and the water beneath it. And when you have that kind of condition, water in contact with something it’s not attracted to, well, you get low friction. You can slide very easily.

IRA FLATOW: Are there things you can do to predict or prevent things from sticking to each other, like prevent ice formation?

ROBERT CARPICK: Well, that’s a key question that us tribologists wrestle with. Can we actually control friction and stickiness and adhesion? There’s all sorts of applications where you want to do that. And I actually wanted to point out a really interesting study that just came out this past summer.

This is the group of Professor Kevin Golovin and his coworkers at the University of Toronto. They came up with a way to detect if ice is forming on a surface. And they, in particular, point out this is really useful for airplanes and even things like UAVs, drones. If ice collects even on those small propeller wings of drones, it can cause them to crash. Well, they used a funny method called a Triboelectric Nanogenerator, TNG. What that does–

IRA FLATOW: Sounds like something Back to the Future-ish.

ROBERT CARPICK: Yeah, it really is. It actually comes almost out of Star Trek, you would think. Yeah, well, these tribal nanogenerators are cool devices. When you apply friction to them or other forces, they generate electricity.

And Golovin and co-workers found a really neat use for them. They found that when water condenses on the surface of one of these nanogenerators and it freezes, there’s a frozen front, a line between the liquid and the solid phase of the water and ice. And that line spreads out. So it’s like that contact line, It’s called, spreads out across the surface.

It’s creating a little bit of friction and force on the nanogenerator. That creates some charge. The charge gives you a signal. You can read, and you can say, hey, look, ice is forming. And even better, the charge that is created can even be used to help melt the ice itself. So they are using it both as a detector to see if ice is forming and as a way to get rid of the ice once it’s there.

IRA FLATOW: That is pretty cool. Speaking of ice and slipping, we’re about a month out from the start of the Winter Olympics. And I know you’re into curling, right?

ROBERT CARPICK: I am.

IRA FLATOW: That’s got to be– two surfaces, rock on ice. Are there science questions to be answered still there?

ROBERT CARPICK: There are indeed. There has been a hot debate over the years as to what is it that makes a curling rock curl. And for anybody who doesn’t know, in the sport of curling, when you throw that granite rock down the ice, you put a spin on it.

And if you put a clockwise spin on it, the rock will move, or, we say, curl to the right. And if it’s a counterclockwise spin, it moves to the left. This lets you have some control over the final position of that curling stone, so you can hide a rock ultimately behind another one that’s further up. Or you can knock a rock out that might look like it’s hidden, protected behind another one. And this is why people call it chess on ice.

Just a couple of years ago, a researcher in Japan outfitted some curling stones with these high-precision position sensors, and it’s pretty interesting. These sensors– he borrowed it from another research project. They’d been used for some high-precision gravity measurement experiments and also for some optical alignment experiments from a high-energy particle accelerator, so completely different use.

He takes these things, he puts them on some curling stones. He takes them to Karuizawa ice park in Nagano, and then he just measures the motion of all of these curling stones.

And he gets an athlete, an Olympic athlete, to help him with this. And what he finds is by tracking the motion and doing some analysis, he can figure a bunch of things out, including how the friction force changes with speed and how the motion of the curling rock changes. It’s not just spinning around the same central point. The point about which it spins changes as it moves down the ice.

So the summary of all this is he confirmed something people had proposed, but he measured much more precisely how friction between that granite rock and the ice changes with speed. And it goes way down as the speed goes up. So friction drops like a rock, pardon the pun, as you increase the speed.

And the thing that was new is they found it goes way up at the lowest speeds. Nobody had ever seen that before. This, combined with some other analysis they did, does shed some new light on the problem. And I think that it could even be useful for athletes to plan how they might want to throw the rock based on these studies. So very cool study just came out a couple years ago.

IRA FLATOW: So much to talk about. So great to have you. Thank you for taking time to be with us today.

ROBERT CARPICK: Thank you so much, Ira. I appreciate it. Have a good one, and happy 2026.

IRA FLATOW: Thank you. Dr. Robert Carpick is the John Henry Towne Professor in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania in Philadelphia.

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