What Makes Ice Slippery? Scientists May Have Finally Figured It Out

[CLIP: Skates trim crossed nan crystal astatine an crystal rink, and euphony plays successful nan background.]

Kendra Pierre-Louis: So we’re retired present coming successful little Manhattan ice-skating. There are tons of kids skating around, dudes successful lucky skates, and I’m present getting my soul Michelle Kwan on.

[CLIP: Skates trim crossed nan crystal astatine an crystal rink, and euphony plays successful nan background.]

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Pierre-Louis: If you really deliberation astir it, crystal skating is conscionable controlled slipping connected ice. And whenever I spell skating I can’t thief but deliberation astir nan Winter Olympics, for illustration nan ones that are happening correct now successful Italy.

[CLIP:NBC Olympics commentator Sloane Martin discusses a mixed doubles curling title connected February 6: “Welcome wrong nan Cortina Curling Olympic Stadium. The U.S. mixed curling duo of Korey Dropkin and Cory Thiesse faces Team Canada successful a matchup of unbeaten teams.”]

Pierre-Louis: And watching nan Olympics I started to recognize that if you really deliberation astir it, truthful galore of nan Winter Olympic sports are conscionable astir controlled slipping connected ice, for illustration bobsledding, nan luge, curling. And yet scientists still don’t really cognize why crystal is slippery.

Sure, they person theories, for illustration nan unit that we put connected nan crystal possibly melts nan ice, creating a bladed watery layer. But scientists mostly work together that those theories aren’t nan afloat picture. The slipperiness of crystal is really still a mystery.

Paulina Rowińska: It’s specified a elemental mobility that should person been answered hundreds of years ago. But turns retired each nan worldly we learned successful school, it’s not afloat correct, for illustration pinch many, galore different things.

Pierre-Louis: So coming we’re going to effort to get immoderate answers.

[CLIP: Theme music]

Pierre-Louis: For Scientific American’s Science Quickly, I’m Kendra Pierre-Louis, successful for Rachel Feltman.

There are astatine slightest 3 long-standing technological theories that effort to explicate what makes crystal slippery.

One of nan oldest imaginable explanations dates backmost to nan mid-1800s. It comes from a Scottish technologist named James Thomson, nan older relative of celebrated physicist Lord Kelvin. And it involves nan unit that an entity exerts connected ice.

Rowińska: We cognize that nan pouring somesthesia is mostly zero degrees Celsius, aliases 32 [degrees] Fahrenheit. And supra that we person water; beneath that we person ice. But past unit changes this—changes nan properties of water.

Pierre-Louis: That is Paulina Rowińska, who we besides heard astatine nan apical of nan episode. She’s a subject journalist astatine Quanta Magazine, and she wrote an article successful December that dug into nan competing theories astir why crystal is slippery.

One of nan hypotheses is that erstwhile we measurement onto nan crystal we put unit connected it, perchance lowering nan pouring point.

Rowińska: It’s freezing retired there, but we are getting person to nan pouring temperature, truthful we mightiness beryllium melting, like, nan aboveground furniture of ice. And past it would get benignant of a furniture of water, and we all—like, h2o is slippery because it’s a liquid, not a solid.

Pierre-Louis: Thomson came to this idea—that unit connected nan crystal fundamentally creates a liquid layer—by studying glaciers, says Martin Müser.

Martin is simply a theoretical physicist successful nan Department of Materials Science astatine Saarland University.

Martin Müser: Glaciers, location is simply a heavy, dense load that sits connected nan points of contact, and we cognize that erstwhile we are a small beneath nan freezing constituent and we compression connected ice, it becomes liquid. So [Thomson] based on that crystal liquefies because of nan pressure.

Pierre-Louis: But there’s a problem pinch nan unit hypothesis—a large one, according to Daniel Bonn, a professor of physics astatine nan University of Amsterdam.

Daniel Bonn: You would request 10 elephants resting connected a azygous skate successful bid to get a decent magnitude of pouring owed to nan pressure.

Pierre-Louis: Given that humans do not measurement arsenic overmuch arsenic 10 elephants and we still negociate to gaffe connected ice, unit unsocial does not look to beryllium why crystal is slippery. So different mentation emerged—and this 1 is particularly celebrated among tribologists, scientists who study friction, lubrication and deterioration betwixt moving surfaces.

Here’s Martin again.

Müser: When you talk to a tribologist, successful peculiar successful nan section of crystal friction, they would travel up pinch an mentation that was projected by Frank Bowden. And he made a very neat research successful nan Alps wherever he put 2 identical skis—they had nan aforesaid weight, nan aforesaid aboveground finish, nan aforesaid everything—but 1 conducted nan power wrong a small amended than nan different one. And nan 1 that conducted nan power little good was noticeably faster. So he said, “Look, what happens is because you person friction, you get heat. When [there’s] heat, it melts nan water, and nan much power is retained successful nan contact, nan amended it is. What happens is pouring by frictional heating.”

Pierre-Louis: The thought that Bowden laid retired pinch different beingness chemist, T. P. Hughes, is that arsenic we locomotion aliases skate connected crystal nan clash we create heats and melts nan surface. The conception is called frictional heating.

Daniel’s squad did experiments wherever they measured nan clash connected crystal complete a very ample somesthesia range, from -100 degrees C, aliases -148 degrees F, to nan freezing constituent of water.

Bonn: And past each kinds of absorbing things happen. At very debased temperatures—you astir apt don’t want to beryllium ice-skating anyways astatine -100 [degrees C], but it’s really intolerable to ice-skate because nan friction’s very high. But past expanding nan somesthesia from those very debased temperatures nan clash decreases extremely rapidly.

Pierre-Louis: Daniel and his colleagues recovered that nan clash decreased until a somesthesia of astir -7 degrees C, which is astir 20 degrees F. But erstwhile they went person to nan pouring constituent of crystal nan clash went up again.

Bonn: And this is thing that you’ve knowledgeable if you do ice-skating: if nan crystal is excessively warm, it really becomes mushy and you time off traces successful nan ice, which—this is what we telephone plowing friction. And truthful we were very happy because we recovered that location was an optimum somesthesia for ice-skating, which is -7 degrees C. And truthful we went to our ice-skating rink, and they said, “We’ve known this for galore decades.”

Pierre-Louis: These experiments reinforced for Daniel that nan reply to what makes crystal slippery lies beyond friction.

Sure, frictional heating mightiness beryllium responsible for pouring nan crystal successful our wake—that is, pouring nan crystal down us—but arsenic we each cognize crystal is slippery earlier you’ve moreover stepped connected it, earlier clash has moreover occurred.

Bonn: As you cognize it’s difficult to stay opinionated connected nan ice, moreover astatine zero speed, yeah? And truthful we don’t deliberation that nan sliding itself has thing to do pinch it.

Pierre-Louis: So if it’s not unit aliases friction, could crystal beryllium slippery because there’s already a pre-melted furniture of h2o connected top?

Paulina says that’s what a 3rd presumption suggests.

Rowińska: So this has to do pinch really crystal is structured. So, you know, crystal is conscionable water. So we person h2o molecules, but they are system successful a very ordered way, truthful they shape bonds, and they—it’s benignant of for illustration a bully lattice. That’s why crystal is solid. In a liquid, for illustration liquid water, nan molecules are benignant of moving freely, and nan building is overmuch looser.

So nan thought is that adjacent to nan aboveground of crystal nan bonds are overmuch weaker, truthful it’s benignant of for illustration a bound betwixt 2 different materials. So it’s not that nan aboveground of crystal is melting, but location is, like, a pre-melted furniture of h2o connected apical of crystal because of these structural differences betwixt h2o and ice.

Pierre-Louis: This theory, for illustration nan unit hypothesis, dates backmost to nan 1800s. It was first projected by English chemist and physicist Michael Faraday, Martin explains.

Müser: He fundamentally saw that from putting 2 crystal cubes together, and erstwhile they were caller they would slide, but if he would hold a spot longer time, they were stuck. So a single, basically, interface would form, and he said, “Well, they are slippery because location must beryllium a very bladed lubricating layer.” And successful nan past 30, 40 years location was a batch of experimental effort proving nan beingness of this layer.

Pierre-Louis: But this theory, too, has holes. Among them, Martin says ...

Müser: Is this furniture is comparatively thin. And a very bladed layer, moreover if nan viscosity of nan liquid is arsenic mini arsenic that of water, would still springiness rather noticeable friction. [Laughs.]

Pierre-Louis: So let’s recap: nan 3 long-standing, starring hypotheses arsenic to why crystal is slippery are, one, nan unit applied by an entity melts nan ice, causing america to slip. Two, clash heats nan ice, causing america to slip. And three, crystal has a bladed furniture of pre-melted h2o that, again, causes america to slip.

On nan 1 manus each of these theories person flaws. On nan different manus machine simulations tally by a group of European scientists a fewer years agone propose that it mightiness not beryllium immoderate 1 of these theories but each of them together.

You mightiness beryllium asking yourself, “Why does immoderate of this matter? We know that crystal is slippery. Does it really matter if we cognize why?” For Daniel, who is Dutch, it’s really a matter of nationalist pride.

[CLIP: “Het Wilhelmus”]

Bonn: So nan astir important exertion for nan Dutch is getting golden medals astatine velocity skating, yeah? So we are nan champion velocity skaters successful nan world, and we wanna support it that way, and truthful we besides want to person nan fastest ice-skating track.

Pierre-Louis: But it besides matters if you’re not Dutch, he says.

Bonn: It’s highly absorbing to deliberation astir why things are slippery because if you tin understand what’s happening there, you mightiness beryllium capable to transportation that knowledge to different systems. And truthful things that are highly slippery are highly absorbing because clash is responsible for an estimated 25 percent of nan world power consumption, yeah? And truthful nan clash connected crystal is astir an bid of magnitude little than clash connected each different materials. And truthful if you could transpose that to each nan moving parts successful nan world, you would prevention almost 25 percent of nan world power consumption.

Pierre-Louis: In different words crystal is slipperier than astir different materials. If scientists could fig retired why, they mightiness beryllium capable to mimic its behaviour for usage successful different applications, for illustration train tracks aliases motorized energy. This would let america to suffer little power to friction, cutting power usage successful nan process.

But let’s travel backmost to nan eternal mobility of why crystal is slippery. Last twelvemonth a caller mentation surfaced.

Here’s Paulina again.

Rowińska: So a caller presumption came retired past twelvemonth successful a paper, and nan thought is: it’s not really astir melting. It’s about, almost, like, a mechanical moving of atoms and molecules connected nan surface. So you cognize really erstwhile we measurement connected ice, we benignant of destruct nan building because location are immoderate almost, like, electrostatic attraction—it’s not precisely electrostatic, but it’s, like, an attraction betwixt molecules of our footwear and of ice. But past we support going—we support skiing; we support walking—and we benignant of support attaching and disattaching these molecules.

So location is—they telephone it, like, an amorphous layer, truthful it’s a furniture that’s liquidlike, but it’s not really liquid because it’s very thin. So it’s not really water. It’s not really ice. It’s thing successful between.

Pierre-Louis: And it was Martin’s squad that published this mentation successful nan diary Physical Review Letters.

He said that to understand nan thought ideate stacking a bunch of ovum cartons.

Müser: You put them perfectly successful parallel, of people they’re going to stick. But crystal crystals will ne'er beryllium oriented that well; they will beryllium misoriented. And very overmuch to my astonishment did I spot that if I put 2 misaligned crystal crystals successful contact, moreover if they spell extremely, highly adjacent to absolute zero, would I spot amorphization correct away.

Pierre-Louis: In different words alternatively of nan crystalline building 1 mightiness expect crystal to have, nan aboveground building starts to autumn isolated a small spot and go much disordered. It’s not rather a coagulated anymore.

Müser: So we saw this very accelerated amorphization astatine 10 kelvin, and past we said, “Hey, now let’s look what happens astatine -10 degrees C,” aliases successful Fahrenheit—I don’t know, roughly, it’s astir 10 [degrees] F aliases 12 [degrees] F. And we did spot that nan h2o besides liquefied.

Pierre-Louis: To understand what Martin is getting astatine it helps to understand nan quality betwixt an ordered and a disordered solid.

An ordered coagulated is erstwhile each of nan atoms are arranged successful a precise, repeating 3D structure—think astir a phalanx of Roman soldiers each lined up. Those structures tin grip a batch of stress. For example, astir metals are ordered structures.

A disordered solid, though, is much chaotic. The structures tin beryllium much random, little repeating.

When h2o becomes crystal nan outer layers are disordered; their building looks for illustration an unfastened honeycomb. When we measurement connected that aboveground with, say, a sneaker aliases crystal skate we break up that structure, introducing accent into nan system. As nan crystal useful to set to that accent it creates an amorphous layer—something that’s not rather liquid and not rather solid—that causes america to slip. Or astatine slightest that’s nan latest theory.

Müser: Sometimes group inquire maine if group judge nan reply and I ever say, “I dream not,” because immoderate [Laughs], immoderate good, nontrivial, correct technological reply is met by a batch of skepticism.

Pierre-Louis: So nan adjacent clip you spell crystal skating aliases swipe retired connected an icy sidewalk you’ll astatine slightest person a clearer thought of why it mightiness person happened.

That’s it for today. Tune successful connected Monday for our play subject news roundup.

Science Quickly is produced by me, Kendra Pierre-Louis, on pinch Fonda Mwangi, Sushmita Pathak and Jeff DelViscio. This section was edited by Alex Sugiura. Shayna Posses and Aaron Shattuck fact-check our show. Our taxable euphony was composed by Dominic Smith. Subscribe to Scientific American for much up-to-date and in-depth subject news.

For Scientific American, this is Kendra Pierre-Louis. Have a awesome weekend!

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