# Ice skating, how does it really work?

Okay, some textbooks I came across, and a homework assignment I had to do several years ago, suggested that the reason we can skate on ice is the peculiar $p(T)$-curve of the ice-water boundary. The reasoning is that due to the high pressure the skates put on the ice, it will melt at temperaturs below $273 K$ and thus provide a thin film of liquid on which we can skate. It was then mentioned as fun fact that you could ice-skate on a planet with lakes of frozen dioxide because that gas has the $p(T)$-curve the other way round.

My calculations at that time told me that this was, pardon my french, bollocks. The pressure wasn't nearly high enough to lower the melting point to even something like $-0.5$ degrees Celsius.

I suppose it is some other mechanism, probably related to the crystal structure of ice, but I'd really appreciate if someone more knowledgeable could tell something about it.

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I'm pretty sure this was a problem on the final exam for my undergrad thermodynamics class ;-) so at least I think your calculation is reasonable. I don't know/remember what the real reason is though. –  David Z Dec 7 '10 at 5:31
Well, this analysis completely ignores that when you skate you are not standing but you are actually moving. There should be some friction between the skates and ice and this should provide enough heat to melt the ice and create a thin water film. At least this is my intuition (perhaps completely wrong). –  Marek Dec 7 '10 at 11:44
The binding energy near a surface is different than the binding energy in bulk, and it is possible that you melt a thin surface layer without melting the bulk. –  Ron Maimon Aug 21 '11 at 14:14

Yup, this is true that the pressure is too small, but the true explanation is not justified yet. Nevertheless the common sense is that there is a lubricating film of water or at least anomalous ice. For an overview, see: http://lptms.u-psud.fr/membres/trizac/Ens/L3FIP/Ice.pdf

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The pressure is too small for bulk melting, but surface melting is different, and this is the relevant issue. Compressing a surface of water-ice will melt some surface, but compressing the surface of other materials will solidify any surface liquid, because liquid ice has a smaller volume. The explanation is fundamentally correct, the bulk melting is irrelevant. –  Ron Maimon Sep 23 '11 at 17:10
@RonMaimon: If there are any irregularities in the ice, or the blade, wouldn't the pressure at those points be nearly infinite unless or until the H2O underneath them gave way? I would think at least some of the weight of the skater would be borne by liquid water unless the compressed water liquified, reshaped itself to a lower-pressure configuration, and refroze. Would skis be effective at -35 on a polished frozen sheet of ice? –  supercat Nov 29 '14 at 16:14

It was shown that surface water molecules vibrate more strongly than those in the bulk, having less neighbor molecules to interact with. Apparently, this creates a nanometric film of quasi-liquid water that reduces friction.

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this is only true up to certain temperature below which you need not have a layer of water. –  mythealias Nov 10 '12 at 18:41

I remember reading in a book (on surface physics) during my grad study on this topic. There was a diagram on friction of a steel "skate" on solid argon at and below argon melting temperature. The diagram was qualitatively identical to the same experiment for ice. Friction dropped to low values when temperature aproached melting point. Argon melts regularly, for that reason pressure melting is not possible. I regret that I did not memorize the title and author of that book :=( Georg

Another fact against 2pressure melting": how does skiing work? The pressure under a ski is very low.

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That doesn't argue against pressure melting at all. Why would you expect skiing and skating to exploit the same mechanism? Why would you expect snow and solid ice to have the same properties? –  David Richerby Feb 12 '14 at 14:41