# How does iceskating work?

I was willing to understand a little bit better the physics behind iceskating.

Messy thoughts

From New Scientist (1964) (probably not the most up-to-date reference $\ddot \smile$), I found a diagram showing the relationship between both the static and kinetic coefficients of friction and the temperature (x-axis is reversed) for ice.

When we ice skate, we put a big pressure on the ice. As the ice is less dense than liquid water, this pressure may eventually be high enough to turn ice into liquid water. Intuitively I would think that high pressure would yield to high temperature but I suppose that this would hold true exclusively for gazes ($PV=nRT$). In any case, I don't quite have a good intuition for why warmer (or more pressurized eventually) ice would decrease the coefficient of friction of ice.

Questions

• What is the intuition behind the fact the coefficient of friction of ice decreases as temperature increases?

• Does the coefficient of friction of ice decreases with higher pressure too? For the same reason as above? As a correlated-side effect of increasing temperature?

• Why do ice skates slide so well? Because it puts a high pressure on the contact surface or because it puts a high pressure on the contact surface which in turn increases the temperature?

• Possible duplicate of What makes ice slippery? Feb 21 '16 at 8:51
• I don't think this is a duplicate. The linked question is related but this one is specifically about the effects of temperature and pressure, which the linked question does not address.
– bon
Feb 21 '16 at 9:56
• Maybe it has something to do with the effect of pressure on the melting point of ice. Check the phase diagram. As the pressure increases, the melting temperature decreases. I just don't know if the pressures under the blade of a skate are high enough to allow the ice to melt so that a layer of water could provide lubriction. You might check this using an estimate of the contact area and the weight of the person. Feb 21 '16 at 12:47
• Similar question on Physics: physics.stackexchange.com/q/1720
– user7951
Jun 28 '20 at 15:47
• Note that the topic is much trickier than it appears, consisting from several levels. The first level is obvious reason of decreasing melting point under pressure, working down to about -20 deg C. Next one is heating up by friction. But the last levels are not understood at all. Jun 29 '20 at 7:35

## 4 Answers

Calderon & Mohazzabi give an excellent summation of the various theories proposed through the years to explain why ice is so slippery in their 2018 paper "Premelting, Pressure Melting, and Regelation of ice revisited" in the Journal of Applied Mathematics and Physics.

They offer both theoretical and experimental evidence that neither pressure melting nor friction melting explain the phenomenon and conclude from atomic force microscopy, among other evidence, that there is a pre-melting Quasi-​Liquid surface layer with special properties that make ice skating possible.

References

1. Calderon, C. and Mohazzabi, P. (2018) Premelting, Pressure Melting, and Regelation of Ice Revisited. Journal of Applied Mathematics and Physics, 6, 2181-2191. doi: [10.4236/jamp.2018.611183.][1]

There's a very nice—and fairly accessible—discussion of your third question in Nature Magazine's News and Views Section[1]. In this article, Bonn discusses the results of a fairly recent work that appeared in Physical Review X[2]. Bonn summarizes the key points as follows:

The idea that a thin film of meltwater wets the surface of ice has been accepted since the nineteenth century...[but]...Water is not a good lubricant, because its low viscosity means that it is easily squeezed out of gaps. The idea that a layer of water is sufficient to lubricate a skate on ice is therefore strange. It doesn’t even make intuitive sense, given that it is impossible to skate on a road or a kitchen floor with a layer of water on it...[The explanation thus seems to be that water and ice together]... form a viscoelastic, liquid–solid third body in response to friction and wear. (Emphasis mine)

References

1. Bonn, Daniel. The physics of ice skating. Nature 577, 173-174 (2020). link: https://www.nature.com/articles/d41586-019-03833-5

2. Canale, L., Comtet, J., Niguès, A., Cohen, C., Clanet, C., Siria, A. and Bocquet, L. Nanorheology of interfacial water during ice gliding. Physical Review X, 9(4), p.041025 (2019). link: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.041025

• @Nilay Ghosh: Thanks for making my references pretty :). However, the phrase "and fairly accessible" is parenthetical, and thus needs to be separated by parentheses, commas, or dashes. Hence I've restored the dashes you removed. The test for whether a phrase is parenthetical is whether you can remove it without changing the essential meaning of a sentence. For more information, see: englishclub.com/grammar/sentence/parenthetical-expression.htm Jul 3 '20 at 1:36

My understanding of ice skating is that it has to do with the pressure of the skate, melting the ice and you skate on the melted water. Look at the phase diagram of the water. At 1 atm, the water is solid, but at higher pressures (keeping the temp. the same) the water changes to a liquid.

• The effect is called regelation. You can cut ice with a thin steel wire and weights. Since ice expands when it freezes, pressure at the freezing point can cause thawing. May 12 '18 at 20:06

This question has more physics into it than chemistry but the ice melting to provide a slippery surface (and guess subsequently re-freeze) makes sense. We can also try asking someone to glide over dry-ice made out of carbon-dioxide instead of regular ice and see if there is any increase or decrease in the friction .

Two possibilities -

• (a) If water contributes then the idea is that dry-ice under pressure would turn directly to gas and as such not contribute to being slippery or
• (b) The process of solid turning to gas makes the surface even more slippery than normal - but the process would be inherently destructive since the gas will not re-solidify back into ice.

Alternatively we can try out an experiment using ice-cubes out of water and dry-ice and run it over a butter knife and apply some pressure to simulate a person skating on ice.