Borek said:
Ice melts under pressure, creating a thin layer of water that makes friction much lower. Perhaps you should search along these lines, for sure that's how it works in at least some cases.
That is a widely used explanation for why ice is so slippery. You can find this meme expressed all over the internet, and even in some physics classes and physics texts. It is however completely wrong. The math just doesn't add up with the pressure melting explanation for why ice is slippery.
- Consider an 80 kg hockey player who has all of his weight on a 10 mm by 3 mm section of a single hockey skate blade. This would lower the melting point of the ice under that blade by a couple of degrees. Yet the best ice for hockey is about -9°C, well below the melting point under that single blade. The pressure explanation doesn't work.
- Consider a petite 40 kg figure skater who has her weight on both blades. Figure skates are considerably flatter and a bit wider than hockey skates; I'll use 100 mm×5mm as the contact area for one blade. She reduces the melting point under her skates by a mere 0.03°C. The best ice for figures is about -5°C, well below this -0.03°C figure. Once again the numbers don't add up.
- Consider a 20 kg young boy who walks on ice at -10°C in his Sunday dress shoes. He reduces the melting point of the ice by an imperceptibly small amount. That the melting point of the ice under the boy's shoes is well above the ice temperature doesn't stop the boy from slipping and falling.
- Pressure melting cannot be responsible for the low friction of ice. See "resources" below.
So if it isn't pressure, what is it that makes ice so slippery? There is indeed a layer of water between the blade and the solid ice, but it isn't pressure that makes this happen. Three factors come into play.
- There is a thin water-like layer at the ice/air boundary, even at temperatures down to -157°C. This layer is just a molecule thick at this extreme cold. It's significantly thicker at warmer temperatures. Near 0°C, that water-like layer becomes a bit too thick for proper skating. Instead of gliding over the ice, the skate has to slosh through that now thick water-like layer.
- The surface in contact with the ice has to be somewhat flat so that water-like layer can be present between the surface and the ice. Ice isn't very slippery at all to a needle dragged across ice. This also explains why ice cleats work. Note that the pressure melting explanation fails completely here; a pinpoint surface maximizes pressure.
- Frictional heating also appears to play some role. The water-like layer in ice below -35°C is thinner than the microscopic roughness of even the most finely ground ice skate blade. Extremely cold ice or snow isn't all that slippery if one isn't moving. Start moving, however, and friction warms the ice below the blade to the point where a water-like layer thick enough to mask that microscopic roughness can form.
Some resources:
S.C. Colbeck,
Pressure melting and ice skating, American Journal of Physics, 63:10 (1995)
Pressure melting cannot be responsible for the low friction of ice. The pressure needed to reach the melting temperature is above the compressive failure stress and, if it did occur, high squeeze losses would result in very thin films. Pure liquid water cannot coexist with ice much below -20 °C at any pressure and friction does not increase suddenly in that range. If frictional heating and pressure melting contribute equally, the length of the wetted contact could not exceed 15 μm at a speed of 5 m/s, which seems much too short. If pressure melting is the dominant process, the water films are less than 0.08 μm thick because of the high pressures.
R. Rosenberg,
Why is ice slippery?, Physics Today 58:12 (2005)
Everyday experience suggests why ice surfaces should be slippery: Water spilled on a kitchen floor or rainwater on asphalt or concrete can create the same kinds of hazards for walkers and drivers that ice can. Presumably, the liquid makes the surface slippery because liquids are mobile, whereas solid surfaces are relatively rigid. Asking why ice is slippery is thus roughly equivalent to asking how a liquid or liquid-like layer can occur on the ice surface in the first place.
You can find this article on the internet, but given the strict rules about copyright at this site, I'm not going to post the link. Search and yea shall find.
Y. Li and G. A. Somorjai,
Surface Premelting of Ice, J. Phys. Chem. C 111:27 (2007)
In this review, we summarize the available experimental data from recently developed molecular level techniques on the surface structure, surface premelting layer thickness, and friction of ice. We conclude that surface premelting of ice is responsible for the unique surface properties of the important substance.
Also see
Science of Hockey: Why is Ice Slippery at
http://www.exploratorium.edu/hockey/ice2.html
Explaining Ice - The Answers are Slippery at
http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all&_r=0
