Premelting - Ice Skating

Ice Skating

It should be noted first that the friction coefficient for ice, without a liquid film on the surface was measured to be . Friction coefficients for ice skaters are around or below 0.005. So there is a need for a water film on the ice surface in order to explain the small friction appearing in ice skating. Three possible mechanisms could account for a film of liquid water on the ice surface:

• Pressure: Because of the abnormality water ice can be melted by raising the pressure in the system
• Premelting: Due to premelting there is always a thin film of liquid water on the ice surface
• Friction: The sliding on the surface causes heat and melts the ice to a liquid water film

To make it clear before: Until now scientific community is not sure which mechanism makes ice skating possible. And it could easily be that all three effects play a role but people want to know whether one effect could it all do on its own. For several decades it was common to explain the low friction of the skates on ice by pressure melting. But till today several arguments were found which contradict this thesis. The strongest argument against pressure melting as the only effect responsible for the low friction on ice is that ice skating is still possible below temperatures of -20 °C under which ice can not be melted due to pressure anymore (see phase diagram). But pressure below about 0.2 GPa could still result in a lowering of the melting temperature. Nowadays scientists mostly look for the origin of the water film in the two later effects and there are again arguments in favour or against the one or the other. The thickness of the film is an argument which is all in favour of the friction argument, as these thicknesses could reach the orders of whereas film thickness due to premelting at lower temperatures, e.g. -10 °C, is in the orders of nm. De Koning et al. found in their measurements that the adding of impurities to the ice can lower the friction coefficient in orders of 15%. When assuming that impurities do not play such a big role for friction melting, this indicates that a film of premelted water could play a certain role in ice skating. On the contrary they found that the friction coefficient increases with skating speed what would not support the friction argument, but could also be due to a different skating technique at higher speeds. This is not the end of the story as there are several more arguments in favour or against premelting or friction. But one can summarize that the main problem of the premelting argument is the thin layer it provides. On the other hand one could say that 0.6 is a friction coefficient comparable to rubber and bitumen (roughly 0.8). Hence the question is whether one needs as much force to push a resting ice skater as pushing a resting normal person.