Experiments show that ice has a minimum kinetic friction at −7°C (19°
F), and many indoor skating rinks set their system to a similar temperature. The low amount of friction actually observed has been difficult for physicists to explain, especially at lower temperatures. On the surface of any body of ice at a temperature above about −20°
C (−4°
F), there is always a
thin film of liquid water, ranging in thickness from only a few molecules to thousands of molecules. This is because an abrupt end to the
crystalline structure is not the most
entropically favorable possibility. The thickness of this liquid layer depends almost entirely on the temperature of the surface of the ice, with higher temperatures giving a thicker layer. However, skating is possible at temperatures much lower than −20°
C, at which temperature there is no naturally occurring film of liquid.
When the blade of an ice skate passes over the ice, the ice undergoes two kinds of changes in its physical state: an increase in pressure, and a change in temperature due to kinetic friction and the heat of melting. Direct measurements
[3] show that the heating due to friction is greater than the cooling due to the heat of melting. Although high pressure can cause ice to melt, by lowering its melting point, the pressure required is far greater than that actually produced by ice skates. Frictional heating does lead to an increase in the thickness of the naturally occurring film of liquid, but measurements with an atomic force microscope have found the boundary layer to be too thin to supply the observed reduction in friction.
