It is a common misconception
that the "slipperiness" of ice
by a localized high pressure exerted by an object on the ice. While it
is true that ice melts under pressure
, this factor by itself cannot
explain why skates move so well over ice.
Experiments have shown that in order to lower the melting point of
ice by one degree Celsius (1.8 F), approximately 12.2 MPa
is required. That is more than 120 times atmospheric pressure; a truly
large quantity. An ice skater would only be able to exert about 10% of
this pressure. Besides, much lighter objects (think about hockey pucks) glide on ice just as good.
The slipperiness of ice is caused by an effect called Surface
Melting. This concept was first postulated by Michael Faraday
(1791-1867). Molecules in a solid lattice have a limited degree of
freedom to move, since they are surrounded by other molecules. However,
molecules at a free (solid/gas) boundary have more freedom, and thus
show more disorder. Because of their less restricted freedom, these
molecules show liquid like behavior in a quasi-liquid surface
layer. Surface Melting can occur at temperatures below the bulk melting
point of the solid. Thus, the slipperiness of ice is actually
caused by lowering the friction at the solid interface.
Surface Melting of a solid is dependent of temperature. As the
temperature is decreased, the quasi-liquid surface layer becomes
smaller. Hockey players, Speed skaters and Figure skaters should already
know this; from experience, the temperature of the ice floor is very
important for optimum performance.