Science of Hockey: Why Is Ice Slippery?

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Slippery All the Time

Why is the ice slippery in the first place? Is it more slippery when it's "fast ice" or is something else going on? How would a chemist explain the difference between "fast ice" and "slow ice"? We asked Professor Gabor Somorjai of the Lawrence Berkeley National Laboratory these questions and discussed his research into ice.

Somorjai's recent discoveries have explained why skaters and pucks slide on the ice. These new findings challenge long-held theories about why ice is slippery. In the past, scientists believed that either pressure or friction melted the ice, creating a water lubricant that allows skates and pucks to slide. Berkeley chemist Michel van Hove, a colleague of Somorjai's, has done calculations which show that skates and pucks do not generate enough pressure to instantly liquefy ice. Somorjai has discovered that ice has a "quasi-fluid layer" that coats the surface of ice and makes it slippery. Even ice that is 200 degrees below zero Fahrenheit (-129 Celcius) or more still has this layer.

Professor Somorjai explains his latest findings about the nature of ice.

External Forces?

External forces, such as pressure and friction, can melt the ice. But Professor Somorjai's findings indicate that ice itself is slippery. You don't need to melt the ice to skate on it, or need a layer of water as a lubricant to help slide along the ice.

An image of an ice crystal. Notice the "hexagonal stable structure" of the crystal.

Slippery Layers

According to Professor Somorjai, the "quasi-fluid" or "water-like" layer exists on the surface of the ice and may be thicker or thinner depending on temperature. At about 250 degrees below zero Fahrenheit (-157 centigrade), the ice has a slippery layer one molecule thick. As the ice is warmed, the number of these slippery layers increases. This may help explain in part the difference between "fast ice" and "slow ice." As the number of layers increases, the players' skates need to "slosh" through more of these "water-like" layers; more friction occurs in these conditions, slowing the players down. These extra layers would also "soften" a landing for a figure skater--who skates on warmer ice than a hockey player. There is more on the structure of this "quasi-fluid" layer at the beginning of the "Skating" section. But before we get too technical, let's examine how ice is made.

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