The New York times has a reasonably good article today on the mystery of ice skating, which is explained incorrectly in many (if not most) high school chemistry and physics textbooks. The canonical explanation is that pressure depresses the freezing point of the ice, and that the skaters are really skating along on a liquid film. Great story, but it is almost certainly incorrect. Pressure-depression of the freezing point can’t explain skating on extremely cold days, and the pressures that can be generated by a skater are not particularly high.
Another possible explanation for skating is that the skates use friction to create a liquid-like layer on top of the ice. However, this hypothesis can’t explain why ice is slippery even when a person is standing still.
The article cites another explanation for skating; ice has a thin liquid-like film at the vapor interface all of the time. This explanation was first put forward by Michael Faraday. This explanation is supported by the experiments by the Somorjai group at Berkeley, but the Salmeron group’s AFM experiments (which show a high degree of friction) appear to contradict this.
One hypothesis the article doesn’t cover is the role of entrapped gas molecules between the skate and the ice surface. I’ve seen simple friction models in the physics literature that show how a few entrapped gas molecules can act like “ball bearings” and can prevent two solid surfaces from sticking to each other as one slides over the other.
The article concludes with a summary of some of the other crystalline phases of ice (although they missed the most important one….)
[tags]ice, skating, friction[/tags]
