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Feb 21 2006

Why is ice slippery?

I have been meaning to write about this for some time but got sidetracked by all the other topics. It formed the basis of an article by Robert Rosenberg in the December 2005 issue of Physics Today (pages 50-55).

How people respond to such a question can tell you a lot about their relationship to science in general. Some people will think that such an everyday question should have a simple answer that has been known for a long time. Others will answer that that is just the way ice is. Intelligent design and other types of creationists might respond that ice was made slippery for a purpose that we cannot comprehend. Yet others will say “who cares?” and wonder why we should bother with trying to answer such a question at all. But ask a group of scientifically-minded people this question and you will get a lively discussion going about the various possible explanations.

The basic reason why ice is slippery is not hard to understand. It is caused by the presence of a thin layer of water on the surface of ice

But this begs the next question of what causes that layer of water to coexist with the ice. Why doesn’t it also freeze? And seeking the explanation for this has formed the basis for a long series of research programs that have lasted for well over a century.

One popular explanation is that the pressure of the feet or skis or skates on the ice lowers the melting point temperature of ice. So when ice skaters glide on the ice, the high pressure caused by their weight on the thin blades (nearly 500 times atmospheric pressure) causes the melting point to drop below the normal freezing point of 0oC, and thus the surface of the ice, finding itself about that temperature, melts. Joly examined this idea in 1886 and calculated that such a high pressure would lower the melting point temperature to -3.5oC.

The catch is that the optimum temperature for figure skating is -5.5oC while ice hockey players favor harder, faster ice at -9oC. In fact skating is possible at temperatures as low as -35oC. However, the chief scientist of the 1910 Scott expedition to the Antarctic reported that at -46oC, the snow surface became sandlike, making skiing difficult. But this pressure explanation, despite its flaws, remained the dominant explanation for nearly a century.

Another competing explanation (originating in 1939) was that the friction caused by the walking or sliding generated heat that caused the ice surface to melt. Experiments done between 1988 and 1997 to test this showed that skiing and skating over ice and snow did create enough frictional heating to create a water layer.

But this still did not explain why ice was slippery even when you were simply standing on it. The current explanation originated as far back as in 1850 from suggestions by that great scientist Michael Faraday who did some experiments. He found that even though cubes of ice seemed to have a surface water layer, when you held two cubes together, their common surface froze together to form a single ice block. This suggests the phenomenon of pre-melting which says that the surface layer of ice is unstable (because of the lack of molecules above it) and becomes liquid at temperatures below the commonly accepted melting point. But when two cubes are brought together, the common surface is no longer a surface layer and thus freezes, binding the two.

Faraday’s explanation was supported by others at the time but challenged by critics who said that holding the cubes together created pressure between them and it was this (and not premelting) that caused them to bind together. Faraday lost out in the end and his theory was pretty much forgotten until more sophisticated experiments were done beginning around 1950. In 1969, experiments confirmed that premelting of ice started occurring at -35oC and yet more sophisticated experiments have been done since and are still continuing that suggests that premelting is the explanation.

Is this something peculiar to ice? It turns out that premelting occurs in other materials as well. Lead, for example, melts at 327oC. But premelting causes the surface to become unstable at about 307oC, so you would be able to skate on lead at that lower temperature, in the few seconds before you were burned to a crisp. (Kids, don’t try this at home!) Scientists have found similar liquid surface layers below the melting point for other metals, semiconductors, molecular solids and rare gases, suggesting that this is a common phenomenon.

Some interesting features of this history stand out. It shows how science and scientists work. Although Michael Faraday was a famous scientist, and his explanation ultimately turned out to be correct, it was not accepted at the time, despite some experimental support. It was only much later, with solid quantitative data coming in from a variety of sources, that resulted in the premelting hypothesis being accepted. Scientific prestige only assures you that your ideas will be taken seriously, not that they will be accepted.

Second, ideas by themselves, however appealing or aesthetically appealing, need their own supporting data if they are to be accepted. It is not enough to simply discredit other people’s ideas. Solid support for Faraday has only been coming in the last half-century.

Third, if you have a theory that explains one phenomenon, then it should also predict other phenomena. This is what was done by extending the liquid layer surface idea to other materials and seeing if they showed the same thing. When they did, the idea gained credibility. In other words, it was not an ad hoc theory to explain just one result, that of ice. This is the essential hypothetic-deductive (if-then) reasoning at work in science, which says that if something explains result A, then it should be possible to use it to predict something in context B.

I emphasize these three points to contrast the process with what intelligent design creationist (IDC) advocates are trying to do with their idea. They are violating points two and three of those criteria. They should not be surprised that IDC is being rejected by the scientific community.

POST SCRIPT: Motown at the Super Bowl

Some critics argued that the NFL showed bad judgment by selecting some aging British rock band to play at half-time at the Super Bowl, and completely ignoring Detroit’s Motown sound.

Meanwhile, Alexander Pollack, Detroit’s city architect, had proposed a truck horn band in which the air horns of big trucks could be tuned to play the musical scale and thus could serve as an orchestra. He had proposed that this collection of trucks be driven in during half time to play music on their horns but his idea was rejected because of the weight of the trucks. I like the Rolling Stones but this would have been a far more novel halftime show.

But he did get the trucks lined up to play somewhere outside the stadium and if you go here, you can hear the air horns playing “Stop in the name of love.” So Motown did have a presence at the game. Sort of.

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