The ice cometh: Why is it slippery?

One of my great winter joys has been ice skating – on a lake or river rather than a rink (especially an indoor rink). Nothing beat the first venture with blades onto brand new ice, barely thick enough to be safe, often clear as glass. And I’m not just talking about childhood here. As recently as 10 years ago we lived on a dammed section of river that by Thanksgiving usually had frozen solid. I would skate on it as often as I could, winding upstream through marsh and woods, until snow came and covered the ice. I am hoping one of these years our Birch Lake will freeze and stay clear of snow for at least a few days. It is fascinating to look down into a lake through sheet of ice.

Do you skate on your lake, conditions permitting? Have you ever wondered why you’re able to skate – that is, why ice is slippery? After all, the solid phases of most compounds are not. Perhaps you don’t wonder because you think you know the answer. If you’re like me, you were taught in grade school that we can skate because the pressure of the blades lowers the melting temperature (32 degrees F) at the ice surface, creating a thin film of water on which we glide. But if that’s true, why are we able to slide across ice while wearing flat-soled shoes, which exert much less intense pressure?

It turns out science has pretty well rejected the pressure explanation. In an article in the New York Times (http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=all&_r=0) Robert Rosenberg, an emeritus professor of chemistry at Lawrence University in Appleton, Wis., explained why: “The explanation fails, he said, because the pressure-melting effect is small. A 150-pound person standing on ice wearing a pair of ice skates exerts a pressure of only 50 pounds per square inch on the ice. (A typical blade edge, which is not razor sharp, is about one-eighth of an inch wide and about 12 inches long, yielding a surface area of 1.5 square inches each or 3 square inches for two blades.) That amount of pressure lowers the melting temperature only a small amount, from 32 degrees to 31.97 degrees. Yet ice skaters can easily slip and fall at temperatures much colder.”

There are now two other explanations – apparently not mutually exclusive. One is that friction from the skate blade (or shoe) is what heats and melts the ice and creates the slipperiness. The other is that the ice surface is inherently slippery. “This argument holds that water molecules at the ice surface vibrate more, because there are no molecules above them to help hold them in place, and they thus remain an unfrozen liquid even at temperatures far below freezing,” the Times article said.

Among scientists who believe in inherent slipperiness is Dr. Gabor Somorjai at Lawrence Berkeley National Laboratory. His sophisticated tests support the theory and so does a basic observation: A person standing motionless on ice is not creating friction – yet still may easily slip and fall.

One who disputes the importance of the liquid layer is Dr. Miquel Salmeron, also of Lawrence Berkeley. The Times reports that he and colleagues dragged a device similar to a tiny phonograph needle across ice and found the friction to be “very high.” And high friction of course would generate meaningful heat.

“Dr. Salmeron said this finding indicates that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature. In his view, friction is the primary reason ice is slippery,” the Times article said.

Which theory is right? Maybe one or the other, maybe both. Scientists disagree. For all the advances in scientific knowledge, ice remains a mysterious substance. So, let’s not worry too much about the explanation. Let’s get out on the ice and slide around!