Why ice is slippery

It’s a wonder more hockey players don’t have a God complex. They are, after all, walking on water.

Well, actually we all do that when it comes to ice. It’s the reason skating is possible.

Ice — whether it’s a cube you pull from the freezer or a sheet in a NHL arena — may appear to be the completely solid form of water, but there is a small amount of the liquid form on top. That allows hockey players to skate.

“They’re sliding on a ridiculously thin layer of water, but nonetheless it’s still there,” said Michael Kruger, a UMKC physics professor. “They’re not generating it themselves. It’s not due to their pressure.”

If that sounds unlikely, consider why it’s not possible to skate on glass. That surface is completely solid, but you wouldn’t get far on skate. Ice has the liquid layer because of surface melting, which is also known as premelting.

But how is this possible? Even someone with a rudimentary knowledge of science can tell you that water freezes below 32 degrees Fahrenheit.

Kruger said the water molecules in the center of a piece of ice are constrained, sort of like a bunch of people crammed into an elevator. But at the top of the ice, the molecules are less confined. Because there are fewer intra-molecular forces acting on the surface molecules, the molecules act more like a liquid than a solid.

And it’s a thin, thin layer. One that gets thinner with colder air temperatures.

“If it were 3 inches, everyone would know about it,” Kruger said. “But if the temperature is at … 28 or 29 degrees Fahrenheit, that thickness is something like 50 nanometers. Facial hair grows 1 nanometer per second. The wavelength of light is about 550 nanometers. So this thickness is 1/10th the wavelength of light. So this is very thin. As it gets colder and colder, by (negative-13 degrees), that layer is like 1 nanometer thick, which is how much your beard has grown in a second.”

If that sounds too small to be the reason for skating, it may be. There are others who believe that friction also plays a part.

According to a New York Times article, Miquel Salmeron, a scientist at Lawrence Berkeley National Laboratory, ran an experiment in which he dragged the tip of an atomic force microscope, resembling a tiny phonograph needle, across the surface of ice.

Salmeron reported that the microscope tip was so small that its friction melted only a tiny bit of water, which immediately refroze and therefore did not provide the usual lubrication. He told the Times that the friction of ice was very high, which showed that while the top layer of ice may be liquid, it is too thin to contribute much to slipperiness except near the melting temperature.

One thing that most agree on is the old theory on why ice is slippery is wrong. It was once commonly believed that pressure exerted by the blades of a skate would melt the ice.

“If you were to calculate the pressure your skates apply to the ice, what you’d find is it’s not enough to melt the ice,” Kruger said. “Indeed, a bit of pressure can melt ice, but what you apply through your skates is not enough to melt ice. That’s not the explanation.”

Kruger believes it’s the liquid form of water caused by surface melting.

“It’s really very thin, but you don’t need a lot,” Kruger said. “The thinnest of layers will act as lubricant and allow you to slide. If you go down to incredibly low temperatures, ice isn’t going to be particularly slippery.”

To reach Pete Grathoff, send e-mail to pgrathoff@kcstar.com