A Slippery Subject

Thursday, May 01, 1997
Why is ice slippery? This seemed to be one question that scientists had satisfactorily answered years ago: the pressure of, say, a skate blade or the sole of a shoe melts the very top layer of ice, making a wet surface for the blade or shoe to glide over. When the pressure is gone, the liquid surface freezes again. This explanation made so much sense that until recently no one took the trouble to test it carefully. Now some researchers finally have, and they’ve found that the old explanation is not quite right.

Chemist Gabor Somorjai and physicist Michel Van Hove, both at the University of California at Berkeley, believe they now understand the slipperiness of ice on an atomic scale. They bombarded a thin layer of ice with a stream of electrons. Detectors recorded the angles and velocities of the electrons reflecting off the ice. To the researchers’ surprise, the electrons didn’t follow the simple trajectories expected of an object bouncing off a solid surface. Instead they seemed to be hitting a constantly changing surface that scattered them.

The only way to explain the result, says Somorjai, is to assume that the uppermost layers of the ice aren’t really frozen at all but consist of a thin film of water that serves as a permanent lubricant. When something pushes down on the ice, like a shoe, Somorjai explains, the water molecules compact into underlying interstices and create a smooth, flush surface, which means you can slide over it more easily.

The researchers also found that ice drastically decreases in slipperiness as temperatures fall below about -22 degrees. At that temperature, they believe, the number of molecular layers that behave like a liquid diminishes, reducing the slipperiness, so that the ice resembles any other solid.

Van Hove suspects that ice is much more slippery than other solids because the chemical bonds that hold water molecules are much weaker than the bonds joining other solids. An icy surface behaves as if it were boiling, says Van Hove, and the weak bonds between neighboring molecules make it unstable. There’s always something coming off and coming back on, in the sense that evaporation and condensation are occurring all the time.
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