No One Is Quite Sure Why Ice Is Slippery

They concluded that molecules near the surface behaved differently than those deep within the ice. Ice is a crystal, which means that each water molecule is locked in a periodic lattice. However, on the surface, water molecules have fewer neighbors to bind to and thus have greater freedom of movement than in solid ice. In that layer, called the pre-melted layer, the molecules are easily replaced by a shoe or boot.

Today, scientists generally agree that the melting layer exists, at least near the melting point, but they disagree about its role in ice sliding.

A few years ago, Lewis McDowell, a physicist at the Complutense University of Madrid, and his collaborators ran a series of simulations to determine which of three hypotheses—pressure, friction, or prior melting—best explained ice sliding. “In computer simulations, you can see the atoms moving,” he said, something that is not possible in real experiments. “And you can actually look at the neighbors of those atoms” to see whether they are spaced periodically, as in a solid, or irregularly, as in a liquid.

They noticed that the ice mass they simulated was actually covered by a liquid-like layer only a few molecules thick, as the premelting theory predicts. When they simulated a heavy object sliding on the surface of the ice, the thickness of the layer increased, which is consistent with pressure theory. Finally, they discovered frictional heating. Near the melting point of the ice, the molten layer was already thick, so frictional heating did not affect it significantly. But at lower temperatures, the sliding object produced heat that melted the ice and thickened the layer.

“Our message is: The three controversial hypotheses work more or less simultaneously,” McDowell said.

Hypothesis 4: Crystallization

Or perhaps surface melting is not the primary cause of ice sliding.

Recently, a team of researchers at Saarland University in Germany outlined the arguments against the three prevailing theories. First, for the pressure to be high enough to melt the ice surface, the contact area between the skis and the ice must be “unreasonably small,” they wrote. Second, for a skater moving at a realistic speed, experiments show that the amount of heat generated by friction is insufficient to cause melting. Third, they found that at very cold temperatures, the ice remains slippery despite the lack of a previously melted layer. (Surface molecules still have a scarcity of their neighbors, but at low temperatures they don’t have enough energy to overcome the strong bonds with solid ice molecules.) “Either the ice sliding comes from a combination of all of them or a few of them, or there is something else that we don’t know yet,” said Ashraf Atila, a materials scientist on the team.

Scientists looked for alternative explanations in research conducted on other materials, such as diamonds. Gem polishers have long known from experience that some aspects of a diamond are easier to polish or “softer” than others. In 2011, another German research group published a paper explaining this phenomenon. They created a computer simulation of two diamonds sliding against each other. Atoms on the surface were mechanically pulled from their bonds, allowing them to move, form new bonds, and so on. This sliding formed a non-structural “amorphous” layer. In contrast to the crystalline nature of diamonds, this layer is irregular and behaves more like a liquid than a solid. This shifting effect depends on the orientation of the molecules on the surface, so some sides of the crystal are softer than others.

Attila and his colleagues believe that a similar mechanism occurs in ice. They simulated ice surfaces sliding against each other, while keeping the temperature of the simulation system low enough to ensure no melting. (Therefore any slippage will have a different interpretation.) In the beginning, surfaces attracted to each other, much like magnets. This is because water molecules are dipoles, with varying concentrations of positive and negative charges. The positive end of one molecule attracts the negative end of another molecule. The attraction in the ice created tiny welds between the sliding surfaces. As the surfaces slid over each other, the seams separated and new layers formed, gradually changing the structure of the ice.