They discovered that molecules near the surface behave differently from molecules inside the ice. Ice is a crystal, which means that each water molecule is locked in a periodic lattice. However, at the surface, water molecules have fewer neighbors to bond with and so they have more freedom of movement than solid ice. In that so-called premelted layer, molecules are easily displaced by skates, skis or boots.
Today, scientists generally agree that a pre-melt layer exists, at least close to the melting point, but they disagree on its role in ice slippage.
A few years ago, physicist Luis McDowell and his colleagues at the Complutense University of Madrid ran a series of simulations to establish which of three hypotheses – pressure, friction or premelting – best explains ice slip. “In computer simulations, you can see 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 periodically spaced, like in a solid, or disordered, like in a liquid.
They observed that their simulated block of ice was actually coated with a liquid-like layer a few molecules thick, as the premelting theory predicts. When they simulated a heavy object sliding across the ice surface, the layer thickened, consistent with the pressure theory. Ultimately, he discovered frictional heating. Near the melting point of the ice, the pre-melted layer was already thick, so frictional heating had no significant effect on it. However, at lower temperatures, the sliding object generated heat that melted the ice and thickened the layer.
“Our message is: All three controversial hypotheses work together on one level or another,” McDowell said.
Hypothesis 4: Amorphization
Or perhaps surface melting is not the main cause of ice slippage.
Recently, a team of researchers from Saarland University in Germany identified arguments against all three prevalent theories. First, for the pressure to be sufficient to melt the snow surface, the area of contact between skis and snow must be “unduly small,” he wrote. Second, for skis moving at realistic speeds, experiments show that the amount of heat generated by friction is insufficient for melting. Third, they found that in extremely cold temperatures, ice is still slippery even if there is no pre-melted layer on it. (The surface molecules still lack neighbors, but at low temperatures they don’t have enough energy to overcome strong bonds with solid ice molecules.) “So either the ice slippage is coming from a combination of all of them or some of them, or there’s something else we don’t know about yet,” said Achraf Attila, a materials scientist on the team.
Scientists looked for alternative explanations in research on other substances such as diamonds. Gem polishers have long known from experience that some edges of diamonds 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. The atoms on the surface were mechanically pulled out of their bonds, allowing them to move, form new bonds, etc. This sliding created a structureless, “amorphous” layer. In contrast to the crystalline nature of diamond, this layer is disordered and behaves more like a liquid than a solid. This demorphization effect depends on the orientation of the molecules on the surface, so some edges of the crystal are softer than others.
Attila and his colleagues argue that a similar mechanism exists in ice. They simulated ice surfaces sliding against each other, keeping the temperature of the simulated system low enough to ensure the absence of melting. (So any slippage will have a different explanation.) Initially, the surfaces were attracted to each other like magnets. This was because water molecules are dipolar, having unequal concentrations of positive and negative charges. The positive end of one molecule attracts the negative end of another molecule. Attraction in the ice caused small welds to form between the sliding surfaces. As the surfaces slid past each other, welds broke and new ones formed, gradually changing the composition of the ice.
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