They had the intuition that molecules close to the surface behave differently from those located deep in 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 bond with and therefore have more freedom of movement than in solid ice. In this so-called pre-melted layer, the molecules are easily moved by a skate, a ski or a shoe.
Today, scientists generally agree on the existence of the pre-melted layer, at least close to the melting point, but they disagree on its role in the slipperiness of ice.
A few years ago, Luis MacDowellphysicist from the Complutense University of Madrid, and his collaborators led a series of simulations determine which of the three hypotheses – pressure, friction or pre-melting – best explains the slippery nature of ice. “In computer simulations, you can see the atoms moving,” he said, which is not achievable in real experiments. “And you can actually look at the neighbors of these atoms” to see whether they are periodically spaced apart, like in a solid, or disordered, like in a liquid.
They observed that their simulated ice block was indeed covered by a liquid layer only a few molecules thick, as predicted by premelt theory. When they simulated a heavy object sliding across the ice surface, the layer thickened, consistent with pressure theory. Finally, they explored friction heating. Near the melting point of ice, the pre-melted layer was already thick, so frictional heating had no significant impact on it. However, at lower temperatures, the slippery object produced heat that melted the ice and thickened the layer.
“Our message is this: All three controversial hypotheses operate simultaneously to one degree or another,” MacDowell said.
Hypothesis 4: Amorphization
Or maybe surface melting isn’t the main cause of the ice’s slipperiness.
Recently, a team of researchers from Saarland University in Germany identified arguments against all three dominant theories. First, for the pressure to be high enough to melt the ice surface, the contact area between (let’s say) the skis and the ice would have to be “unreasonably small.” they wrote. Second, for a ski moving at a realistic speed, experiments show that the amount of heat generated by friction is insufficient to cause melting. Third, they found that in extremely cold weather, ice remains slippery even if there is no pre-melted layer. (Surface molecules still lack neighbors, but at low temperatures they don’t have enough energy to overcome the strong bonds with solid ice molecules.) “So either the slipperiness of ice comes from a combination of all or a few of them, or there is something else we don’t know yet,” said Ashraf Atilaa materials scientist on the team.
Scientists have sought alternative explanations by looking for other substances, such as diamonds. Gemstone polishers have long known from experience that some faces of a diamond are easier to polish, or “softer,” than others. In 2011, another German research group published an article explaining this phenomenon. They created computer simulations of two diamonds sliding against each other. The atoms on the surface were mechanically torn from their bonds, allowing them to move around, form new bonds, etc. This sliding formed an “amorphous” layer without structure. Unlike the crystalline nature of diamond, this layer is disordered and behaves more like a liquid than a solid. This amorphization effect depends on the orientation of the molecules on the surface, so that some faces of a crystal are softer than others.
Atila and his colleagues say a similar mechanism occurs in ice. They simulated ice surfaces sliding against each other, keeping the temperature of the simulated system low enough to ensure no melting. (Any slipperiness would therefore have a different explanation.) Initially, the surfaces attracted each other, a bit like magnets. This is because water molecules are dipoles, with unequal concentrations of positive and negative charges. The positive end of one molecule attracts the negative end of another. The attraction exerted by the ice created tiny welds between the sliding surfaces. As the surfaces slid past each other, the welds broke and new ones formed, gradually changing the structure of the ice.
