They sensed that molecules near the surface of ice behave differently than those deep within it. Ice is a crystal, meaning each water molecule is arranged in a periodic lattice. However, at the surface, the water molecules have fewer neighbors to bond with, giving them more freedom of movement compared to solid ice. In that premelted layer, molecules can be easily pushed aside by skates, skis, or shoes.
Today, scientists widely accept that the premelted layer exists, especially close to the melting point, but there’s still debate about its role in making ice slippery. A few years ago, Luis MacDowell, a physicist at the Complutense University of Madrid, and his collaborators conducted simulations to determine which of three hypotheses—pressure, friction, or premelting—best explains ice’s slipperiness. “In computer simulations, you can see the atoms move,” he noted, something that’s difficult to track in real experiments. “You can actually examine the neighbors of those atoms” to check if they are in a regular pattern like solid ice or disordered like a liquid.
They found their simulated block of ice was indeed covered with a liquid-like layer just a few molecules thick, aligning with the premelting theory. When they simulated a heavy object sliding on the ice, the layer thickened, supporting the pressure theory. They also investigated frictional heating. Near the melting point, the premelted layer was already thick, so frictional heating didn’t significantly affect it. However, at cooler temperatures, the sliding object generated heat that melted the ice, further thickening the layer. “Our message is: All three controversial hypotheses operate simultaneously to some degree,” MacDowell explained.
Hypothesis 4: Amorphization
Or perhaps the melting of the surface isn’t the main reason ice is so slippery. Recently, a team at Saarland University in Germany raised questions about the three main theories. First, for pressure to be high enough to melt ice’s surface, the contact area between skis and ice would need to be “unreasonably small,” they pointed out. Second, at realistic speeds, experiments show friction generates insufficient heat to cause melting. Lastly, they found that at extremely low temperatures, ice remains slippery even without a premelted layer. (Surface molecules still lack neighbors, but at lower temperatures, they don’t have enough energy to break free from their strong bonds with solid ice molecules.) “So either the slipperiness of ice comes from a mix of these factors or there’s something else we don’t yet understand,” said Achraf Atila, a materials scientist on the team.
Materials scientists at Saarland University demonstrated through simulations that as two blocks of ice slide against one another, an amorphous layer in the middle gradually becomes thicker. The researchers looked for alternative explanations by examining other materials, like diamonds. Gemstone polishers have long observed that some sides of a diamond are easier to polish, or “softer,” than others. In 2011, another German research team published findings on this. They created simulations of two diamonds sliding against each other. Surface atoms were mechanically pulled from their bonds, allowing them to shift, form new bonds, and so on. This sliding created a disordered “amorphous” layer, which behaves more like a liquid than a solid. The amorphization effect varies based on the surface molecule’s orientation, which explains why some sides of a crystal are softer.
Atila and his colleagues propose that a similar process occurs in ice. They simulated ice surfaces sliding past one another, ensuring the temperature remained low enough to prevent melting. (Any slipperiness would then need a different explanation.) Initially, the surfaces attracted each other like magnets because water molecules are dipoles, with uneven positive and negative charges. The positive end of one molecule pulls in the negative end of another. This attraction in ice created tiny bonds between the sliding surfaces. As the surfaces moved against each other, these bonds snapped and new ones formed, gradually altering the structure of the ice.
