Watch a group of children in a schoolyard as they take running leaps onto a bare patch of ice and you know you’re witnessing a kind of joy that’s been around a long, long time.
Our fascination with ice and its slippery nature may date back to the depths of the Pleistocene, when the first Homo sapiens left Africa and pushed northward into the glacier-bound world beyond. During the course of that epic journey, someone had to have been the first to see the latent fun in a frozen pond or a snow-covered hill.
Fast forward a few hundred thousands years and the 2014 Winter Games are set to begin. Like its summer counterpart, the quadrennial event is humanity’s most recognized expression of the athletic ideal. But it’s also an acknowledgment that the coolest thing about winter is that we’ve invented so many ways to play with frozen water.
One way or another, every competition in Sochi will come down to some kind of man-made object sliding over ice or snow – an almost magical phenomenon that makes the winter sports we love possible. And like every love affair, it comes with an element of mystery.
“I see ice as an old friend,” says Ed Lozowski, a physicist and professor emeritus at the University of Alberta. “But it’s an old friend I still don’t understand very well.”
Dr. Lozowksi speaks for a long succession of researchers who have pondered and probed the science of slide and its role in winter athletics.
It’s a topic he began exploring a decade ago during a sabbatical at the National Research Council in Ottawa. While skating on the Rideau Canal, he started to wonder how it can be that the friction between ice and skates is so incredibly low. He knew there were broad descriptions for what was going on, but found there was much less known in terms of predicting how different materials interact with ice under different conditions.
“There’s been a lot of hand waving but very little in the way of quantitative theory,” says Dr. Lozowski, who has been studying the problem ever since.
Ice is unusual as a solid because in the normal range of winter temperatures it’s never too far from its melting point. Just a small amount of energy applied to ice in the right way can melt a thin surface layer. This creates a temporary film of liquid water – typically no more than one-thousandth of a millimetre thick – that another solid surface, like the bottom edge of a skate, can slide over with little resistance.
“In a car engine, when we have solid parts in contact with one another, we put in oil for lubrication,” says Anne-Marie Kietzig, an assistant professor of chemical engineering at McGill University who studies friction. “Ice is self-lubricating, due to the water film.”
Where does the energy to make the water film come from? Scientists once thought pressure was the main source. The pressure from a skater’s weight pushing down on the ice raises the ice temperature as much as half a degree, which contributes to melting. But that doesn’t explain how a six-ounce hockey puck can seem to slide as easily as a 200-pound hockey player. Instead, the friction created by dragging something over ice is now thought to provide most of the necessary heat.
Even at temperatures well below the freezing point, ice continues to be remarkably slippery. This is in part because as ice get colder, it gets harder. Blades and runners don’t sink down into the ice so much, which allows them to ride on top more easily, even if it takes more heat to create the liquid layer. And even in extreme Antarctic cold, a sled can still work because the molecules on the top layer of the ice are only loosely tied to the molecules below, allowing a runner to push across them almost like over miniature ball bearings, which is enough give to get frictional heating started.
But that’s just the beginning. The science of slide also depends on whether the frozen surface is smooth or bumpy or packed snow. And it depends on what is doing the sliding.
“It’s more about how you can manipulate the materials so that they do what you want them to do,” Dr. Kietzig says.
That in essence, is where all Winter Olympics competition leads: to the finely honed triangle of frictional forces, equipment design and athletic performance. The degrees of freedom in that arrangement are what make the Games so engaging. But mastery over them makes victory attainable.
Polyethylene-base skis on snow
How it works