There Is Still Hot Debate Around How Ice Skating Actually Works

Figure skating is the oldest sport in the Winter Olympics. But the debate behind how ice skating actually works dates back even further than its 1908 Olympic debut. You'd think we know by now why ice is slippery, right? Well, this matter of physics has proven difficult to get a grip on.

Lickity Slick

There is nothing inherently slippery about ice itself. It's all about the combination of ice and a tiny bit of water that makes it so slick, or in more scientific terms, reduces its friction. But how that thin layer of water gets there in the first place is what has baffled scientists for years. Our first theory of what's happening comes courtesy of Michael Faraday: pressure. In June 1850, Faraday explained to an audience at London's Royal Institution how pressing two ice cubes together could make them form into a single block. He believed the pressure between the two cubes created a thin layer of water that quickly refreezes.

But pressure doesn't pass muster in terms of ice skating. Even an overweight ice skater wouldn't be able to create enough pressure between skate and ice to get it melting, Anne-Marie Kietzig of McGill University tells New Scientist.

According to Kietzig, the answer is in frictional heating. That is, the friction between the moving skate and the ice heats up the ice enough to create a bit of water to glide on. But hold that thought. Friction doesn't explain why ice is slippery when you're not shooting around a rink like Tara Lipinski. Anyone who's ever wiped out while trying to stand up on a frozen pond knows that.

Changqing Sun of Nanyang Technical University in Singapore has yet another theory. In a November 2014 study, Sun introduced the idea that the thin layer of water that's largely believed to make ice slippery may not even be a liquid layer at all. Instead, he says it's a "supersolid skin." He proposes this skin is a solid because the weak bonds between H2O molecules are stretched, but not broken like they are in liquid water. "I believe the problem has been completely resolved," Sun told New Scientist. But not everyone is convinced.

Gen Sazaki of Hokkaido University made the first direct observations of this slippery maybe-liquid, maybe-supersolid layer in 2013. He calls it a quasi-liquid, and definitely doesn't call it resolved. Sazaki's answer has this quasi-liquid act as a transitional stage between solid and liquid, but, for him, it's still an unsatisfactory answer. "Even with something as familiar as ice," Sazaki tells New Scientist, "reality is much more complicated than we expected"

The World May Never Know

In 2015, a new theory emerged as to why ice is slippery when a hard material, like an ice skate, moves across it. Warning: This one is the headiest yet.

The main breakthrough by Bo Persson, a scientist at Germany's Jülich Research Center, was to connect a theoretical description of ice friction to the hard data they had from experiments. If, as previous research suggested, sliding on the ice at a certain speed actually created a film of melted water, you'd expect that once you got to that magical sliding speed the ice would get way more slippery. But Persson's experimental data said that wasn't the case. Instead, the ice gets gradually more slippery as you slide faster and faster.

His research suggested something "paradoxical": when you're not sliding fast enough to form that film, the ice has a lot of friction, which helps create enough heat to form the film anyway. If you are sliding fast enough to form that film, the ice doesn't have much friction, which creates very little heat and leads to the film freezing. That means that there's a sweet spot in sliding speed where the ice's surface is made up of both water-like and ice-like substances that switch roles over and over really quickly, not unlike Sazaki's quasi-liquid.

Still, the exact reason as to why ice skates can glide so smoothly and easily is a mystery. The problem lies in the inability to get in between the skate and the ice, a plane called the buried interface, while the skate is contacting the ice in motion. "It is nearly impossible to study directly at the molecular level what happens at the buried interface," Persson told Even if you're not completely wowed by an Olympic figure skater's triple Salchow, just the fact that they are skating on ice at all is itself an impressive feat of physics.

The Science of Figure Skating

Written by Joanie Faletto February 5, 2018

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