Women's short track speed skating at the 2014 Winter Olympics. Image by Pawel Maryanov.

In the Summer Olympics, athletes skim through water, dash across the earth, and twirl through the air, but the Winter Olympics are all about ice. Whether it’s a skier cruising down a snowy mountainside or a luge sled hurtling down a frozen ramp at 80 miles per hour, ice plays a starring role.

We take for granted that ice is slippery enough to make these sports possible, but this property of ice is actually rather unusual for natural materials. You couldn’t skate without wheels on a floor made of stone or wood, so why is ice different? Over the years, many physicists have asked themselves this same question, and come up with a number of intriguing possibilities.

Pressure Melting

As early as 1850, scientist James Thomason was considering scientific explanations for ice’s slippery nature. He suspected that differences in pressure might be at play. While most solids are denser than their liquid form, ice is actually less dense than water. He thought that if a skater stood atop the ice, the pressure from her skates might compress and apply enough pressure to the ice to melt a thin layer of water at the surface, giving ice its characteristic slide.

When Thomason put his theory to the test the melting point of ice did indeed lower when more pressure was applied. However, it didn’t entirely clear up the question. According to his equations, an ice skater wouldn’t be capable of applying enough pressure to significantly change the melting point of ice. Given that items as small as coins will slide on ice as well and certainly are not imparting any great pressure changes, his explanation further falls apart.

Though pressure melting does occur, it can’t tell us why a curling stone glides serenely across the ice’s surface.

A curler at the 2012 Junior Olympics. Photo by Ralf Roletschek

Friction Heating

In 1939, Frank P. Bowden and T.P. Hughes published a scientific article detailing another idea about why ice might be slippery. Rather than pressure, they thought friction might be key.

When two objects rub against each other, friction converts their kinetic energy to heat. These two scientists thought that the friction of a skate sliding across the ice might produce enough heat to melt a tiny layer of water, water that would help the skater glide forward.

They too found that their hypothesis was supported by the data, but like pressure melting, friction heating doesn’t fully explain the phenomenon of slippery ice. People can still slip and fall on ice if they are standing still but shift their weight, so it doesn’t seem that movement is a perquisite to sliding. Something else is going on with ice, a mysterious factor of intrinsic slipperiness.

Pre-melting

At the same time as James Thomason was investigating pressure melting, Michael Faraday was considering other icy options. He didn’t think ice needed pressure or friction to have a thin layer of water at the surface. He thought this occurred naturally to all ice (and other solids) near its melting point. His idea was not widely explored upon until 1949, when C. Gurney took another look at it. He reasoned that molecules at the surface of a solid had less pressure and support to keep them in the solid phase, so they might revert to liquid when near their melting point. If ice constantly had a small layer of water atop it, it would explain why ice was so slippery regardless of motion or pressure.

This concept has been supported by a number of researchers since. In 1987, scientists were able to use x-rays to detect quasi-liquid molecules of water on the surface of ice. This layer was only one thousandth the width of a single bacteria, at 1 – 94 nanometers. This phenomenon was examined again in 2004, when Katsuyuki Kawamura found that molecules behaved like a liquid on the surface of ice because they had fewer molecular bonds holding them steady.

How the Magic Happens

So when a skate (or your foot) steps on the ice, what makes it spontaneously start moving? Probably a combination of all three hypotheses!

The microscopic layer of water on top of the ice gets the skate going. As the skate moves and bites into the ice, friction between the skate and the ice adds heat and melts the ice further. The skater begins hydroplaning over the surface of the ice, riding both on the water already there and the additional water her friction produces.

Theoretically, this process will stop working so well at around -30oC (-22oF). That’s the temperature at which the liquid layer on top of ice acts more solid, and cold enough that frictional heating would have a hard time too. It is probably more difficult to ice skate at these temperatures, though it’d be so cold outside at that point we recommend you stay indoors anyways.

If you really like skating in the cold and it reaches around -40oC (-40oF), you could have different skating options. The metal mercury, most often seen as a beautiful silvery liquid, will freeze at this temperature. If mercury was in solid form but near its melting point, it probably would also have a microscopic layer of liquid mercury at the surface, creating a beautiful metal skating rink.

Unfortunately, mercury inhalation can be a pretty severe health risk. If you make yourself a mercury ice skating rink, you might want to incorporate a gas mask into your skating costume.

- Kate Dzikiewicz, Paul Griswold Howes Fellow