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(Image: Josh Haner/eyevine)

FOR physicists no less than figure skaters, ice is remarkably hard to get a grip on. The overwhelming consensus is that ice has low friction because of a thin film of liquid water coating its surface. Hence skaters balanced on thin metal blades can glide smoothly across the ice rink, but grind to a halt on the wooden floor beyond. The tricky part is how this liquid layer forms. More than a century of research has brought us little closer to a definitive answer.

It all started in June 1850, when Michael Faraday told an audience at London’s Royal Institution of how pressing two ice cubes together led to them forming a single block. He attributed this to the appearance of an intervening film of water that quickly refreezes. For many years, the appearance of this layer of water was put down to pressure. In fact, even a person of above-average weight on a single skate generates far too little pressure to account for the observed melting, says of McGill University in Montreal, Canada. “The mathematics doesn’t work out.”

Instead, Kietzig argues that the main player is frictional heating. The movement of a blade across the ice, for instance, easily generates enough heat to melt some of it.

You might think that would be the end of it. But of Nanyang Technical University in Singapore has other ideas. He argues that since ice is slippery even when you’re standing still, friction cannot be the whole story. “Mechanisms such as friction heating and pressure melting have been ruled out,” he says.

According to Sun, the assumption that the slippery layer coating ice is a liquid is also fundamentally flawed. He says this layer should properly be called a because the weak bonds between H₂O molecules at the surface are stretched, but unlike in liquid water none of them are broken. He also argues that this elongation of bonds ultimately produces a repulsive electrostatic force between the surface layer and anything it comes into contact with (see diagram).

He compares the effect to the electromagnetic force that levitates Maglev trains, or the air pressure a hovercraft generates beneath its hull. If he’s right, his model helps to explain many of the layer’s properties, including its remarkably low friction. “I believe the problem has been completely resolved,” says Sun.

Most in the ice field are not convinced. Gen Sazaki at Hokkaido University in Sapporo, Japan, who made the first direct observations of this layer in 2013, prefers to call it a quasi-liquid. He thinks it represents a transitional stage between solid and liquid as the temperature goes up.

For Sazaki, understanding how this mysterious sheet of H₂O forms is still some way off. Even when it comes to something as familiar as slipping on ice, he says, “reality is much more complicated than we expected”.

“Even with something as familiar as ice, reality is much more complicated than we expected”

Read more: “10 mysteries that physics can’t answer… yet”

This article appeared in print under the headline “Why is ice slippery?”