No One Is Quite Sure Why Ice Is Slippery

The original version of this story appeared in Quanta Magazine.

The reason we can gracefully glide on an ice-skating rink or clumsily slip on an icy sidewalk is that the surface of ice is coated by a thin watery layer. Scientists generally agree that this lubricating, liquidlike layer is what makes ice slippery. They disagree, though, about why the layer forms.

Three main theories about the phenomenon have been debated over the past two centuries. Last year, researchers in Germany put forward a fourth hypothesis that they say solves the puzzle.

But does it? A consensus feels nearer but has yet to be reached. For now, the slippery problem remains open.

Hypothesis 1: Pressure

In the mid-1800s, an English engineer named James Thomson suggested that when we step on ice, the pressure we exert melts its surface, making it slippery. Under normal conditions, ice melts when the temperature rises to 0 degrees Celsius (32 degrees Fahrenheit). But pressure lowers its melting point, so that even at lower temperatures, a layer of water might form on the surface. This theoretical relationship between melting point and pressure was experimentally confirmed by Thomson’s younger brother William, better known as Lord Kelvin.

In the 1930s, though, Frank P. Bowden and T. P. Hughes of the Laboratory of Physical Chemistry at the University of Cambridge cast doubt on the pressure melting theory. They calculated that an average skier exerts way too little pressure to significantly alter ice’s melting point. To do so, the skier would have to weigh thousands of kilograms.

Hypothesis 2: Friction

Bowden and Hughes suggested an alternative explanation for the formation of the water layer: that it melts because of heat generated by friction caused by whatever is sliding against it.

They tested their theory in an artificial ice cave in the Swiss Alps, using a complex contraption to measure the friction between ice and other materials. They found that the friction was higher with materials that are good at conducting heat, such as brass, than with poor conductors like ebonite. From this, they concluded that when ice is rubbed by a material that easily absorbs heat, less heat is available to melt the ice, making it less slippery. This supported their theory that frictional melting is responsible for ice’s slipperiness.

Although this explanation still appears in textbooks, many scientists disagree with it. “The problem with that is you only melt the ice behind you, not the ice you are actually skating on,” said Daniel Bonn, a physicist at the University of Amsterdam. Ice can be slippery the moment we step on it, before any motion has occurred that could cause frictional heating.

To test the friction hypothesis, Bonn and his team created a microscopic ice-skating rink. They rotated a piece of metal (standing in for the blade of a skate) at different speeds, each time measuring the force required to move the metal and the force that the metal exerted on the ice. The ratio of these forces gave them a measure of the ice’s slipperiness. The scientists found that the slipperiness did not depend on the speed, suggesting that frictional heating—which should increase with speed—isn’t what makes ice slippery.

Hypothesis 3: Premelting

There’s another possibility: that ice’s surface is wet even before anything makes contact with it.

In 1842, the English scientist Michael Faraday observed that two touching ice cubes will freeze to each other, and even a warm hand will stick to ice. He attributed this phenomenon to a thin, premelted layer that sits on ice’s exposed surface, and that freezes again when covered up. Faraday couldn’t explain why it happens, and it took almost a century for other scientists—notably Charles Gurney and Woldemar Weyl—to propose why “surface premelting” might occur.