Original version With this story appeared in Quanta Magazine.
The reason we can glide gracefully on an ice rink or slip awkwardly on an icy sidewalk is because the surface of the ice is covered with a gaunt layer of water. Scientists generally agree that it is this lubricating, liquid-like layer that makes ice slippery. However, they disagree on why this layer forms.
Over the last two centuries, three main theories regarding this phenomenon have been debated. Last year, scientists from Germany he put forward a fourth hypothesis which, as they say, solves the mystery.
But is it true? Consensus appears to be getting closer, but has not yet been reached. For now, the slippery problem remains open.
Hypothesis 1: Pressure
In the mid-19th century, English engineer James Thomson suggested that when we walk 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). However, pressure lowers its melting point, so that even at lower temperatures a layer of water can 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.
However, in the 1930s, Frank P. Bowden and T. P. Hughes of the Physical Chemistry Laboratory at the University of Cambridge questioned the theory of pressure melting. They calculated that the average skier applies too little pressure to significantly change the melting point of ice. To accomplish this, the skier would have to weigh thousands of kilograms.
Hypothesis 2: Friction
Bowden and Hughes proposed an alternative explanation for the formation of the water layer: it melts due to the heat generated by the friction caused by anything that slides on it.
They tested their theory in an artificial ice cave in the Swiss Alps, using a complicated device to measure friction between ice and other materials. They found that friction was higher for materials that conduct heat well, such as brass, than for materials that conduct heat poorly, such as ebonite. From this, they concluded that when ice is rubbed with a material that readily absorbs heat, less heat is available to melt the ice, making it less slippery. This confirmed their theory that frictional melting was responsible for the slipperiness of ice.
Although this explanation still appears in textbooks, many scientists disagree with it. “The problem is you’re only melting the ice behind you, not the ice you’re actually driving on,” he said Daniel Bonnphysicist from the University of Amsterdam. Ice can become slippery the moment we step on it, before any movement occurs that could cause frictional heating.
To test the friction hypothesis, Bonn and his team created a microscopic ice rink. They rotated a piece of metal (replacing the blade of a skate) at different speeds, each time measuring the force needed to move the metal and the force the metal exerted on the ice. The ratio of these forces gave them a measure of the slipperiness of the ice. The researchers found that slipperiness did not depend on speed, suggesting that frictional heating – which should escalate with speed – is not what makes ice slippery.
Hypothesis 3: Premelting
There is another possibility: the ice surface is moist even before anything touches it.
In 1842, English scientist Michael Faraday observed that two ice cubes touching each other freeze to each other, and even a balmy hand sticks to the ice. He attributed this phenomenon to a gaunt, pre-melted layer that sits on the exposed surface of the ice and which refreezes when covered. Faraday couldn’t explain why this was happening, and it took other scientists – especially – almost a century to figure it out Charles Gurney AND Woldemar Weyl—propose why “pre-melting of the surface” might occur.