For more than a century, students around the world have been told that pressure and friction make ice melt. The familiar winter slip on a frozen sidewalk is often blamed on body weight pressing through the sole of your (still warm) shoe. New research from Saarland University indicates this view is incomplete, finding that slipperiness stems from interactions between molecular dipoles in the ice and those in the contacting surface, such as a shoe sole, rather than from pressure or friction.

The study by Professor Müser and colleagues Achraf Atila and Sergey Sukhomlinov challenges a model put forward nearly two hundred years ago by the brother of Lord Kelvin, James Thompson, who suggested that pressure and friction, along with temperature, cause ice to melt.

“It turns out that neither pressure nor friction plays a particularly significant part in forming the thin liquid layer on ice,” explains Martin Müser. Instead, computer simulations by the team reveal that molecular dipoles are the key drivers behind the formation of this slippery layer, which so often causes us to lose our footing in winter.

But what exactly is a dipole?