Why we slip on ice: Physicists challenge centuries-old assumptions
Saarland University
image:
The illustration shows what happens on the surface of ice when another object, such as skis, ice skates or shoe soles, comes into contact with it: the previously orderly crystal structure of the water molecules is suddenly disrupted.
view moreCredit: AG Mueser
For over a hundred years, schoolchildren around the world have learned that ice melts when pressure and friction are applied. When you step out onto an icy pavement in winter, you can slip up because of the pressure exerted by your body weight through the sole of your (still warm) shoe. But it turns out that this explanation misses the mark. New research conducted at Saarland University reveals that it’s not pressure or friction that causes ice to become slippery, but rather the interaction between molecular dipoles in the ice and those in the contacting surface, such as a shoe sole.
This insight from Professor Müser and his colleagues Achraf Atila and Sergey Sukhomlinov overturns a paradigm established nearly two centuries ago by the brother of Lord Kelvin, James Thompson, who proposed that pressure and friction contribute to ice melting alongside temperature.
'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? A molecular dipole arises when a molecule has regions of partial positive and partial negative charge, giving the molecule an overall polarity that points in a specific direction.
To get a better understanding of what is going on, it helps to know how ice is structured. Below zero degrees Celsius, water molecules (H₂O) arrange themselves into a highly ordered crystal lattice in which the molecules are all aligned neatly with one another creating a solid, crystalline structure. When someone steps onto this orderly structure, it’s not the resulting pressure or friction of the shoe that disrupts the top layer of molecules, but the orientation of the dipoles in the shoe sole interacting with those in the ice. The previously well-ordered structure suddenly becomes disordered. ‘In three dimensions, these dipole-dipole interactions become “frustrated”,’ says Müser, referring to a concept in physics where competing forces prevent a system from achieving a fully ordered stable configuration. At the microscopic level, the forces between the dipoles in the ice and those in the shoe sole material disrupt the orderly crystalline structure at the interface between ice and shoe, causing the ice to become disordered, amorphous and ultimately liquid.
In addition to overturning nearly 200 years of accepted knowledge, the team‘s research also debunks another misconception. ‘Until now, it was assumed that skiing below –40°C is impossible because it’s simply too cold for a thin lubricating liquid film to form beneath the skis. That too, it turns out, is incorrect,’ explains Professor Müser.
‘Dipole interactions persist at extremely low temperatures. Remarkably, a liquid film still forms at the interface between ice and ski—even near absolute zero,’ says Müser. However, at such low temperatures, the film is more viscous than honey. We'd hardly recognize it as water and skiing on it would be practically impossible—but the film nevertheless exists.
For someone who is nursing an injury because they slipped and fell in winter, it hardly matters whether pressure, friction or dipoles were to blame. But for physics, the distinction is crucial. The implications of this discovery by the Saarland research team are still unfolding, and the scientific community is taking notice.
Journal
Physical Review Letters
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Cold Self-Lubrication of Sliding Ice
