For nearly two centuries, students have been taught that ice is slippery because pressure or friction melts a thin surface layer into water. But a team at Saarland University has now overturned this long-standing explanation. Their work, published in Physical Review Letters, shows that slipperiness is instead caused by subtle molecular dipole interactions—a force hidden at the microscopic level that disrupts the crystalline structure of ice.
The Long-Standing Theory Under Challenge
The original idea dates back almost 200 years, to James Thomson, brother of Lord Kelvin, who suggested that pressure and friction trigger surface melting. It was a neat answer to a common experience—why we slip on frozen pavements or glide across an ice rink. But experiments never fully lined up. At extremely low temperatures, for example, skating should have been impossible because frictional melting wouldn’t occur, yet people still managed to move on ice.
The Saarland group, led by Professor Martin Müser, revisited the problem using advanced computer simulations. Their findings make clear that pressure and friction are far less significant than once believed.
Dipoles and the Hidden Slipperiness of Ice
At the heart of the discovery is the concept of a molecular dipole. Water molecules carry a partial positive and negative charge, giving them polarity. In solid ice, these dipoles are neatly ordered in a rigid lattice. But when a shoe sole, ski, or skate comes into contact, the dipoles in those materials interact with the ones in the ice.
Instead of sliding smoothly into alignment, the forces between them become “frustrated”—a term physicists use when competing forces prevent a system from settling into an orderly state. The result is that the once-structured surface layer collapses into a disordered, amorphous film. It behaves like a liquid, even at temperatures where no melting should occur.
“It turns out that neither pressure nor friction plays a particularly significant part in forming the thin liquid layer on ice,” Müser explains. “Dipole interactions are the key drivers.”
Skiing at Near Absolute Zero? Not Quite
One of the most surprising outcomes of the study is that this liquid-like film persists even in extreme cold. The team showed that dipole interactions remain active near absolute zero, meaning a lubricating layer can still form. But there’s a catch: the film becomes as viscous as honey. Technically liquid, yes—but far too sluggish to allow skiing or skating.
That challenges another old belief—that skiing below –40 °C is impossible because no liquid film could exist. As Müser points out, “That too, it turns out, is incorrect.”
Why It Matters Beyond Winter Sports
While this might sound like a quirk of physics, the implications are far broader. A better grasp of ice friction could influence everything from transport safety on frozen roads to the design of equipment for exploring icy worlds such as Europa or Enceladus. Understanding how molecular forces behave at interfaces may also inspire new materials that mimic ice’s slipperiness for industrial or medical use.
For now, though, the work of Müser and his colleagues Achraf Atila and Sergey Sukhomlinov forces physics textbooks to be rewritten. What once seemed a simple story of melting turns out to be a subtler dance of hidden molecular forces—reminding us that even the most ordinary everyday experiences can conceal unexpected science.