Why ice is slippery: Scientists overturn 200 years of assumptions

Every winter, countless people step onto icy sidewalks and suddenly find themselves sliding. For generations, the explanation was simple: pressure from shoes or skis melted a thin layer of ice, making it slippery. Friction was also thought to play a role.

This idea, taught for more than a century, seemed convincing enough. But new science shows a very different story.

Researchers at Saarland University have uncovered that ice slipperiness is not caused by pressure or friction. Instead, it’s the interaction between molecular dipoles in the ice and those in materials like shoe soles.

The study challenges a belief rooted in the 19th century, when James Thomson, Lord Kelvin’s brother, first proposed the old theory.

“It turns out that neither pressure nor friction plays a particularly significant part in forming the thin liquid layer on ice,” said Professor Martin Müser, who led the research.

Dipoles make ice slippery

So what are dipoles? A dipole forms when a molecule has partial positive and negative charges, creating polarity. Water molecules naturally have this polarity, and in ice they align in a crystalline lattice.

When someone steps onto ice, it is not the force of weight but the orientation of dipoles in the shoe sole disrupting the ordered structure of ice molecules. This disturbance causes disorder at the surface.

“In three dimensions, these dipole-dipole interactions become ‘frustrated,'” said Müser.

The result is that the once-orderly lattice breaks down into an amorphous, liquid-like film. This process explains why ice remains treacherously slippery.

Old explanations overturned

The new theory also challenges long-standing explanations involving pressure melting and frictional heating. For decades, scientists debated whether skiers glide because pressure lowers the melting point or because frictional heat generates a thin water film.

But simulations show that ice can liquefy without significant heating or pressure. Instead, displacement-driven amorphization occurs.

The crystalline surface becomes disordered purely through sliding motion, with molecules shifting from their fixed lattice positions into unstable, fluid-like arrangements that mimic melting yet arise entirely from motion-induced disruptions rather than thermal energy.

In fact, ice at extremely low temperatures can amorphize faster than ice closer to its melting point. This shows that slipperiness does not depend only on thermal effects but on molecular disorder created during sliding. It highlights a deeper physical mechanism that rewrites how scientists understand everyday icy surfaces.

Slippery even in extreme cold

The research also refutes another long-standing belief. “Until now, it was assumed that skiing below -40°C (-40°F) 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,” Müser said.

“Dipole interactions persist at extremely low temperatures. Remarkably, a liquid film still forms at the interface between ice and ski – even near absolute zero.”

At such extremes, the film is thicker than honey, making skiing impractical, but the layer still exists.

Surface materials matter

The Saarland study highlights how ice friction depends not only on molecular dipoles but also on surface properties. Hydrophobic surfaces, which repel water, reduce friction more effectively than hydrophilic ones, which attract it.

This suggests that the material of skis, skates, or shoes plays a major role in how slippery ice feels. Even with disorder-induced films, friction remains small only if the surfaces allow water to slip easily.

As a result, design choices in sporting equipment, footwear, and transportation materials can directly affect performance, safety, and stability on frozen surfaces during winter and beyond everyday human movement.

Rethinking slippery ice

For everyday life, whether pressure, friction, or dipoles are responsible may not change much for someone who slips on a sidewalk. But in physics, this distinction matters deeply.

The work of Müser and his colleagues Achraf Atila and Sergey Sukhomlinov overturns nearly two centuries of accepted understanding.

It reveals that slipperiness comes from disorder, not just melting, and that cold ice can lubricate itself by forming amorphous films, reshaping how scientists think about motion, energy, and structural changes under stress at the most fundamental molecular scale.

The implications stretch beyond icy sidewalks and ski slopes. They open a new window into how materials deform, how friction works at the molecular level, and why even the coldest ice can still betray our balance, inspiring further research into novel materials and practical safety solutions.

The study is published in the journal Physical Review Letters.

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