Ice doesn’t need to melt to stay slippery — new simulations show its crystal structure breaks down under motion, creating a disordered, water-like layer even at minus 20°C. This overturns decades of theory and explains why skiing remains possible in extreme cold.
For more than a century, scientists have debated why ice stays slippery — especially when temperatures plunge below freezing. The dominant theory has been that a thin film of liquid water forms between the ice and the object sliding over it. But what has never been settled is how that water appears in the first place. A new study led by Martin Müser at Saarland University offers a definitive answer: ice doesn’t need to melt to become slippery. Instead, its crystal structure can locally break down under motion, forming a disordered, water-like layer even at extreme cold.
The findings, published in Physical Review Letters, challenge long-held assumptions. Textbooks have long cited three mechanisms: pressure melting, surface melting, and frictional heating. Each explains part of the phenomenon, but none fully accounts for the behavior observed in real-world conditions. Experiments show little warming during fast sliding, and pressure melting alone cannot explain skiing at minus 20°C.
Looking at Ice One Molecule at a Time
To close the gap between theory and experience, the research team turned to molecular dynamics simulations. These computer models track the motion of individual water molecules under controlled conditions. The researchers used a well-tested water model known as TIP4P/Ice, which accurately reproduces the known properties of ice and liquid water.
They began with the simplest setup: two flat ice crystals pressed together at 10 kelvins above absolute zero. Even before sliding started, tiny regions formed at the interface where molecular energy was lower than in the perfect crystal. These spots appeared where the electric dipoles of surface water molecules lined up favorably across the contact.
Once sliding began, those same regions became weak points. The crystal structure around them started to deform and lose its order. Old patches disappeared, and new ones formed farther along the sliding path. Importantly, this breakdown did not require classic crystal defects or large bursts of heat. Ice’s open structure allows local rearrangements with only small energy changes.
Why Perfect Ice Crystals Still Grip
The team also tested a popular idea from surface physics known as structural lubricity — the notion that two atomically flat but mismatched crystals can slide with almost no friction because lateral forces cancel out. If this applied to ice, perfectly smooth crystals might glide effortlessly.
The simulations showed that this does not happen for realistic ice. Even when two ice crystals were misaligned and kept dry, the shear stresses remained high, often above 100 megapascals at minus 10°C. Low friction only appeared once a sufficiently thick disordered or liquid-like layer was present at the interface.
To understand why, the researchers compared TIP4P/Ice with a simpler water model called mW. That model lacks molecular orientation effects and can show very low shear stress under ideal conditions. Yet even there, adding a small number of extra particles or defects sharply increased friction. This echoed earlier findings that tiny amounts of contamination can destroy superlow friction between smooth surfaces.
Motion, Not Heat, Drives the Change
One of the study’s central results concerns how the disordered layer grows. Once amorphization began, its thickness increased with the square root of the sliding distance. This pattern points to a process driven by displacement rather than temperature. Each small sideways motion gives surface molecules another chance to escape their crystal positions, while a thicker layer slows further growth.
“To test whether heat played a role, our team compared sliding-induced disorder with ordinary melting. Shearing one nanometer of ice at minus 10°C took about as long as melting the same amount of ice at rest when the crystal was artificially heated above freezing. Yet during sliding, the local temperature never rose above about minus 5°C,” Müser told The Brighter Side of News.
“Strain turned out to matter more than heat. A small in-plane stretch of the crystal sped up melting far more effectively than frictional warming. Even more surprising, very cold ice amorphized faster than warmer ice. At 10 kelvins, structural breakdown under sliding happened about six times faster than at minus 10°C,” he continued.
This finding challenges the common belief that skiing becomes difficult in extreme cold because ice fails to liquefy. Instead, the problem appears to be the opposite. The disordered layer that forms at low temperatures behaves like a very thick fluid with high resistance to flow.
From Simulations to Real Friction
Connecting molecular behavior to real-world slipperiness is not simple. Friction depends on surface roughness, contact geometry, water squeeze-out, and how ice deforms under load. To bridge that gap, the researchers simulated a rigid, corrugated surface pressing into ice and then sliding over it at realistic speeds.
When a hydrophilic, or water-attracting, surface was driven into ice at high pressure, it left a permanent indentation. During later sliding, the friction coefficient peaked around 0.5, similar to values measured with atomic force microscopes. Friction dropped as the surface began to glide within its own track, shaped by thin water layers and capillary forces.
When the same surface was made hydrophobic, meaning it repelled water, friction fell dramatically. Both the initial sticking force and the steady sliding resistance were about half as large. Away from the indentation, friction dropped below 0.1 and sometimes approached values associated with very slippery ice.
The reason lies in how water behaves at different surfaces. At hydrophobic walls, water can slip more easily, and stress fluctuations that dissipate energy are reduced. Although the atomic structures looked similar, the effect on friction was large.
What Makes Ice Truly Slippery
The simulations point to a clear recipe for low friction on ice. A sliding surface must allow a disordered water-like layer to form, but it must also be smooth and weakly interacting with water. Roughness and strong adhesion amplify energy loss, raising friction.
Using typical stresses and loads, the researchers estimated a lower bound for ice-on-ice friction of about 0.05 at minus 10°C. For other materials, such as steel on ice, even lower values around 0.01 are consistent, because adhesion is weaker and slip is easier.
Taken together, the results overturn several long-held assumptions. Thin premelted films matter only at the very start of sliding. After that, thicker amorphous layers created by motion dominate. Pressure melting can still play a role when roughness creates strong stress gradients, but large temperature rises are not required.
“Until now, it was assumed that skiing below minus 40 degrees Fahrenheit is impossible,” Müser said. The simulations suggest otherwise. Ice can remain slippery at such temperatures, but the disordered layer becomes so viscous that glide turns into drag.
Research findings are available online in the journal Physical Review Letters.
For skiers, this means the science of ice friction is more forgiving than previously thought. The key to smooth gliding isn’t just temperature — it’s surface chemistry and the ability to form a lubricating, disordered layer. For engineers designing ice rinks or winter machinery, it means friction control can be optimized without relying on temperature manipulation. And for anyone who’s ever slipped on a sidewalk, it means the mystery of why ice feels so slippery — even in the bitter cold — is finally solved.
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