Why Are Icy Surfaces Slippery?

Winter Storm Fern, a rare convergence of Arctic cold and Southwest moisture, seems set to bring arctic weather to many parts of the U.S. this weekend. With it, storm warnings included familiar messages: slow down, watch for black ice and assume the sidewalk is plotting against you.

But the true issue isn’t the storm itself: it’s the molecular “deal” ice strikes with everything it touches. Unlike most solids, ice refuses to act like a rigid crystal. Instead, it behaves as a self-made lubricant — especially as temperatures hover near freezing.

To help us understand why we lose our grip in icy conditions, Penn Today sat down with Robert Carpick, the John Henry Towne Professor in the School of Engineering and Applied Science. An expert in tribology — the study of friction, wear and lubrication — Carpick analyzes what happens when surfaces meet at the atomic scale. He also happens to be an avid curler, a sport where mastering the “slip” is the difference between a win and a wipeout.

Why is Ice Slippery?

Ice has an unusual property: it can melt when you apply pressure to it, whereas most materials behave the other way around — pressure usually makes liquids become solid. For a long time, people thought pressure caused slipperiness. But pressure-induced melting only happens in a very narrow temperature range, while ice remains slippery well outside those limits.

Others suggested friction from sliding — think rubbing your hands in the cold to stay warm or a shoe making contact with ice — heated the ice enough to create a melt layer. But that’s sort of a chicken-and-egg problem: generating enough heat requires some extended sliding with high friction — ice is slippery without having to slide hardly at all.

Eventually researchers realized that ice has another funny property: its surface can pre-melt, meaning it naturally has a thin layer of water on top of it, well below the melting temperature. The layer gets thicker as the temperature gets closer to the melting temperature.

However, this idea—based on ice being self-lubricated—doesn’t explain why some materials have lower friction against ice than others; if the water layer is always there, everything should be equally slippery. It’s an oversimplification, but that’s the basic argument.

Recently, researchers in Germany used simulations to show that when something touches ice, the water molecules at the surface rearrange from an ordered crystal into a disordered, amorphous structure. This isn’t caused by pressure or friction, but by microscopic electrical charges. Water molecules have positive and negative ends, and when they touch another surface, they react to the atoms in that material. They believe this electrical ‘push and pull’ disrupts the ice’s rigid structure, creating that slippery, disordered layer, which would explain why ice is slippery across different temperatures and why some materials slide on it better than others.

But in short, we don’t yet know for sure. As many have observed, despite the commonality of water and ice, their physical properties are remarkably unique.

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The top image is by mustafahacalaki via Getty Images.