Why are icy surfaces slippery?

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.

Your research focus is tribology. Could you explain what that means, and has how it’s made you think differently about curling?

Tribology comes from the Greek tribos (rubbing or sliding) and is the study of interacting surfaces in motion: adhesion, friction, lubrication, and wear.

Ice friction is complex, but with curling, you add a spin on the material: a granite rock sliding over it. The spin causes the rock to ‘curl’—to follow a path that’s not straight.

Adding to the fun, the ice is deliberately textured (pebbled), and this helps the rock curl even more. Sweeping in front of the ice makes it curl less, to let players ‘steer’ the curling rock a bit, ultimately getting the rock to be exactly where you want it to be to score points.

There’s still a debate over this too: science doesn’t have a unified understanding of why curling rocks curl, why pebbling promotes it, and why sweeping inhibits it. Perhaps the amorphization model could help explain it. I might think about it more during the snowstorm.

How far away are scientists from engineering a ‘nonslip’ ice?

I recently went skating at an indoor rink made of ‘glice,’ which are slabs of a polymer whose friction is so low that it almost feels like skating on ice—but not quite. You get a lot of little shavings that scrape off from the contact of the skates that stick to your clothes and are annoying, but overall, it’s not bad, especially given that it works at room temperature. So, an interesting challenge would be to engineer synthetic ‘ice,’ meaning a material that has friction as low as ice but without needing any cooling.

As far as nonslip ice goes, I don’t know if we can ever tame water and ice to do what we want.

However, ice friction increases if you move slowly or stand still. Cross-country skiers know that while waxed skis are slippery, standing still provides initial traction. Recent curling research shows this increase in friction at low speeds is very strong. So, the best way to avoid slipping is to be patient. On an icy sidewalk, we walk slowly partly for balance, but also to avoid the drop in friction that occurs as soon as you start to slide.

So be careful in the storm—don’t rush, if you can stand the cold that long.