Why Is Ice So Slippery?

A thin layer of not-quite-water lends ice its slipperiness. fotoliza/Shutterstock

Ice is slippery.

That's a given, just like water is wet. But have you ever wondered exactly why you have to channel your inner penguin to navigate it safely?

We're probably a lot more focused on avoiding a spectacularly embarrassing, or even injurious, tumble than the scientific strangeness of ice.

Fortunately, scientists won't let a good paradox slip past them. And ice is a fascinating paradox.

Most researchers agree that ice owes its slipperiness to the thinnest layer of water resting on its surface. And yet, it's not quite the water we know — rather, it has a sticky, almost glutinous texture.

So how does something that's actually slush-like send us spiraling out of control?

Surprisingly, scientists haven't quite nailed the answer. There are at least a couple of theories on how that layer comes to be in the instant we step on it. One rather nebulous theory is that by standing on ice, we create pressure. And that pressure may be enough to melt the top layer of ice, creating a film of water that causes us to skitter uncontrollably.

"I think everybody agrees that this cannot possibly be," Mischa Bonn of the Max Planck Institute for Polymer Research in Germany, tells Live Science. "The pressures would need to be so extreme, you can't even achieve it by putting an elephant on high heels."

Another more prominent theory suggests that a film of water is created by friction — our boots hitting the ice generate just enough heat for a slight, rapid melt.

But that doesn't solve the question of why that layer of water is so slick. You could pour gallons of water on your kitchen floor and still not have an ice rink. What is it about that film of viscous water that sends us tumbling? Thanks to research published this month in the journal Physical Review X, we may finally have the answer.

The French researchers suggest that the film isn't "simple water" at all. But rather, as they note in a news release, a mix of ice water and crushed ice — similar to the properties of a snow cone. That film is water that's neither here nor there. Not quite water and not quite ice — but entirely slippery.

Person skating on ice.
Of course, depending on your approach to ice, its trademark slipperiness could be a lot of fun. Lipskiy/Shutterstock

To reach that conclusion, the researchers had to literally bend an ear to the sound that ice makes. They built a kind of tuning fork that can listen to the sounds generated as we slide on ice. As you might imagine, the device had to be sensitive enough to pick up sound generated on a molecular level.

That sound revealed a fascinating and complex profile for ice. For one thing, their research confirmed that friction is in fact responsible for creating that filmy layer. And the layer is incredibly thin — about a hundredth the thickness of a strand of hair.

But that ultra-lean layer of not-quite-melted water packs all of ice's slippery potential. It's enough to turn even the most unassuming puddle into a winter landmine. And, as the researchers suggest, deciphering its molecular properties may be key to defusing them.

Slick, dangerous roads could be rendered a lot safer in winter — and perhaps without the environmental toll we pay when we bury our streets and sidewalks in salt.

Indeed, we may soon have a cure for slipperiness.