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Why ice is so slippery? Because it is not just ice – it is also some liquid water

Posted December 1, 2017

It is winter and it is time to enjoy it. One of the popular winter sports that all age groups love is ice skating or, as it would be more accurately to say, water skating. Why? As scientists explain, ice behaves a lot like liquid close to its surfaces. In fact, it doesn’t even matter how cold it is. Researchers proved this using modern surface-sensitive measuring technique, but you might have noticed this phenomenon yourself.

Two ice cubes in the freezer will stick together, because of thin layer of liquid water between them. Image credit: Kinnek via Wikimedia(CC BY-SA 4.0)

Ice is very slippery, but why? Some other smooth surfaces are not so slippery, while ice doesn’t even have to be particularly smooth. It turns out it is because there is always a thin layer of water on ice, even at -30 degrees Celsius. If you placed to cubes of ice together in the freezer, they will stick together very rigidly. Meantime, if you place two pieces of gold together, they will never stick. This is also because ice is always coated in a thin film of water. But is it possible to freeze ice completely?

Scientists from the University of Amsterdam used very sensitive equipment, which is able to distinguish between the outer layer of ice and its hard structure. They found that the film is four molecular layers thick at –3 degrees to two molecular layers at –30 degrees Celsius. Dropping the temperature lower eventually reduces water layer to completely nothing. That is why ice becomes less slippery as the temperature drops. When it is freezing below -30, skating is extremely difficult, but you can walk on ice without even worrying about slipping and falling. This discovery was only made possible by extremely precise equipment – previous attempts to reach the same level of accuracy were unsuccessful because older instruments could not detect this thin layer of water.

The technique that scientists were now using is called sum-frequency generation spectroscopy. It allows analysing the very surface alone, without interference from what is underneath it. Two rapid (femtosecond) lasers fire at the object from different angles at the right conditions. They only interact with the molecules of the surfaces, if everything is set-up right. This takes a lot of effort and time, but at the end a specific colour of light is revealed, which tells a lot about the molecular structure of the surface. In this case, this method allowed distinguishing between liquid and hard layers of the ice.

Everyone knows that water freezes at 0 degrees. However, now you know that just a tiny little bit of it stays liquid till -30. This may be a small interesting fact, but at least you know one of the reasons why ice is slippery.


Source: University of Amsterdam

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