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Why is Ice so Slippery?

It is not easy to slide one solid over another. You have to deal with a very high friction force. An exception to this situation is ice. Ice is so slippery that ice skaters can slide on the ice with almost no resistance.

In the past, the slipperiness of ice was explained by the formation of a liquid layer on the surface. According to this hypothesis, the heat caused by the friction force causes the ice on the surface to become liquefied and thus the material becomes slippery. However, the accuracy of this explanation, and whether or not such a fluid addition really exists, has been controversial in the past. Moreover, one important problem with this explanation was that liquid water is not a good “lubricant”: If you want to make a surface slippery, you pour oil, not water. So, how could the extreme slipperiness of ice be explained by the presence of liquid water, which is a bad lubricant?

A group of researchers who wanted to understand why ice is extremely slippery first developed a device that can detect friction forces on the nanometer (one billionth of a meter) scale. Then, using this device, they examined how the structure of the ice changed due to friction forces. The results show that surface liquefaction has occurred in accordance with the description which is assumed to be correct in the past. However, the resulting thin film is much thinner than expected according to the theoretical predictions made in the past. Moreover, it does not seem possible that this layer, whose thickness varies from a few hundred nanometers to a micrometer, consists only of liquid water. Because the viscosity of the thin film (resistance of fluids to flow) is similar to that of oils rather than that of liquid water. Researchers suggest that the resulting thin film is a mixture of ice water and crushed ice. This structure is compared to the structure of the snow cone, one of the popular foods on summer days.

Detailed information about the research, Dr. You can find the article published in L. Canale et al. In Physical Review X.


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