The traditional theory of the question "Why do you slide on ice?" Is overturned

byChris Hau

Winter sports to be done on ice, such as speed skating competing for sliding speed on ice, is a lot. However, unexpectedly the mechanism itself "Why do you slide on ice?" Has not been elucidated until now. Finally, researchers at Max Planck Polymer Laboratories have unraveled old mysteries.

Molecular Insight into the Slipperiness of Ice - The Journal of Physical Chemistry Letters (ACS Publications)
https://pubs.acs.org/doi/10.1021/acs.jpclett.8b01188

The slipperiness of ice explained - ScienceDaily
https://www.sciencedaily.com/releases/2018/05/180509121544.htm

The old legendary view on the question "Why do skateboard shoes' blades slide on ice?" Is that the ice melts due to the pressure rising when the blade presses the ice. From the rare characteristic of water that "solid (ice) is lower in density than liquid (water)", thermodynamically it changes into high density liquid water in the direction to let it escape when pressure is applied to ice The mechanism works, it slides by the water which changed from ice. However, with this idea, we can not explain that a ground contact area like a shoe sole that is not a blade is wide and slips even in a state where the pressure is relatively small.

Research on solving the question "Why do you slide on ice?" Was done by Dr. Yuki Nagata of Max Planck Institute of PolymerResearch Team. The research team focused on the structure of "layers" that can be thinned on the surface of the ice and examined how this layer changes when the ice slides.

In the research, I discovered that by lowering the temperature of the iron ball on the ice which changed the temperature from minus 100 degrees to 0 degrees, the frictional force increases as the ice temperature decreases. In order to investigate the cause of the change in the frictional force depending on the temperature, the structure of the water molecules on the surface of the ice was analyzed by spectroscopic method,Molecular dynamics method(MD method) with the simulation results.


As a result of comparison, we found that there are two kinds of water molecules on the surface of ice. One is a water molecule which constitutes a molecule by three hydrogen bonds and enters under the ice, and the other is a water molecule which moves relatively freely by two hydrogen bonds. Common ice water molecules are bound by four hydrogen bonds, but there are water molecules with fewer hydrogen bonds on the ice surface. And it turned out that these mobile water molecules move on the ice continuously as small spheres due to thermal vibration.

We also found that these two solid water molecules are interconverted and the proportion varies with temperature change. In the experiment, it is confirmed that when the temperature exceeds minus 70 degrees, two water molecules start to increase, and the frictional force on the ice surface decreases with temperature rise. As mentioned above, it turned out that the surface of the ice slippage was caused not by the molten liquid water, but by the "mobile water molecules" present on the ice surface.


Although the friction coefficient keeps decreasing as the temperature rises, it has been confirmed that it takes the minimum value at minus 7 degrees. For this reason, researchers have found that the ice becomes soft in the temperature range of minus 7 degrees to 0 degrees, the substance on the surface of the object sliding on the ice deepens into the ice, so that the friction increases and the slip becomes difficult thinking about.