Why is Ice Slippery? Not a Simple Question!

by Yuanzheng (Bruce) Mao

 

Figure 1 The smoothness of the curling stone sliding along the ice sheet is enormous. Picture is from https://wonderopolis.org/wonder/what-is-curling.

In winter, don’t walk on the surface of a frozen lake, even though the ice is thick and strong enough to support you. You are very likely to fall over and break your bones, because ice is extremely slippery! One the other side, this slipperiness gives us the opportunity to do exciting winter sports such as ice skating, curling (Figure 1), ice hockey, etc. None of them would be possible without slippery ice! That ice is slippery seems to be a common sense. But have you ever thought of—why is ice slippery?

Richard Feynman, winner of The Nobel Prize in Physics in 1965, reckoned that ice is unusual. Normally, when you compress a liquid a lot, it becomes solid. However, surprisingly, water expands when it solidifies—i.e., it becomes bigger when it turns into ice. So, when you stand on the ice, the pressure exerted by you compresses the ice, undoing the solidification. The ice melts! Therefore, there is a layer of water on the top of ice, which slips you over. [1] Elegant explanation, isn’t it?

However, more recent research disapproves this suggestion.

Scientists have spent decades searching for a satisfying explanation. They have made much progress but have not ended in a universal agreement. Nowadays, they are still debating on this seemingly understandable problem. The conditions when you stand on ice are in fact more complex than we imagined. There are many factors influencing the friction force between you and the ice, such as the shape of the contact surface (your shoes), the temperature of the ice, your weight, your speed of moving, etc... [2] The slightest change of the condition may lead to a huge impact on the friction.

But there is still a relatively convincing explanation: Ice is composed of a huge amount of water molecules. These molecules arrange themselves in a regular, organised and neat order. They are compact but still vibrating—no molecules in the universe ever stop! However, in these water molecules, there are still some loose molecules on the top. They can move around across the surface of ice—like marbles on the floor. Scientists call them mobile molecules. It is these mobile molecules that contribute to the extreme slipperiness of ice. Imagine that, if you step on the marbles accidently, they will roll out under your feet. And you tip over! Similar things happen for ice. If you step on these mobile water molecules, they will make you fall over. [2] [3] [4]

Scientists have also find that the ice is the most slippery at around -7℃. Too cold, there are fewer mobile water molecules. If you step on only 1 marble on the floor, compared to 100 marbles, that would be relatively luckier—much safer. Too hot, there are too many mobile water molecules, causing the ice to be softer. When you step on the ice, you carve into it. Your feet are ‘stuck’ in the ice. In this way, the friction is increased truly dramatically. [4] [5] At -7℃, there is a perfect balance between these two factors—neither too cold, nor too hot—beautiful slipperiness.

Why is ice slippery? This question is actually not as simple as you might expect. Scientists are still working hard on this question. I hope they will work it out in the near future.

Reference

𝜷: interesting

𝜷 𝜷: very interesting

[1] firewalker (2012), Richard Feynman. Why. [online] Last accessed 8 December 2021: https://www.youtube.com/watch?v=36GT2zI8lVA

𝜷 [2] Yuen Yiu (2021), Why Is Ice Slippery? It’s Not a Simple Question [online] Last accessed 8 December 2021: https://www.insidescience.org/news/why-ice-slippery-its-not-simple-question

𝜷 [3] Max Planck Society (2018), The slipperiness of ice explained [online] Last accessed 8 December 2021: https://phys.org/news/2018-05-slipperiness-ice.html

𝜷 𝜷 [4] B. Weber, Y. Nagata, S. Ketzetzi, F. Tang, W. J. Smit, H. J. Bakker, E. H. G. Backus, M. Bonn, and D. Bonn (2018), ‘Molecular Insight into the Slipperiness of Ice’, Journal of Physical Chemistry Letters https://doi.org/10.1021/acs.jpclett.8b01188

[5] Leah Crane (2021), Physicists finally worked out why ice is slippery after 150 years [online] Last accessed 8 December 2021: https://www.newscientist.com/article/2267653-physicists-finally-worked-out-why-ice-is-slippery-after-150-years/

 

Some interesting readings

1.     Richard Feynman explaining why: https://www.youtube.com/watch?v=36GT2zI8lVA

2.     Why Is Ice Slippery? It’s Not a Simple Question by Yuen Yiu: https://www.insidescience.org/news/why-ice-slippery-its-not-simple-question

3.     Physicists finally worked out why ice is slippery after 150 years by Leah Crane: https://www.newscientist.com/article/2267653-physicists-finally-worked-out-why-ice-is-slippery-after-150-years/#ixzz7EU57nFbr

4.     The slipperiness of ice explained by Max Planck Society: https://phys.org/news/2018-05-slipperiness-ice.html

5.     Molecular Insight into the Slipperiness of Ice by Bart Weber et al.: https://pubs.acs.org/doi/10.1021/acs.jpclett.8b01188

6.     Why is ice so slippery? by CNRS: https://phys.org/news/2019-11-ice-slippery.html

7.     NATURE CURIOSITY: WHY IS ICE SO SLIPPERY?: https://www.reconnectwithnature.org/news-events/the-buzz/nature-curiosity-why-is-ice-so-slippery

8.     Ice skating on water, even when it is really cold by AMOLF: https://phys.org/news/2017-11-ice-skating-cold.html

9.     Nanorheology of Interfacial Water during Ice Gliding by L. Canale et al.: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.9.041025

10.  Friction on Ice: How Temperature, Pressure, and Speed Control the Slipperiness of Ice by Rinse W. Liefferink et al.: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.011025

11.  The Surface of Ice Is Like Supercooled Liquid Water by Dr. Wilbert J. Smit et al.: https://onlinelibrary.wiley.com/doi/10.1002/anie.201707530