Ice skating is one of the most popular pastimes in the Netherlands. Everyone knows how to do it, yet physicists are only now beginning to understand the underlying processes that make it possible. In a News and Views article that appeared in the journal Nature this week, physicist Daniel Bonn explains what is known and what is unknown about the physics of ice skating.
The main mystery when it comes to the physics of ice skating is why ice gets so slippery when it is touched by a skate. It has long been thought that a layer of water forms between the skate and the ice, acting as a lubricant and allowing the skate to move over the ice with very little resistance. However, this can’t be the whole story – if it was, then why can’t we skate on a wet road or a recently mopped kitchen floor?
Bonn: ‘We are starting to find out that the layer that forms between the ice and the skate is neither liquid water nor solid ice. The layer is much more gooey than liquid water: like thick grease, it cannot easily be squeezed out from the gap between the skate and the ice. This is because, unlike liquid water, which is only viscous, the lubrication layer also appears to be elastic. It is this very special combination of properties that ultimately allows us to skate.’
In his article, Bonn refers to an experiment recently performed by a group of French physicists. The group connected a tiny glass bead to a vibrating piece of metal similar to a large tuning fork. By then allowing the glass bead to ‘skate’ over a piece of ice and measuring how the frequency of the tuning fork changed, the physicists were able to very precisely probe the properties of the lubricating layer that formed.
Bonn: ‘Experiments like this are starting to explain why skating is so specific to ice. Many interesting questions remain unanswered, though. Does the gooey layer form rapidly enough on pristine ice to lubricate immediately on contact? And why is –7°C the optimal temperature for ice skating? Speed-skating rinks across the world are kept at this temperature, simply because we know that this is what works best. What we would love to understand is why.’