# Why does Ronald Mulder experience hardly any ice friction at 60 km/h?

How can Ronald Mulder run his skates across an ice layer at 60 km/h? His skating blades get help from a lubricating layer of meltwater. In Leiden, physicists found this explanation to be incomplete. Theoretician Hans van Leeuwen and experimental physicist Tjerk Oosterkamp searched for a deeper answer.

It was already clear that ice skating is special because a skate creates its own lubricant. Friction causes the ice to melt slightly, so that the skate is always surrounded by a thin layer of water which provides lubrication at exactly the right place. Ice skating is actually aquaplaning. For aquaplaning goes: the more speed, the larger the friction. And the thicker the water layer, the smaller the friction. A skate creates a thicker water layer at higher velocity, resulting in only a slow increase of friction when a skater gains speed. Leiden physicists Hans van Leeuwen and Tjerk Oosterkamp didn’t settle for this explanation; at 60 km/h you would still experience significant friction. Now they have found a new factor which explains why Mulder indeed hardly feels increased friction while he approaches top speed.

## Independent

Emeritus professor Hans van Leeuwen conducted a careful analysis of the friction and pressure in the layer of meltwater. He found a factor which nobody noticed before. The ice gets strongly deformed by the enormous pressure built up by Mulder. At higher velocity, less deformation occurs, because the skate has less time to deform the ice. Because of this, the skate lies less deep in the ice and therefore doesn’t need to plow as much. Now to what extend can we subtract this lower ‘plow friction’ from the larger friction as a result of a higher speed? According to Van Leeuwen’s theory, both factors practically cancel each other out, causing the friction to be virtually independent of the speed. Of course air resistance plays a bigger role, but a similar increase of friction with the ice is less than previously thought.

## Parameter

To reach this explanation, Van Leeuwen collected all available literature and deduced a number of estimates for the parameters in his theory. The most important parameter is the rate of deformation for ice. He wants to see this confirmed experimentally, to verify his conclusion.

## Friday afternoon

This brings us to professor and devoted ice skater Tjerk Oosterkamp. Next to his work on quantum mechanics, he has set up an ice skating experiment together with his students ‘for Friday afternoons’. They use it to move a skate across an ice layer and measure precisely how the ice deforms at variable speed, temperature and weight. They try to find the missing value which should confirm Van Leeuwen’s conclusion. Oosterkamp: ‘This is a typical Friday afternoon experiment that we conduct with students. It is very hard to find the money to have a PhD student work on this fulltime. That is why it’s fantastic that Hans has patiently worked out his theory, which is a difficult and time-consuming job, but has also been a wonderful project.

## Puzzle

‘When I came to Leiden in 2000, Hans was already emeritus professor. Now he works with just as much passion as back then. In the past five years, we have had many conversations and Hans kept on insisting that we couldn’t get around the fact that ice not only deforms elastically, but also plastically. The pressure under the ice skate runs so high that the compressed ice doesn’t fully rebound after the skate has passed. I didn’t have the patience to puzzle the pieces together, but Hans analyzed the problem for as long as it took to get to the bottom if it. The formulas are not even that complicated, but there are a lot of them and he had to use all the tricks in the book to solve them.

## Crowdfunding

‘It’d be super if his work would be rewarded with a subsidy, so that we can do a proper thorough experiment to confirm Hans’ theory experimentally. After all this time, the experiment has become pretty complex. We need a new engine to test things at realistic velocities. That’s why we are contemplating to start a crowdfunding campaign.’

Then Oosterkamp points out a picture of king Willem-Alexander at his coronation. In between his numerous medals we perceive an elfstedentocht cross. ‘Or we’d have to search for a wealthy Leiden alumnus with a passion for ice skating…’