In the Olympic questions series, researchers from the University of Twente explore how technology, systems and human behaviour shape elite sport. The series offers a scientific perspective on what happens behind Olympic performance, and what that means beyond medals and results. In this story, UT researcher Alvaro Marin explains the physics of ice that make ice skating possible.
Many people learn the same explanation in school. When a skate presses on the ice, the pressure melts the ice slightly. A thin layer of water forms, and the water is slippery. It sounds logical. But it turns out to be far too simple. “If pressure melting were the full explanation, skating would not work at all,” says physicist Alvaro Marin from the University of Twente.
Ice does not behave like a normal material
Skating becomes easier when you go faster. That is already unusual. And at extremely low temperatures, around minus 40 degrees Celsius, skating becomes almost impossible. Pressure melting alone cannot explain either effect.
The phase diagram of water makes this even clearer. At the typical temperatures that one finds in a skating rink, we would need to increase a person's weight to around 700 kg to reach the liquid phase part of the diagram and see some actual pressure melting. So if skating works on Olympic rinks for normal human weights, something else must be going on.
Ice breaks the usual rules of friction
To understand what that “something else” is, it helps to look at how friction normally works. In liquids, friction increases with speed. Move your hand slowly through water, and it feels easy; move it faster, and resistance grows. With solid objects, sliding friction is mostly independent of speed. Push a box slowly or quickly across the floor, and the friction stays roughly the same.
On ice, friction actually decreases as speed increases. Every skater recognises this. Starting from a standstill takes effort, but once you are moving fast, gliding suddenly becomes smooth and easy. “That already tells us that ice is doing something unusual,” says Marin. “It is not acting like a normal solid or a normal liquid.”
The magic nano layer under your skates
The explanation lies at a scale far too small to see. When a skate moves over ice, it does not simply slide across a hard surface. The blade breaks up tiny ice crystals at the surface. These fragments form extremely small particles that together create an ultra-thin layer on top of the ice. “A magic nano layer,” as Marin likes to call it. Exactly how this layer forms, and how much of it is solid, liquid or something in between, is still an active topic of research.
This layer is only tens to hundreds of nanometres thick. It is not fully solid and not fully liquid, but something in between. And crucially, the faster the skate moves, the more this layer starts to behave like a lubricant. That is why speed skating works so well. Higher speeds strengthen the lubricating effect, reducing friction even further. Ice skating turns out to be a balance between speed, temperature and microscopic material behaviour.
This balance also explains why Olympic ice rinks are so carefully tuned. Figure skating ice is usually kept at around minus 3 degrees Celsius. That slightly higher friction gives skaters more control during jumps and landings. Speed skating ice is slightly colder. This makes the ice faster with less resistance.
But colder is not always faster. At very low temperatures, the magic nano layer becomes too thin or disappears altogether. The ice becomes too hard, and friction starts behaving like normal solid friction again. Without that special layer, skating simply stops working. “It would be like skating on pavement,” says Marin.
From Olympic ice to the classroom
At the University of Twente, ice skating doubles as a physics lesson. Marin uses elite sports in his teaching within Applied Physics and the Sport Science Minor to show students how complex physics can hide in plain sight.
Ice skating combines friction, temperature, materials science and motion, and it still holds open questions. Olympic skating only works because of a microscopic magic layer of ice crystals. It is not solid. It is not liquid. And without it, those effortless Olympic laps would never happen.
