Ice is slippery – you know that. The explanation to this was eluding physicists’ for the past 150 years. But the answer could be in now. The cause of this slipperiness of ice is governed by water molecules bouncing around in the topmost layer of the ice.
A series of experiments was carried out by Physicist Rinse Liefferink at the Institute of Physics, University of Amsterdam in the Netherlands along with his colleagues Feng-Chun Hsia, Bart Weber, and Daniel Bonn. They used spherical objects sliding across ice kept at various temperatures. Studies brought about three important factors that contribute to the complex dependence of ice friction:
(1) the temperature of ice,
(2) the pressure put by the surface of an object on the ice, and
(3) the speed of the object.
If the temperature is higher than -10°C, the ice got too soft and the sliding objects started to gouge into it, slowing them down. Below -10°C the friction between the ice’s surface and the objects rose. And we know that as friction increases, the objects will slow down. Computer simulations showed that the motion of molecules in the ice could cause this.
According to physicist Liefferink, there are water molecules dancing at the top of the ice, and this really high mobility of the water molecules, makes the ice slippery. At lower temperatures of -100°C, these water molecules are not dancing, but kind of standing still.
If there is too much pressure on the sliding object it restricts the molecular motion, decreasing the slipperiness (Physical Review X, doi.org/fvf9). Earlier it was thought that ice is primarily slippery because of a surface layer of liquid water, but Liefferink’s team did an experiment that contradicted that. The researchers used a dense plastic with similar properties to ice and dripped water over it to simulate that layer. They found that the objects only glided along easily after reaching a speed of about 1 metre per second.
Ice friction is low due to the high mobility of the water molecules at the interface of the sliding object and ice, at temperatures close to the ice melting point. This slipperiness can be suppressed by increasing the local contact pressure. The exceptionally high hardness of ice, at temperatures close to its melting point, enables the slipperiness of ice and distinguishes ice from other solids. This means that the optimal ice skate is very smooth and has sharp edges. When the smooth surface makes contact with the ice, the contact pressure, and therefore the sliding friction, is low. When the skate is tilted, the sharp edge plastically penetrates the ice, leading to high ploughing friction that enables grip, which is necessary to accelerate and turn.
- Friction on Ice: How Temperature, Pressure, and Speed Control the Slipperiness of Ice. https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.011025#fulltext
- Why is Ice so slippery? https://www.sciencedaily.com/releases/2019/11/191105104416.htm
- The Physics of Ice Skating https://www.nature.com/articles/d41586-019-03833-5