A problem that has been slipping through physicists鈥� fingers for the past 150 years is finally nearing an answer. It has long been a mystery why ice is slippery, and it seems to be mostly governed by water molecules bouncing around in the topmost layer of the ice.
To try to illuminate which properties of the ice and the slider are key to slipperiness, Rinse Liefferink at the University of Amsterdam in the Netherlands and his colleagues performed a series of experiments using various spherical objects sliding across ice kept at various temperatures. They found that there are three important factors: the temperature of the ice, the pressure put on the surface of the object that touches the ice and the speed of the object.
In terms of temperature, there is a sort of sweet spot around -10掳C. At higher temperatures, the ice got too soft and the sliding objects started to gouge into it, slowing them down. And at temperatures much lower than -10掳C, the friction between the objects and the ice鈥檚 surface increased dramatically, again slowing them down.
Advertisement
Simulations showed that this may be due to the motion of the molecules in the ice. 鈥淭here are these water molecules dancing at the top of the ice, and this dancing, the really high mobility of the water molecules, makes the ice slippery,鈥� says Liefferink. 鈥淚f you go to -100掳C, they鈥檙e not dancing, but kind of standing still.鈥�
Putting too much pressure on the sliding object also confines the motion of these molecules, decreasing the slipperiness.
Previous work suggested that ice is primarily slippery because of a layer of liquid water that lets objects slide over it, but Liefferink and his colleagues performed an experiment that contradicts that. Because ice is made of water and readily melts, it is difficult to tell whether there is a liquid layer on top or not, so they used a dense plastic with similar properties to ice and dripped water over it to simulate that layer. They found that the sliding objects only glided along easily after reaching a speed of about 1 metre per second.
鈥淚f there is a water layer on ice, it鈥檚 only interesting at a high-speed regime,鈥� says Liefferink. 鈥淏ut kids, who are not in this high-speed regime, are also able to skate on ice, so it can鈥檛 be this water layer.鈥�
However, the researchers weren鈥檛 able to perform all their tests at high speeds. This means that, while we now have a solid understanding of why ice is slippery if you are moving across it slowly, there is more work yet to be done to figure out if the same mechanism keeps it slippery at high speeds.
Physical Review X
