Sunday, November 10, 2019
Why is the ice so smooth? The question sounds banal – but the exact mechanism is surprisingly complicated and controversial for more than 150 years among scientists. However, a research team from France is now providing new insights.
Skaters glide effortlessly across the ice. What is physically underlying the gentle glide is not yet as clear as many would suspect. Now French researchers are presenting a new study. Ice is therefore so slippery, because the thin film of water on it is viscous like oil and thus represents a good lubricant, they report in the journal "Physical Review X". The fact that a thin film of water on the ice surface makes it possible to glide with ice skates or runners, is generally agreed among scientists. But although the process has been studied for more than 150 years, the exact mechanism is still controversial. "The low friction of the ice, however, remains paradoxical and contrary to the expectation that liquid water is usually considered as a poor lubricant due to its low viscosity," write the researchers around Lydéric Bocquet and Alessandro Siria from the French research institute ENS. They used an aluminum device in the form of a tuning fork to which they applied a bead of borosilicate glass with a diameter of 1.5 millimeters. By means of electromagnetic excitation and taking advantage of the natural vibrations of the tuning fork different sensors measured the vibrational motion of the glass ball. While gliding the ball over ice, the researchers were able to study the mechanical properties of meltwater to the order of nanometers (millionths of a millimeter). Cold ice is slipperier. Bocquet, Siria and colleagues found that the friction of the ice with temperature (minus 15 to zero degrees) – colder ice is so slippery. "The increase in viscosity with temperature can be interpreted as the increasing density of ice fragments as the ice softens near the melting point," explain the physicists. The depth of the ball in the ice, and thus the pressure exerted on the ice, almost did not matter how viscous the water film was. The researchers also investigated the effect of water repellent surfaces on frictional resistance – which is growing in winter sports practice of skis. They found that the friction is indeed greatly reduced, so an object slides better. The strength of the water film does not change, it always stays at a few hundred nanometers. A detailed explanation for the better gliding waxed skis leave the scientists open. Ice crystals in the water film thus ensure that the melt water on the ice becomes viscous and thus a good lubricant. Tests with the plastic polyethylene glycol (PEG) 1000 showed that the toughness in the surface melt was also greater in this material than in the completely liquid state. "Beyond ice friction, our results suggest new ways to self-healing lubricants to achieve ultra-low friction," the team said.