| Press Release

Why is ice so slippery?

The answer lies in a film of water that is generated by friction, one that is far thinner than expected and much more viscous than usual water through its resemblance to the “snow cones” of crushed ice we drink during the summer. This phenomenon was recently demonstrated by researchers from the CNRS and ENS-PSL, with support from École Polytechnique, in a study that appeared in Physical Review X on November 4, 2019.
The “slippery” nature of ice is generally attributed to the formation of a thin layer of liquid water generated by friction, which for instance allows an ice skater to “surf” on top of this liquid film. The properties of this thin layer of water had never been measured: its thickness remained largely unknown, while its properties, and even its very existence, were the subject of debate. What’s more, since liquid water is known to be a poor lubricant, how could this liquid film reduce friction and make ice slippery?
To solve this paradox, researchers from the Laboratoire de physique de l’ENS (CNRS/ENS-PSL /Sorbonne Université/Université de Paris), in collaboration with a team from the Laboratoire d’hydrodynamique (LadHyX,CNRS/École Polytechnique), developed a device equipped with a tuning fork—similar to those used in music—that can “hear” the forces at work during ice gliding with remarkable precision. Despite the instrument’s size, which measures a few centimetres, it is sensitive enough to probe ice and analyse the properties of friction on a nanometric scale.
Thanks to their unique device, the scientists were able to clearly demonstrate for the first time that friction does indeed generate a film of liquid water. This film nevertheless offered a number of surprises: with a thickness measuring a few hundred nanometres to a micron, or one hundredth the thickness of a strand of hair, it is much thinner than theoretical estimates had suggested. Even more unexpectedly, this film is not at all “simple water,” but consists of water that is as viscous as oil, with complex viscoelastic properties. This unexpected behaviour suggests that surface ice does not completely transform into liquid water, but instead ends up in a mixed state similar to “snow cones,” a mix of ice water and crushed ice. The mystery of sliding on ice can therefore be found in the “viscous” nature of this film of water.
These results show that a thorough overhaul is needed of the theoretical descriptions that have been proposed to describe friction on ice. The unusual properties of meltwater are a key factor that has not been taken into consideration until now. This will help better understand the phenomenon of ice gliding, in winter sports for example, and will also help propose innovative solutions for increasing friction in order to avoid skidding on icy roads.
Nanorheology of interfacial water during ice gliding. L. Canale, J. Comtet, A. Niguès, C. Cohen, C. Clanet, A. Siria, L.Bocquet. Physical Review X, 4 november 2019. DOI : 10.1103/PhysRevX.9.041025
CNRS researcher | Lydéric Bocquet l lyderic.bocquet@ens.fr
CNRS researcher | Alessandro Siria l alessandro.siria@ens.fr
CNRS press | Alexiane Agullo | T +33 1 44 96 43 90 l alexiane.agullo@cnrs.fr

About École Polytechnique

École Polytechnique, also known as L’X, is the leading French institution combining top-level research, academics, and innovation at the cutting-edge of science and technology. Its various undergraduate and graduate-level programs – Bachelor of Science, Ingénieur Polytechnicien (Master’s level program), Master’s, and PhD – are highly selective and promote a culture of excellence with a strong emphasis on science, anchored in humanist traditions. As a widely internationalized university, École Polytechnique offers a variety of international programs and attracts a growing number of foreign students and researchers from around the globe (currently 41% of students and 40% of faculty members).

École Polytechnique offers an exceptional education to prepare bright men and women to excel in top-level key positions and lead complex and innovative projects which meet the challenges of 21st century society, all while maintaining a keen sense of their civil and social responsibilities. With its 23 laboratories, 22 of which are joint research units with the French National Center for Scientific Research (CNRS), the École Polytechnique Research Center explores the frontiers of interdisciplinary knowledge to provide major contributions to science, technology, and society. École Polytechnique is a founding member of Institut Polytechnique de Paris.

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