Computer simulation looking at how solids move across the ice confirms the presence of a self-lubricating layer on the surface of the ice, making it very slippery, according to a study published in Proceedings of the National Academy of Science (PNAS).
Slippery ice is a curious subject: ice skaters want to maximise it to improve their performance, while drivers wish to reduce it to avoid accidents on the road. “It is important to understand the origin of this widely known property of ice, both in order to improve the performance of Olympic athletes and to ensure vehicle safety during the winter,” added the researchers.
Scientists have been wondering why ice is slippery for centuries. This has attracted names like Michael Faraday, James Thomson, Osborne Reynolds, and Philip Browden, all trying to come up with the answer. This study shows that there was a bit of truth coming from many different theories. “What we, in fact, find is that the key principles of the slippery nature of ice are the surface melting phenomenon proposed by Faraday; the gradual melting caused by pressure, reminiscent of Thomson’s hypothesis, and the melting caused by friction, as proposed by Bowden,” said Luis González MacDowell, a researcher at the Complutense University of Madrid (UCM) Physical Chemistry Department.
In this study, the team conducted a computer simulation down to the atomic level of the ice. “Our analysis of how the ice molecules are collectively organised to give them their peculiar lubricant power offers us a privileged insight into the process that could not be achieved through conventional experiments, given the huge difficulty in conducting an experimental observation of a lubricating layer of a thickness of a billionth of a metre,” said MacDowell.
The authors suggest the presence of a self-repairing lubrication layer. When pressure increases, the melted lubricant layer is forced away from the opposing materials, which leaves them in direct contact. However, as this happens, the ice beneath the material melts and reforms the lubricant layer.
This knowledge could be useful beyond understanding why ice is slippery. The authors suggest the same idea could be applied in designing better lubricants in other systems. “It is important to remember that more than half the energy generated worldwide is lost through friction. Improved lubrication processes would mean a huge saving in fuel, money, and environmental impact,” concluded Pablo Llombart from the UAM’s Nicolás Cabrera Institute.
Baran L, Llombart P, Rzysko W et al (2022) Ice friction at the nanoscale. PNAS, 119 (49) e2209545119, https://doi.org/10.1073/pnas.2209545119