Abstract
The slipperiness of ice is an everyday-life phenomenon, which, surprisingly, remains controversial despite a long scientific history. The very small friction measured on ice is classically attributed to the presence of a thin self-lubricating film of meltwater between the slider and the ice. But while the macroscale friction behavior of ice and snow has been widely investigated, very little is known about the interfacial water film and its mechanical properties. In this work, we develop a stroke-probe force measurement technique to uncover the microscopic mechanisms underlying ice lubrication. We simultaneously measure the shear friction of a bead on ice and quantify the in situ mechanical properties of the interfacial film, as well as its thickness, under various regimes of speed and temperature. In contrast with standard views, meltwater is found to exhibit a complex viscoelastic rheology, with a viscosity up to 2 orders of magnitude larger than pristine water. The unconventional rheology of meltwater provides a new, consistent, rationale for ice slipperiness. Hydrophobic coatings are furthermore shown to strongly reduce friction due to a surprising change in the local viscosity, providing an unexpected explanation for waxing effects in winter sports. Beyond ice friction, our results suggest new avenues towards self-healing lubricants to achieve ultralow friction.
- Received 2 July 2019
- Revised 4 September 2019
DOI:https://doi.org/10.1103/PhysRevX.9.041025
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Understanding why ice is slippery has evaded researchers for more than 150 years. Slipperiness is usually attributed to the formation of a thin layer of liquid water generated by friction, enabling the slider to glide over the liquid layer. However, this remains a hypothesis and raises many questions: What is the thickness of this layer, and what are its properties? In contrast to oil, water is a bad lubricant, so why would it reduce friction in this case? We develop a new force instrument that opens a new window on the origin of ice friction and allows us to probe this interfacial film at the nanometer scale for the first time.
The force apparatus is based on a tuning fork to which we glue a millimeter-sized bead of glass. By exciting the tuning fork and measuring the horizontal and vertical displacement of the bead, we are able to investigate the liquid layer that is produced as the bead slides over the ice. We find that the liquid film is far smaller than expected, just hundreds of nanometers thick. More unexpectedly, the interfacial meltwater is far from any “simple water”—it is as viscous as oil with complex viscoelastic properties. This “slimy” water film explains why ice is so slippery.
Such behavior requires a fundamental overhaul of existing frameworks describing ice friction, and our experimental results will allow researchers to build such a theory. We hope that any new theory may provide clues on how to increase slipperiness for winter sports, and it may lead to useful solutions for increasing friction, such as in automobile applications.
