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 visco-elastic rheology, with a viscosity up to two orders of magnitude larger than pristine water. The non-conventional 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 ultra-low friction.
Highlights
Ice and snow exhibit outstanding friction properties, with exceptionally low friction coefficients [1]
Our findings suggest to reconsider the standard framework for ice friction, as well as the dissipative mechanisms occurring in the lubricating film
Modeling the intertwinned relationships between the mechanical, rheological, and thermodynamic mechanisms is challenging but our results provide a guide and an experimental benchmark to revisit the standard framework of ice friction
Summary
Ice and snow exhibit outstanding friction properties, with exceptionally low friction coefficients [1]. The pioneering works of Bowden and Hughes [17,18] and later Colbeck [19] have discarded pressure-melting mechanisms in ice and snow friction and suggested frictional melting: viscous dissipation generates heat, which raises the temperature in the contact region up to the melting point, hereby creating a water lubricating film This scenario has been further explored by numerous macroscopic tribological measurements [20,21,22,23], supported—at least partly—by theoretical frameworks [3,16,19,24]. We investigate simultaneously the friction of a millimetric slider on ice and the corresponding interfacial mechanical properties of the meltwater film at the nanoscale To this end, we harvest the possibilities offered by a newly introduced force measurement apparatus [28,29], to realize here a “stroke-probe” tribometer with nanoscopic sensitivity. A millimetric borosilicate glass bead [Fig. 1(a), see Fig. S2 of the
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