Nanorheology of interfacial water during ice gliding
TL;DR
Imagine you're trying to slide a tiny bead across an ice cube. Scientists always assumed the reason it slides easily is because a thin layer of regular water forms underneath it. These researchers built a super-sensitive machine to actually 'feel' that water layer with a tiny bead. They discovered it's not like normal water at all. Instead, it's a 'visco-elastic' fluid, meaning it's thick and gooey, almost like honey, but also springy. This gooey-but-springy nature is the real secret to ice's slipperiness. They also found that if you coat the bead with a water-repellent material, like wax on a ski, it makes this water layer less gooey, which surprisingly reduces friction even more.
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.
- 1Developed a stroke-probe force measurement technique to study ice lubrication.
- 2Measured shear friction and mechanical properties of interfacial water film.
- 3Found meltwater exhibits complex visco-elastic rheology with high viscosity.
- 4Hydrophobic coatings reduce friction by changing local viscosity.
- 5Results suggest new avenues for self-healing lubricants.
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