  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.
