Why ice is slippery

Partly settled. Some of it is understood, some is still argued about.

Faraday first proposed in the 1850s that ice has a naturally liquid-like premelted surface layer even below 0C, sparking a century of competing explanations involving pressure melting and frictional heating. Each theory alone failed key tests: pressure melting cannot explain skating at very cold temperatures, and simple friction heating models could not match measured friction coefficients. In 2019, Canale, Bocquet and colleagues used a nanoscale tuning-fork rheometer to directly measure the viscosity and thickness of the liquid film generated by a sliding object on ice, showing friction-generated premelting dominates but the film is far thinner and more viscous than assumed. Follow-up papers (2023-2024) on 'breakloose friction' and self-lubrication continue to refine the picture, so the mechanistic story is not fully closed.

A phenomenon every child feels on a frozen pond turns out to hinge on a nanometers-thick film of water whose viscosity had never been directly measured until 2019.

Explanations that circulated

• Pressure melting under a skate blade (classic textbook explanation) — contact pressures from a skater are far too low to lower ice's melting point enough to explain the slipperiness
• Frictional heating alone, without accounting for the anomalously thin and viscous premelted water film revealed by nanoscale rheology measurements

References

• Nanorheology of Interfacial Water during Ice Gliding, L. Canale, J. Comtet, A. Niguès, C. Cohen, C. Clanet, A. Siria, L. Bocquet, Physical Review X, 2019