An object moving over a surface usually traps molecules from the surrounding air, which can either lubricate the motion or add to friction. Water molecules, in particular, may add to friction by forming into solid ice — but this has never been proven experimentally.

Credit: © 2008 APS

Now, K. B. Jinesh and Joost Frenken1 at Leiden University have provided the first direct evidence that ice can form when water is confined to a narrow layer, even at room temperature. The researchers used a friction force microscope to monitor the forces on a tungsten tip moving over a surface of graphite, under various levels of humidity. At low humidities, the tip showed a 'stick–slip' motion, getting stuck every 0.25 nm, which matches the spacing of carbon atoms in the graphite surface. When the humidity was increased, the stick–slip distance increased to 0.38 nm, closer to the lattice size of common ice structures.

The researchers believe that when there is more water in the air, ice attaches firmly to both the microscope tip and the graphite surface, and is continually broken and reformed during movement. Therefore, the conditions of confinement can overcome a major obstacle — the fact that water expands when it freezes.