Layering of a liquid metal in contact with a hard wall

When a liquid makes contact with a solid wall, theoretical studies indicate that the atoms or molecules will become layered adjacent to the wall, giving rise to an oscillatory density profile. This expectation has not, however, been directly verified, although an oscillatory force curve is seen for liquids compressed between solid surfaces. Here we present the results of an X-ray scattering study of liquid gallium metal in contact with a (111) diamond surface. We see pronounced layering in the liquid density profile which decays exponentially with increasing distance from the wall. The layer spacing is about 3.8 A, which is equal to the repeat distance of (001) planes of upright gallium dimers in solid α-gallium. Thus it appears that the liquid near the wall assumes a solid-like structure similar to the α-phase, which is nucleated on freezing at lower temperatures. This kind of ordering should significantly influence flow, capillary osmosis, lubrication and wetting properties, and is likely to trigger heterogeneous nucleation of the solid.