LAYERING OF A LIQUID-METAL IN CONTACT WITH A HARD-WALL

When a liquid makes contact with a solid wall, theoretical studies(1-4 ) indicate that the atoms or molecules will become layered adjacent to the wall, giving rise to an oscillatory density profile. This expecta tion has not, however, been directly verified, although an oscillatory force curve is seen for liquids compressed between solid surfaces(5), Here we present the results of an X-ray scattering study of liquid ga llium metal in contact with a (111) diamond surface. We see pronounced layering in the liquid density profile which decays exponentially wit h increasing distance from the wall. The layer spacing is about 3.8 An gstrom, which is equal to the repeat distance of (001) planes of uprig ht gallium dimers in solid alpha-gallium. Thus it appears that the liq uid near the wall assumes a solid-like structure similar to the alpha- phase, which is nucleated on freezing at lower. temperatures. This kin d of ordering should significantly influence flow, capillary osmosis, lubrication and wetting properties(5,6), and is likely to trigger hete rogeneous nucleation of the solid.