Against the wall.

When a liquid makes contact with a wall, theory suggests, the atoms or molecules of the liquid organize themselves into distinct layers next to the solid surface. Now, J. Friso van der Veen of the University of Amsterdam and his coworkers have obtained direct experimental evidence of such a formation when the liquid metal gallium makes contact with a diamond wall.

The researchers observed X rays scattered by a millimetersize drop of gallium sitting on a specially prepared, ultraclean diamond surface. The measurements suggest that gallium atoms adjacent to the wall tend to pair up, with one member of the pair next to the diamond interface and the other at a characteristic distance from the surface. That produces a distinct layer of gallium atoms at the interface and another layer about 0.4 nanometer away from the wall, with relatively few gallium atoms wandering about at intermediate distances. Less distinct layers occur at distances that are small multiples of 0.4 nanometer. The spacing of the layers, which matches that of pairs of gallium atoms in the solid state, suggests that the liquid adopts a solidlike structure near the wall.

"The observed layering of liquid gallium against diamond may have implications for our general understanding of freezing transitions in atomic metals and highlights the possible role of the container wall in triggering the crystallization," the researchers conclude in the Nov. 27 Nature.