Surface layering is a quasi-crystalline structure at the surfaces of otherwise disordered liquids, where atoms or molecules of even the simplest liquid are stratified into well-defined layers parallel to the surface.
While in crystalline solids such atomic layers can extend periodically throughout the entire dimension of a crystal, surface layering decays rapidly away from the surface and is limited to just a few near-surface region layers.
Another difference between surface layering and crystalline structure is that atoms or molecules of surface-layered liquids are not ordered in-plane, while in crystalline solids they are.
[1] Surface layering was predicted theoretically by Stuart Rice at the University of Chicago in 1983 [2] and has been experimentally discovered by Peter Pershan (Harvard) and his group, working in collaboration with Ben Ocko (Brookhaven) and Moshe Deutsch (Bar-Ilan) in 1995 in elemental liquid mercury[3] and liquid gallium[4] using x-ray reflectivity techniques.
More recently layering has been shown to arise from electronic properties of metallic liquids, rather than thermodynamic variables such as surface tension, since surfaces of low-surface tension metallic liquids such as liquid potassium are layered,[5] while those of dielectric liquids such as water, are not.