Ordered Phases at Liquid-Air Interfaces
Department of Physics, University of California San Diego, United States
Surfaces and interfaces often exhibit properties that are strikingly different from those of the bulk materials. I will review recent developments in studies of liquid-air interfaces, with specific focus on metallic liquids. While liquids are well-known to be amorphous in the bulk, we have discovered that the free surfaces of liquid metals exhibit a variety of novel ordered phases, such as surface-induced layering, surface freezing, surface demixing and Gibbs segregation. I will demonstrate how the development of bright synchrotron x-ray sources has enabled us to investigate these surface phases with atomic-level resolution.
Proper interpretation of ordered surface phases requires a detailed understanding of the contribution from thermally excited nanoscale capillary fluctuations decorating free liquid surfaces. While capillary excitations are generally considered a nuisance, since they only obscure the Angstrom-level surface structure, similarly to Debye-Waller factor in solids, I will show that we were able to utilize the thermal capillary waves as a probe of mechanical properties of surface-frozen atomic monolayers and bilayers in AuSi liquid eutectic alloy. Specifically, this approach allows a unique determination of the bending rigidity of the crystalline quasi-2D surface phases in alloys, as well as biologically inspired Langmuir monolayers at water-air interface and other ultrathin films.