Ordered Phases at Liquid-Air Interfaces

Oleg Shpyrko

Department of Physics, University of California San Diego, United States

Abstract

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.