Investigating liquid surfaces down to the nanometer scale using grazing incidence x-ray scattering

TL;DR

This paper uses grazing incidence x-ray scattering to explore liquid surfaces at the nanometer scale, revealing insights into surface roughness, acoustic waves, and monolayer behaviors under various conditions.

Contribution

It provides new measurements of surface bending rigidity and buckling phenomena in surfactant monolayers, extending understanding of liquid surface physics at the molecular level.

Findings

Water surface follows capillary wave model up to high wavevectors

Surface acoustic waves influence scattering at large wavevectors

Monolayer buckling occurs upon compression with characteristic wavelength

Abstract

Grazing incidence x-ray surface scattering has been used to investigate liquid surfaces down to the molecular scale. The free surface of water is well described by the capillary wave model (<z(q)z(-q)> ~ q-2 spectrum) up to wavevectors > 10^8 m^-1. At larger wavevectors near-surface acoustic waves must be taken into account. When the interface is bounded by a surfactant monolayer, it exhibits a bending stiffness and the bending rigidity modulus can be measured. However, bending effects generally cannot be described using the Helfrich Hamiltonian and the characteristic exponent in the roughness power spectrum can smaller than 4. Finally, upon compression, tethered monolayers formed on a subphase containing divalent ions are shown to buckle in the third dimension with a characteristic wavelength on the order of 10^8 m^-1.