Thermally excited capillary waves at vapor/liquid interfaces of water-alcohol mixtures

TL;DR

This study uses synchrotron X-ray scattering to analyze the surface roughness and capillary wave behavior of water-alcohol mixtures, revealing that intrinsic roughness is governed by atomic scales and capillary wave theory fails at bulk correlation lengths.

Contribution

It provides detailed measurements of interfacial roughness and identifies the limits of capillary wave theory at molecular length scales in water-alcohol mixtures.

Findings

Intrinsic roughness dominated by interatomic distances

Capillary wave theory breaks down at bulk correlation lengths

Effective wave-vector cutoff determined for different mixtures

Abstract

The density profiles of liquid/vapor interfaces of water-alcohol (methanol, ethanol and propanol) mixtures were studied by surface sensitive synchrotron X-ray scattering techniques. X-ray reflectivity and diffuse scattering measurements, from the pure and mixed liquids, were analyzed in the framework of capillary-wave theory to address the characteristics length-scales of the intrinsic roughness and the shortest capillary-wavelength (alternatively, the upper wave-vector cutoff in capillary wave theory). Our results establish that the intrinsic roughness is dominated by average interatomic distances. The extracted effective upper wave-vector cutoff indicates capillary wave theory breaks-down at distances on order of bulk correlation lengths.