Decay of interfacial fluid ordering probed by X-ray reflectivity

Abstract

To what extent is the ordering of fluids at interfaces governed by the bulk structure of the fluid? In order to address this question, we have studied a dense, charge-stabilized colloidal suspension at the fluid–air interface and at two different solid–fluid interfaces using specular X-ray reflectivity. The experimental data are well described by a simple model of a stratified fluid, with the wall–particle potential mainly affecting the position of the first particle layer with respect to the interface. The decay of the fluid's density profile can for all the studied interfaces be described by a characteristic wavelength and a decay length, both of which are independently determined from the bulk phase using small-angle X-ray scattering. This latter finding is consistent with theoretical predictions and recent surface-force experiments.