Extending the capillary wave model to include the effect of bending rigidity: X-ray reflectivity and diffuse scattering

TL;DR

This paper extends the capillary wave model to include bending rigidity effects, enabling improved analysis of thin film surface structures via X-ray scattering techniques, especially for soft, liquid interfaces.

Contribution

The authors develop an extended capillary wave model that incorporates bending rigidity, allowing for more accurate characterization of thin film surfaces using X-ray scattering data.

Findings

The extended model accurately describes X-ray scattering for films with bending rigidity.

Pseudo XRR approach enhances resolution and compatibility with existing software.

Experimental validation shows strong agreement with traditional methods.

Abstract

The surface roughness of a thin film at a liquid interface exhibits contributions of thermally excited fluctuations. This thermal roughness depends on temperature (T), surface tension ($\gamma$) and elastic material properties, specifically the bending modulus ($\kappa$) of the film. A non-zero $\kappa$ suppresses the thermal roughness at small length scales compared to an interface with zero $\kappa$, as expressed by the power spectral density (PSD) of the thermal roughness. The description of the X-ray scattering of the standard Capillary Wave Model (CWM), that is valid for zero $\kappa$, is extended to include the effect of $\kappa$. The extended CWM (eCWM) provides a single analytical form for both the specular XRR and the diffuse scattering around the specular reflection, and recovers the expression of the CWM at its zero $\kappa$ limit. This new theoretical approach enables the use of single-shot grazing incidence X-ray off-specular scattering (GIXOS) measurements for characterizing the structure of thin films on a liquid surface. The eCWM analysis approach decouples the thermal roughness factor from the surface scattering signal, providing direct access to the intrinsic surface-normal structure of the film and its bending modulus. Moreover, the eCWM facilitates the calculation of reflectivity at any desired resolution (pseudo XRR approach). The transformation into pseudo XRR provides the benefit of using widely available XRR software to perform GIXOS analysis. The extended range of the vertical scattering vector (Qz) available with the GIXOS-pseudo XRR approach allows for a higher spatial resolution than with conventional XRR. Experimental results are presented for various lipid systems, showing strong agreement between conventional specular XRR and pseudo XRR methods. This agreement validates the proposed approach and highlights its utility for analyzing soft, thin films.