Microscopic View on Short-Range Wetting at the Free Surface of the Binary Metallic Liquid Gallium-Bismuth: An X-ray Reflectivity and Square Gradient Theory Study

TL;DR

This study uses x-ray reflectivity and square gradient theory to analyze microscopic wetting behavior at the free surface of the binary liquid metal gallium-bismuth, revealing how phase separation and thermal fluctuations influence wetting films.

Contribution

It provides the first detailed microscopic analysis of wetting at a binary metallic liquid surface, combining experimental x-ray data with a mean-field gradient model including capillary wave effects.

Findings

Wetting films are pinned near the surface due to a balance of surface and gravitational potentials.

Thermal fluctuations have only a marginal effect on the observed wetting phenomena.

No critical wetting transition is observed in this binary metallic alloy.

Abstract

We present an x-ray reflectivity study of wetting at the free surface of the binary liquid metal gallium-bismuth (Ga-Bi) in the region where the bulk phase separates into Bi-rich and Ga-rich liquid phases. The measurements reveal the evolution of the microscopic structure of wetting films of the Bi-rich, low-surface-tension phase along different paths in the bulk phase diagram. A balance between the surface potential preferring the Bi-rich phase and the gravitational potential which favors the Ga-rich phase at the surface pins the interface of the two demixed liquid metallic phases close to the free surface. This enables us to resolve it on an Angstrom level and to apply a mean-field, square gradient model extended by thermally activated capillary waves as dominant thermal fluctuations. The sole free parameter of the gradient model, i.e. the so-called influence parameter, $\kappa$, is determined from our measurements. Relying on a calculation of the liquid/liquid interfacial tension that makes it possible to distinguish between intrinsic and capillary wave contributions to the interfacial structure we estimate that fluctuations affect the observed short-range, complete wetting phenomena only marginally. A critical wetting transition that should be sensitive to thermal fluctuations seems to be absent in this binary metallic alloy.