Density functional theory of gas-liquid phase separation in dilute binary mixtures

TL;DR

This paper uses density functional theory to analyze gas-liquid phase separation in dilute binary mixtures, explaining stable nanobubbles formation and their dynamics, with detailed profiles and stress calculations.

Contribution

It introduces a theoretical framework for understanding phase separation and nanobubble stability in dilute mixtures, highlighting the role of solute chemical potential differences.

Findings

Stable nanobubbles of about 30 nm radius can be energetically minimized.

Surface tension decreases with increased interfacial solute adsorption.

Bubble oscillations dampen after decompression, indicating dynamic stability.

Abstract

We examine statics and dynamics of phase-separated states of dilute binary mixtures using density functional theory. In our systems, the difference in the salvation chemical potential $\Delta\mu_s$ between liquid and gas is considerably larger than the thermal energy $k_BT$ for each solute particle and the attractive interaction among the solute particles is weaker than that among the solvent particles. In these conditions, the saturated vapor pressure increases by an amount equal to the solute density in liquid multiplied by the large factor $k_BT \exp(\Delta\mu_s/k_BT)$. As a result, phase separation is induced at low solute densities in liquid and the new phase remains in gaseous states, while the liquid pressure is outside the coexistence curve of the solvent. This explains the widely observed formation of stable nanobubbles in ambient water with a dissolved gas. We calculate the density and stress profiles across planar and spherical interfaces, where the surface tension decreases with increasing the interfacial solute adsorption. We realize stable solute-rich bubbles with radius about 30 nm, which minimize the free energy functional. We then study dynamics around such a bubble after a decompression of the surrounding liquid, where the bubble undergoes a damped oscillation. In addition, we present some exact and approximate expressions for the surface tension and the interfacial stress tensor.