Equivalence between condensation and boiling in a Lennard Jones fluid

TL;DR

This study demonstrates that condensation and boiling are fundamentally equivalent processes in a Lennard Jones fluid by analyzing critical nuclei and surface tension, and introduces a method to estimate the Tolman length for curved interfaces.

Contribution

It provides a novel simulation-based approach to unify the understanding of condensation and boiling, linking their nucleation processes through a common surface tension framework.

Findings

Surface tension is the same for drops and bubbles of the same radius.

Condensation and boiling are two sides of the same nucleation process.

Estimated the Tolman length using combined data from drops and bubbles.

Abstract

Condensation and boiling are phase transitions highly relevant to industry, geology or atmospheric science. These phase transitions are initiated by the nucleation of a drop in a supersaturated vapor and of a bubble in an overstretched liquid respectively. The surface tension between both phases, liquid and vapor, is a key parameter in the development of such nucleation stage. Whereas the surface tension can be readily measured for a flat interface, there are technical and conceptual limitations to obtain it for the curved interface of the nucleus. On the technical side, it is quite difficult to observe a critical nucleus in experiments. From a conceptual point of view, the interfacial free energy depends on the choice of the dividing surface, being the surface of tension the one relevant for nucleation. We bypass the technical limitation by performing simulations of a Lennard Jones fluid where we equilibrate critical nuclei (both drops and bubbles). Regarding the conceptual hurdle, we find the relevant cluster size by searching the radius that correctly predicts nucleation rates and nucleation free energy barriers when combined with Classical Nucleation Theory. With such definition of the cluster size we find the same value of the surface tension for drops and bubbles of a given radius. Thus, condensation and boiling can be viewed as two sides of the same coin. Finally, we combine the data coming from drops and bubbles to obtain, via two different routes, estimates of the Tolman length, a parameter that allows describing the curvature dependence of the surface tension in a theoretical framework.