The Birth of a Bubble: a Molecular Simulation Study

We study the nucleation of a bubble in a metastable Lennard-Jones (LJ) fluid, confined to a spherical pore with wetting walls, by a combination of grand canonical, canonical ensemble, and gauge cell Monte Carlo simulation methods complemented by the Voronoi-Delaunay tessellation analysis of statistical geometry of intermolecular cavities. We construct the isotherm of confined fluid in the form of a continuous van der Waals' loop, in which the unstable backward trajectory between the spinodals corresponds to bubble states. We show that as the degree of metastability increases and the fluid becomes progressively stretched, the decrease of fluid density is associated with the evolution of a population of interstitial intermolecular cavities. At the spinodal, the fluid becomes mechanically unstable: Interstitial cavities partly coalesce into a larger cavity located due to the system symmetry around the pore center. This cavity represents a bubble embryo, which grows at the expense of interstitial cavities. The nucleation barrier is calculated by direct thermodynamic integration along the isotherm. We compare our simulation results to the predictions of the classical nucleation theory and experiments on capillary condensation-evaporation of nitrogen in pores of hybrid organic-inorganic mesoporous molecular sieve HMM-3.