Thermodynamics and kinetics of nucleation of phase-separated water bubbles in synthetic quartz (a review)

Abstract

Nucleation is a physical process characterized by localized budding of a different thermodynamic phase by precipitation of vapour from bulk to form a nucleus containing liquid. In nucleation by diffusive transformati-on, the system evolves through clustering of kinetic (growth) units, embryos and nucleus formation; and enlargem-ent or expansion to a microbubble, which grows into stable bubble. Interfacial free energy is the driving force for thermodynamics of nucleation, while cluster dynamics describe the kinetics of nucleation controlled by diffusion and hydrodynamic constraints. When grown-in water in synthetic quartz becomes supersaturated upon heating, the excess dissolved water phase separates upon annealing at critical level of supersaturation, and by interconversi-on reaction, hydroxyl (OH) vapour molecules aggregate and spherical water bubbles nucleate and grow by diffusi-on of OH vapour from bulk quartz in response to Laplace pressure release.

Nucleation in quartz is initiated by aggregation and clustering of OH molecules, controlled by rate of OH vapour diffusion and condensation or precipitation, and rate at which vacancies are supplied at the nucleus surface in order to maintain the nucleus water pressure in equilibrium with the surface tension restraint during the period of annealing. In heterogeneous nucleation, formation of nucleus takes place on defects, surfaces or minute particl-es, where the contact angle becomes a controlling factor. Since hydroxyl groups are almost insoluble in quartz, the OH vapour precipitation and cluster incubation at grown-in defects are assumed to be stable. Homogeneous nucleation requires higher supersaturation than heterogeneous nucleation, as the nucleation involves initiation of new phase directly from supersaturated state by thermal fluctuations.

The main factors which influence nucleation are supersaturation, phase transition, thermal transformation and presence of defects; however, thermodynamics and kinetics models have not been examined in details for nu-cleation of phase-separated water bubbles in annealed synthetic quartz. In the review, the clustering of OH kinetic units leading to nucleation are analyzed from the perspectives of thermodynamics of Gibbs free energy and kineti-cs of OH dynamics. The two different formulations adopted for examining the nucleation energetics are formation energy and Gibbs free energy.

Keywords: Synthetic Quartz, Supersaturation, Phase Separation, Hydroxyl (OH) Kinetic (Growth) Units, Cluster-ing, Gibbs Free Energy, Formation Energy, Homogeneous and Heterogeneous Nucleation