Density Functional Theory Of Homogeneous And Heterogeneous Nucleation Of Droplet Or Bubble

Droplet (bubble) nucleation is ubiquitous in the chemical process. Investigation of the nucleation mechanism accompanying first-order phase transition is of great importance in scientific fields and industrial applications. In this thesis, based on the interactions between fluid molecules, density functional theory was constructed by integrating other statistical mechanics approaches as well as nucleation theory. Microcosmic structures of different fluids were studied in the processes of homogeneous and heterogeneous droplet (bubble) nucleation. The thermodynamic properties of droplets (bubbles), such as free energy barriers, critical nucleation radii and nucleation rate, were analyzed. The main contents are listed as follows:(1) A full density functional theory model was constructed. The global phase equilibria, planar surface tensions of Ar were calculated, and the critical exponent μwas determined. The curvature-dependent surface tensions of droplets forming under the supersaturated conditions were studied, and the Tolman length was obtained. The variation of free energy in the process of droplet nucleation was analyzed. The nucleation rates of Ar were investigated. It is found the prediction accuracy improves greatly when compared with the results of classical nucleation theory.(2) A density functional approach was constructed in order to describe the properties of mixtures. The phase coexisting curve and interfacial tensions under different pressures of a binary Lennard-Jones mixture were studied, and the results agreed well with the simulation data. The gas enrichment and solvent depletion of different mixtures were observed on the solvophobic wall. The free energy of bubble nucleation of different mixtures on the solvophobic wall was calculated. The relationship between the gas enrichment and bubble nucleation was analyzed, and the condition of spontaneous nucleation was obtained.(3) The density functional approach of mixtures was then extended to real mixture systems by integrating statistical associating fluid theory. The phase diagram and interfacial tensions of N2-H2O were calculated, and the results coincided with experimental data. The effects of temperature and surface hydrophobicity on the air enrichment were discussed. The free energy of bubble nucleation of air-water on different hydrophobic walls was researched. The condition forming stable bubbles spontaneously was obtained, which provided direct theoretical basis for explaining the formation mechanism of nanobubbles.(4) A three-dimensional density functional approach was presented. The droplet nucleation process of Lennard-Jones fluid on different walls was discussed, showing the combined effects of solid surface and the curvature of vapor-liquid interface on the microstructures of droplets. The influence of different solid surfaces on the microstructures of droplets was analyzed in the process of nucleation. The free energy variations and the critical nucleus radii in the process of heterogeneous nucleation of droplets were calculated.(5) The three-dimensional density functional approach was applied to mixed systems. The droplet nucleation and bubble nucleation of a binary Lennard-Jones fluid were studied on solid walls. Based on the three-dimensional density distributions of droplets and bubbles on different walls, the contact angles can be determined directly under the condition of vapor-liquid-solid three-phase coexistence. Besides, the line tension and contact angle can be calculated by three-dimensional total energy and two-phase interfacial tension as well as the modified Young’s equation. It was found that the contact angles obtained from the two approaches were consistent with each other, suggesting that the calculations of line tension and contact angle were reliable. Thereby, the new calculation method for line tension was built and the theoretical expression of line tension and surface tension was integrated.