On the Molecular Picture and Interfacial Temperature Discontinuity During Evaporation and Condensation

TL;DR

This paper develops a molecular-level model for the temperature discontinuity at the liquid-vapor interface during phase change, improving upon classical equations and aligning well with experimental data.

Contribution

It introduces new boundary conditions derived from the Boltzmann equation to accurately model interfacial temperature, density, and pressure jumps during evaporation and condensation.

Findings

New boundary conditions connect fluxes to interfacial jumps.

Model aligns well with experimental observations.

Thermomolecular emission model provides molecular insights.

Abstract

Although it has been shown experimentally that a temperature discontinuity exists at the liquid-vapor interface during evaporation and condensation, quantitatively modeling this temperature jump has been difficult. The classical Schrage equation does not give enough information to determine the interfacial temperature jump. Starting from the Boltzmann transport equation, this paper establishes three interfacial boundary conditions to connect the temperature, density, and pressure jumps at the liquid-vapor interface to the interfacial mass and heat fluxes: one for the mass flux (the Schrage equation), one for the heat flux, and the third for the density discontinuities. These expressions can be readily coupled to heat and mass transport equations in the continuum of the liquid and the vapor phases, enabling one to determine the values of the interfacial temperature, density, and pressure jumps. Comparison with past experiments is favorable. A thermomolecular emission model, mimicking thermionic emission of electrons, is also presented to gain more molecular-level insights on the thermal evaporation processes.