Molecular dynamics study on thin film liquid phase change phenomena under ultrafast boundary heating

Abstract

Phase change characteristics of thin film liquid argon subjected to ultrafast boundary heating for different liquid film thicknesses and wettability conditions are the main objectives of the present study. Classical molecular dynamics (MD) simulation has been conducted considering a three-phase domain where liquid and vapor argon (Ar) atoms are placed over the solid platinum (Pt) surface. At first, the whole system is brought to the equilibrium condition at 90 K and then the platinum (Pt) wall temperature is elevated to a target temperature (130 K/250 K) within a range of finite time interval(0.5 ns < th < 5 ns)that results in various boundary heating rates (b) ranging from 8 × 109 K/s to 320 × 109 K/s. Different solid-liquid interaction strength ratios (0.5, 2, and 4) are used to represent hydrophobic, hydrophilic and superhydrophilic surface wetting conditions respectively while liquid argon film thicknesses (δ) are varied from 3 nm to 6 nm. Depending on the combination of boundary heating rate, liquid film thickness, and surface wetting condition, two types of phase change phenomena have been observed namely- diffusive evaporation and explosive boiling. The variations in the system temperature, pressure, net evaporation number and wall heat flux normal to the platinum (Pt) wall over time are closely investigated to explicate the evolution of thin film argon (Ar) phase change characteristics. Besides, to get a better understanding of phase change phenomena of thin film liquid argon (Ar), the time-averaged wall heat flux (qavg) obtained from the molecular dynamics simulation has been compared by calculating the thermodynamic boiling heat flux (qtherm) obtained through classical thermodynamics approach. The thermodynamic heat flux (qtherm) values are in excellent agreement with the time-averaged wall heat flux (qavg) for diffusive evaporation cases whereas for explosive boiling cases it over-predicts the average heat flux. Moreover, a comparative study has been performed on the estimation of cumulative energy density in the liquid film prior to the explosive boiling both from macroscopic as well as molecular dynamics viewpoints. The cumulative energy density within the liquid argon film as obtained from macroscopic viewpoint reasonably matches with that obtained in MD approach for hydrophilic and superhydrophilic surfaces but significant overprediction of the macroscopic energy density is found in case of the hydrophobic surface due to the associated higher thermal resistance.