Session: 11-13-01 Fundamentals and Applications of Evaporation, Boiling and Condensation

Paper Number: 77005

Start Time: Wednesday, 11:05 AM

77005 - Temperature Jump Across the Liquid-Gas Interface of an Evaporating Nanodroplet: A Molecular Dynamics Study 

It is well known that heat flow across material interfaces will result in a temperature drop, ΔT, at the interface due to the presence of interfacial thermal resistance (also known as Kapitza resistance, R_K). R_K can strongly affect heat transfer efficiency in nanoscale systems. The presence of R_K at a liquid-gas interface will also lead to a temperature drop at the interface when heat flows across the interface. Several experimental studies have shown the existence of temperature jumps at liquid-vapor interfaces. Nevertheless, the temperature drop at liquid-gas interface is usually ignored in continuum-level modeling of heat and mass transfer across the surface of an evaporating liquid. In a number of papers, however, it was noted that neglecting temperature jump at an evaporating droplet surface can lead to considerable errors in modeling small droplet heating and evaporation. Hence, the temperature jump at liquid-gas interfaces cannot be neglected in the analysis of evaporation of micro/nanodroplets.

There are two heat transfer modes at the liquid-gas interface, namely, evaporation and heat conduction. Interfacial heat conduction was often overlooked in the thermal analysis of an evaporating liquid surface. Our recent numerical studies show that heat conduction can play an important role in thermal transport across liquid-gas interfaces. To accurately predict R_K or ΔT at liquid-gas interfaces, therefore, interfacial heat conduction should be considered. However, evaporation and interfacial heat conduction occur simultaneously at evaporating liquid surfaces. It remains challenging in experiment to quantitatively analyze the contribution from each of the two mechanisms to the heat transfer across liquid-gas interfaces. One way to mitigate the experimental challenges is to use molecular dynamics (MD) simulations. In MD simulations, we can readily distinguish the heat flux from each of the two heat transfer modes at the evaporating surface.

In this work, we carry out MD simulations to study heating and evaporation of nanodroplets in a non-condensable gas. At quasi-steady state, the MD simulation results show an evident temperature jump across the liquid-gas interface. From the MD simulation and the analysis based on the kinetic theory of gases (KTG), we find the temperature jump is mainly caused by interfacial heat conduction, i.e., heat exchange by collisions between gas molecules and liquid surfaces. The interfacial thermal resistance predicted by the KTG-based theoretical model agrees with the MD simulation results very well. Using the theoretical model for interfacial thermal resistance, we derived the temperature jump boundary condition that can be applied in continuum modeling of evaporation of a liquid droplet in a non-condensable gas.

Presenting Author: Jesus Gutierrez Plascencia California State University, Fresno

Authors:

Zhi Liang California State University, Fresno
Jesus Gutierrez Plascencia California State University, Fresno
Eric Bird California State University, Fresno