The interfacial thermal resistance at liquid–vapor interfaces and the role of intrinsic surface analysis

Abstract

A jump in temperature across the liquid–vapor interface has been observed in both experiments and non-equilibrium molecular dynamics (NEMD) simulations. If capillary fluctuations are removed by evaluating properties in simulations relative to the intrinsic surface, a small peak in the intrinsic density profile—referred to as an adsorbed layer—appears on the vapor side of the interface next to the temperature jump. It has been proposed that this adsorbed layer introduces a resistance to heat transfer that causes the temperature jump. We revisit this explanation by simulating liquid–vapor systems of Lennard-Jones spline particles and analyzing properties relative to the intrinsic surface at both equilibrium and non-equilibrium. We find that the intrinsic density profiles with the same liquid-phase temperature are nearly identical at equilibrium and non-equilibrium, and that the adsorbed layer is very similar in both cases. We show that, when transforming the density to time-averaged coordinates, the particles in the adsorbed layer spread across most of the interfacial region, while the large gradient in temperature is located outside the interfacial region. The reported linear correlation between vapor-side adsorption of the intrinsic density and interfacial thermal conductance is also observed using an alternative definition of adsorption based on time-averaged NEMD profiles, even though the latter contains only about 12% adsorbed particles. We argue that this correlation arises because both quantities increase with interfacial width. Although intrinsic surface analysis provides valuable insight in many contexts, we do not find evidence that the molecular origin of interfacial thermal resistance at vapor–liquid interfaces is governed by the adsorbed layer.