Molecular Dynamics Investigation of Liquid and Vapor Interactions Near an Evaporating Interface: A Theoretical Genetics Perspective

Abstract

Understanding phase transition between the liquid and gaseous states has gained significant interest, and has been ubiquitously observed in many places ranging from natural systems to water–energy nexus and thermal management applications. Phase transition phenomena at liquid–vapor interfaces are greatly governed by intermolecular‐level kinetics, which requires the use of empirical parameters in continuum‐level relations to explain the discrete nature of molecular particles. Despite its significance, it has been a great challenge to find detailed expressions of empirical parameters such as accommodation coefficients, which represent the probabilities for phase transition of liquid or vapor molecules at the interface. Here, direct statistical measurements of accommodation coefficients are reported by tracking the trajectories of liquid and vapor molecules in molecular simulations. The measurements reveal that evaporation and condensation coefficients are different by ≈50%, whereas they have been assumed to be equal in most previous studies. Then, the indirect measurement method is studied from a perspective of theoretical genetics based on the diffusion approximation. A good agreement between two approaches suggests that diffusion approximation can contribute to provide empirical parameters with a cost‐effective method.

Evaporation of Water Nanodroplets on Heated Surfaces: Does Nano Matter?

“…At the end of the equilibration stage, before heating up the substrate to promote evaporation, the nanodroplet is in equilibrium with its vapor and adopts an equilibrium contact angle. Once the systems are in equilibrium, we instantaneously increase the temperature of the substrate to 520 K to promote evaporation, which is commensurate with temperatures previously used in simulation studies , as well as experiments on the evaporation of microscopic droplets on superheated substrates . During this stage, the dynamics of the substrate atoms are integrated in the NVT ensemble, and those of the water molecules in the NVE ensemble to avoid introducing artifacts in their dynamics.…”

“…The equilibrium contact angle, θ eq , is calculated using the last 5 ns of the equilibrium trajectories. After equilibration, we instantaneously increase the temperature of the substrate to 520 K, which is a temperature within the range relevant to droplet evaporation on superheated substrates and of similar value to that used in other previous computational studies. , The center of mass of the substrate is constrained during the simulations.…”

“…In the past decade or so, molecular dynamics (MD) simulations, which do not rely on the major simplifying assumptions of the continuum models, have been used to gain insight into the evaporation and wetting behavior of water at the molecular level. Previous work on the evaporation of water at the molecular scale and nanoscale has been primarily focused on the wetting behavior of sessile nanodroplets as a function of size and temperature, , on the evaporation of films − and free-standing nanodroplets, , and on the quantification of accommodation coefficients, which still remains a controversial issue. , Simulations of sessile nanodroplets have shed some light into the role of hydrophobicity on their evaporative behavior; − however, previous studies have overwhelmingly focused on simple Lennard-Jones fluids, thus offering limited insight into the behavior of water nanodroplets, which is mediated by directional interactions such as hydrogen bonds. To the best of our knowledge, only Reese and co-workers have simulated the evaporation of pure sessile water nanodroplets, which they found to evaporate in CCR mode on platinum surfaces.…”

Interfacial Features Govern Nanoscale Jumping Droplets

“…The goal of this paper is to explore a previously undefined regime using molecular dynamics models. In this study, dynamic characteristics of moving droplets, such as dynamic wetting, molecule positions, and velocities, are directly computed from molecular dynamics simulations, which can not be obtained from either continuum scale models or by experimental methods . Molecular dynamics models allow the understanding of how nanodroplets can physically behave differently from larger droplets, − which have not been observed through experiments.…”

Solid-like Behaviors Govern Evaporative Transport in Adsorbed Water Nanofilms

“…Since the average thickness of the water droplet is not uniform, calculations of the thermal resistances and DoS are excluded for this case and only evaporation rates are reported. The simulation outcomes allow us to calculate the evaporation rates by tracking the number of liquid and vapor molecules in the cell In this equation, N L is the liquid molecule number at each time step, Δ t is the time interval for which the liquid molecule number is counted. N AV , M L , and A LV are Avogadro’s number, water molar weight, and liquid–vapor interface area, respectively.…”