Evaporation Mass Flux: A Predictive Model and Experiments

Evaluation of Evaporation Fluxes for Pesticides and Low Volatile Hazardous Materials Based on Evaporation Kinetics of Net Liquids

Evaporation is the phase transition process that plays a significant role in many spheres of life and science. Volatilization of hazardous materials, pesticides, petroleum spills, etc., impacts the environment and biosphere. Predicting evaporation fluxes under specific environmental conditions is challenging from theoretical and empirical points of view. A new practical method for estimating fluxes is proposed based on our experimental results and previously published data. It is demonstrated that some parameters in theoretical equations for near-equilibrium evaporation can be estimated from experiments, and these formulas can be exploited to predict steady-state evaporation fluxes in the air in a range of 8 orders of magnitude based on a single experiment carried out for nontoxic volatile compounds.

Thermo-osmotic pressure and resistance to mass transport in a vapor-gap membrane

We have investigated the transport of fluid through a vapor-gap membrane. The transport due to a membrane temperature difference was investigated under isobaric as well as non-isobaric conditions. Such a concept is relevant for water cleaning and power production purposes. A coarse-grained water model was used for modelling transport through pores of different diameters and lengths. The wall-fluid interactions were set so as to mimic hydrophobic interactions between water and membrane. The mass transport through the membrane scaled linearly with the applied temperature difference. Soret equilibria were obtained when the thermo-osmotic pressure was 18 bar K-1. The state of the Soret equilibrium did not depend on the pore size or pore length as expected. We show that the Soret equilibrium cannot be sustained by a gradient in vapor pressure. The fluxes of heat and mass were used to compute the total resistances to the transport of heat and mass.

Evaporation in nano/molecular materials

Evaporation is a physical phenomenon with fundamental significance to both nature and technology ranging from plant transpiration to DNA engineering. Various analytical and empirical relationships have been proposed to characterize evaporation kinetics at macroscopic scales. However, theoretical models to describe the kinetics of evaporation from nano and sub-nanometer (molecular) confinements are absent. On the other hand, the fast advancements in technology concentrated on development of nano/molecular-scale devices demand appropriate models that can accurately predict physics of phase-change in these systems. A thorough understanding of the physics of evaporation in nano/molecular materials is, thus, of critical importance to develop the required models. This understanding is also crucial to explain the intriguing evaporation-related phenomena that only take place when the characteristic length of the system drops to several nanometers. Here, we comprehensively review the underlying physics of evaporation phenomenon and discuss the effects of nano/molecular confinement on evaporation. The role of liquid-wall interface-related phenomena including the effects of disjoining pressure and flow slippage on evaporation from nano/molecular confinements are discussed. Different driving forces that can induce evaporation in small confinements, such as heat transfer, pressure drop, cavitation and density fluctuations are elaborated. Hydrophobic confinement induced evaporation and its potential application for synthetic ion channels are discussed in detail. Evaporation of water as molecular clusters rather than isolated molecules is discussed. Despite the lack of experimental investigations on evaporation at nanoscale, there exist an extensive body of literature that have applied different simulation techniques to predict the phase change behavior of liquids in nanoconfinements. We infer that exploring the effect of electrostatic interactions and flow slippage to enhance evaporation from nanoconduits is an interesting topic for future endeavors. Further future studies could be devoted to developing nano/molecular channels with evaporation-based gating mechanism and utilization of 2D materials to tune energy barrier for evaporation leading to enhanced evaporation.

Study on Evaporation Characteristics of Water in Annular Liquid Pool at Low Pressures

In order to investigate the energy transfer mechanism and the nonequilibrium effect during water evaporation in its own pure vapor at low pressures, a series of precise measurements are conducted to obtain the temperature profile near the liquid-vapor interface and the evaporation rates in an annular pool in a closed chamber. The results show that the interface temperature of the vapor side is higher than that of the liquid side when water evaporates in its own pure vapor at low pressures (ranging from 394 to 1467 Pa), the temperature discontinuity across the interface exists in all experimental conditions. The magnitude of the temperature discontinuity is strongly affected by the vapor pressure. A uniform temperature layer with a thickness of about 2 mm is found below the evaporating interface because of the coupling effect of evaporation cooling and thermocapillary convection. The energy required for evaporation is mainly transferred by thermocapillary convection in the uniform temperature layer. Furthermore, the numerical simulation results confirm that the evaporation flux near the cylinders is much larger than that at the middle region, which implies that most of the latent heat required for evaporation is transferred to the interface near the cylinders.

On interfacial viscosity in nanochannels

Capillary driven transport of liquids in nanoscopic channels is an omnipresent phenomenon in nature and technology including fluid flow in the human body and plants, drug delivery, nanofluidic devices, and energy/water systems. However, the kinetics of this mass transport mechanism remains in question as the well-known Lucas-Washburn (LW) model predicts significantly faster flow rates compared to the experimental observations. We here showed the role of interfacial viscosity in capillary motion slowdown in nanochannels through a combination of experimental, analytical and molecular dynamics techniques. We showed that the slower liquid flow is due to the formation of a thin liquid layer adjacent to the channel walls with a viscosity substantially greater than the bulk liquid. By incorporating the effect of the interfacial layer, we presented a theoretical model that accurately predicts the capillarity kinetics in nanochannels of different heights. Non-equilibrium molecular dynamics simulation confirmed the obtained interfacial viscosities. The viscosities of isopropanol and ethanol within the interfacial layer were 9.048 mPa s and 4.405 mPa s, respectively (i.e. 279% and 276% greater than their bulk values). We also showed that the interfacial layers are 6.4 nm- and 5.3 nm-thick for isopropanol and ethanol, respectively.

Influence of Nanoparticles on the Evaporation Behavior of Nanofluid Droplets: A Dh Law and Underlying Mechanism

The evaporation behaviors of droplet containing nanoparticles play an important role in nanofluid combustion, spray drying, and so on. The average evaporation rate of a nanofluid droplet will decrease sharply at the end stage of droplet evaporation because the aggregation of nanoparticles on the surface of the droplet results in a shell. To illustrate the microscopic mechanism for the variation of the average evaporation rate and surface tension caused by the copper nanoparticles on the surface of the water droplet, numerical simulations based on the Brownian dynamics are conducted to study the effects of nanoparticle behaviors on the average evaporation rate and surface tension for various initial volume fractions and various distributions of nanoparticles. The results show us that the nanoparticles' distribution and the initial volume fraction of nanoparticles will greatly affect the average evaporation rate of the nanofluid droplet. Therefore, a D^h law can be expected due to the effects of added copper nanoparticles on the evaporation behavior where h will vary with the initial volume fraction and distribution of nanoparticles. Comparisons to the published results indicate that the exponent h = 2.0 for pure liquid, h < 2.0 when lyophilic nanoparticles are added, and h > 2.0 when lyophobic nanoparticles are added. In general, the most important factors to affect the evaporation rate are the volume fraction, distribution, and lyophilic nature of nanoparticles.

Atomic Scale Interfacial Transport at an Extended Evaporating Meniscus

Recent developments in fabrication techniques have enabled the production of nano- and Ångström-scale conduits. While scientists are able to conduct experimental studies to demonstrate extreme evaporation rates from these capillaries, theoretical modeling of evaporation from a few nanometers or sub-nanometer meniscus interfaces, where the adsorbed film, the transition film, and the intrinsic region are intertwined, is absent in the literature. Using the computational setup constructed, we first identified the detailed profile of a nanoscale evaporating interface and then discovered the existence of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, reducing the excessively estimated evaporation flux values. This work takes the first step toward a comprehensive understanding of atomic/molecular scale interfacial transport at extended evaporating menisci. The modeling strategy used in this study opens an opportunity for computational experimentation of steady-state evaporation and condensation at liquid-vapor interfaces located in capillary nanoconduits.

Ultrahigh Evaporative Heat Fluxes in Nanoconfined Geometries

Advancement in high-performance photonics/electronics devices has boosted generated thermal energy, making thermal management a bottleneck for accelerated innovation in these disciplines. Although various methods have been used to tackle the thermal management problem, evaporation with nanometer fluid thickness is one of the most promising approaches for future technological demands. Here, we studied thin-film evaporation in nanochannels under absolute negative pressure in both transient and steady-state conditions. We demonstrated that thin-film evaporation in nanochannels can be a bubble-free process even at temperatures higher than boiling temperature, providing high reliability in thermal management systems. To achieve this bubble-free characteristic, the dimension of nanochannels should be smaller than the critical nucleolus dimension. In transient evaporative conditions, there is a plateau in the velocity of liquid in the nanochannels, which limits the evaporative heat flux. This limit is imposed by liquid viscous dissipation in the moving evaporative meniscus. In contrast, in steady-state condition, unprecedented average interfacial heat flux of 11 ± 2 kW cm^-2 is achieved in the nanochannels, which corresponds to liquid velocity of 0.204 m s^-1. This ultrahigh heat flux is demonstrated for a long period of time. The vapor outward transport from the interface is both advective and diffusion controlled. The momentum transport of liquid to the interface is the limiting physics of evaporation at steady state. The developed concept and platform provide a rational route to design thermal management technologies for high-performance electronic systems.