Nucleation Of Water Vapor Research Articles

Calculation of the Nucleation Barrier and Interfacial Free Energy of New-Phase nuclei by the thermodynamic integration method using molecular dynamics simulation data

An approach to determining the nucleation barrier and interfacial free energy (surface tension) based on molecular dynamics simulations of structural transformations, in particular, the formation of new phase nuclei, is reported. The approach is based on the thermodynamic integration method, wherein key elements are trajectories characterizing the potential energy change, which are obtained from independent numerical experiments. An important feature of the approach is its applicability to both equilibrium and nonequilibrium systems, as well as the possibility of determining the above thermodynamic characteristics for small new-phase nuclei, with a significant curvature of the surface. For example, we present the temperature dependencies of the surface tension of water droplet nuclei for water vapor nucleation and of the nucleation barrier to crystal nucleation in two model glassy systems, which are computed within the framework of the proposed approach. The calculated values of the surface tension coefficient of water droplet nuclei are compared with the available experimental data.

Targeted Nanoparticle Thermometry: A Method to Measure Local Temperature at the Nanoscale Point Where Water Vapor Nucleation Occurs.

An optical nanothermometer technique based on laser trapping, moving and targeted attaching an erbium oxide nanoparticle cluster is developed to measure the local temperature. The authors apply this new nanoscale temperature measuring technique (limited by the size of the nanoparticles) to measure the temperature of vapor nucleation in water. Vapor nucleation is observed after superheating water above the boiling point for degassed and nondegassed water. The average nucleation temperature for water without gas is 560 K but this temperature is lowered by 100 K when gas is introduced into the water. The authors are able to measure the temperature inside the bubble during bubble formation and find that the temperature inside the bubble spikes to over 1000 K because the heat source (optically-heated nanorods) is no longer connected to liquid water and heat dissipation is greatly reduced.

An adsorption theory of heterogeneous nucleation of water vapour on nanoparticles

Abstract. Heterogeneous nucleation of water vapour on insoluble nuclei is a phenomenon that can induce atmospheric water and ice cloud formation. However, modelling of the phenomenon is hampered by the fact that the predictive capability of the classical heterogeneous nucleation theory is rather poor. A reliable theoretical description of the influence of different types of water-insoluble nuclei in triggering the water condensation or ice deposition would help to decrease uncertainty in large-scale model simulations. In this paper we extend a recently formulated adsorption theory of heterogeneous nucleation to be applicable to highly curved surfaces, and test the theory against laboratory data for water vapour nucleation on silica, titanium dioxide and silver oxide nanoparticles. We show that unlike the classical heterogeneous nucleation theory, the new theory is able to quantitatively predict the experimental results.

A molecular dynamics study of water vapor nucleation in the presence of ions

Ions make a significant contribution to the nucleation dynamics of aqueous aerosol particles, but the understanding about microscopic mechanism is insufficient. In this paper, molecular dynamics simulations are conducted to investigate the condensation of supersaturated water vapor in the presence and absence of ions. The Yasuoka–Matsumoto (YM) method and the kinetic analysis are extended to investigate the influence of ions on the nucleation dynamics. It is observed that the presence of ions has a dramatic influence on the water vapor nucleation, which makes the clustering of water molecules easily. The ion concentration and the supersaturation are key factors that control the phase transition in the simulation system. For a certain supersaturation, the generation rate of clusters is proportional to the ion density. Water molecules tend to condense around ions with negative charge and high electric quantity. In addition, details of the cluster change are explored by the kinetic analysis when the system is added with cations and anions simultaneously. It is found that the change rate of the cluster increases with the rise of ions and decreases with the rise of water molecules. This phenomenon can be related to the microscopic structure of clusters that water molecules wrap around the ions to form a shell. The coalescence effect plays an important role in the nucleation process, and the proportion of the coalescence effect increases as the increase of supersaturation.

Electrically Enhanced Condensation I: Effects of Corona Discharge

The effect of a corona discharge in the steam on the rate of nucleation and condensation of a liquid phase is theoretically and experimentally investigated by means of implementation with a steam condenser for an improved phase-change rate. The phenomena considered include the nucleation of water vapor on mobile charge carriers, the electrohydrodynamic vapor flow toward the condenser wall, and the thermodynamics of the charged microdroplet. The dielectrophoretic interaction of droplets and vapor is presented in terms of additional potential that adds to the evaporation energy barrier on the surface of the droplets. Experimental results obtained on the effect of a corona discharge confirm a 16% improvement in the condensation rate.

Investigation of the Growth of Particles Produced in a Laval Nozzle

This study focuses on numerical modeling of condensation of water vapor in a Laval nozzle, using the liquid drop nucleation theory. Influence of nozzle geometry, pressure, and temperature on the average drop size is reported. A computer program written in MATLAB was used used to calculate the nucleation and condensation of water vapor in the nozzle. The simulation results are validated with the available experimental data in the literature for steam condensation. The model reveals that the average drop size is reduced by increasing the divergent angle of the nozzle. The results also confirm that increasing the inlet pressure has a direct effect on the average drop size while temperature rise has an inverse effect on the drop size.

Molecular dynamics simulations of the nucleation of water: Determining the sticking probability and formation energy of a cluster

We performed molecular dynamics simulations of the nucleation of water vapor in order to test nucleation theories. Simulations were performed for a wide range of supersaturation ratios (S = 3-25) and water temperatures (T(w) = 300-390 K). We obtained the nucleation rates and the formation free energies of a subcritical cluster from the cluster size distribution. The classical nucleation theory and the modified classical nucleation theory (MCNT) overestimate the nucleation rates in all cases. The semi-phenomenological model, which corrects the MCNT prediction using the second virial coefficient of a vapor, reproduces the formation free energy of a cluster with the size ≲20 to within 10% and the nucleation rate and cluster size distributions to within one order of magnitude. The sticking probability of the vapor molecules to the clusters was also determined from the growth rates of the clusters. The sticking probability rapidly increases with the supersaturation ratio S, which is similar to the Lennard-Jones system.

Unusual Temperature Dependence of Heterogeneous Nucleation of Water Vapor on Ag Particles

Copyright 2013 American Association for Aerosol Research

Homogeneous and heterogeneous nucleation of water vapor: A comparison using molecular dynamics simulation

Using simple point charge/extended potential for water–water and a potential model for ion–water interaction, homogeneous and heterogeneous nucleation of water vapor are investigated with molecular dynamics simulation at 350K under supersaturation ratio of 8.9341 and 6.5000. The simulation results show the size and sign preference for ions, which is the rate of nucleation is higher for smaller negative ions. In spite of previous works which the position of ion was fixed in the center of a spherical cluster and the nucleation rate was derived from cluster formation free energy, we consider the ion as the second component which can move with no limitation on its position and the nucleation rate is calculated directly from the simulation data. The effect of supersaturation of water vapor on nucleation rate in presence of positive ions is also studied which indicates the important role of the ion presence in the nucleation process at low supersaturations.

Water vapor nucleation on a crystal surface in a strong electric field

Water vapor nucleation at 260 K in a transverse electric field has been simulated by the Monte Carlo method under conditions corresponding to an internal wall of a spatially extended microcrack in a silver iodide crystal. The bicanonical statistical ensemble method has been employed to calculate, at the molecular level, the free energy of addition and the work of formation of dense phase nuclei in fields with different strengths. In a moderate field, the film mechanism of nucleation characterized by intense distortions on the film surface remains preserved. A domain structure of a film layer in contact with the surface exhibits a high stability with respect to an external field and remains preserved until the film is completely destroyed. In a strong electric field, the nucleation mechanism is fundamentally changed; i.e., the film is destroyed to yield threadlike structures. Therewith, the area of the contact with the surface drastically decreases. The orientation of nanothreads along the electric field lines overcomes a low free energy barrier. The point of equilibrium of nanothreads with vapor depends on the presence of hydrogen bonds, while their stability is determined by longer-range dipole-dipole interactions. The observed form of existence of the condensate as polarized nanothreads seems to be analogous to the superpolarized state previously revealed for water microdroplets, the transition to which has the character of the first-order phase transition.

Nucleation of water vapors at temperatures above the boiling point in the presence of ion admixtures

The dependences of the Gibbs energy, entropy, and work of formation on the sizes of the centers of nucleation of a condensed phase with the size of up to 533 molecules formed on singly charged sodium cations and chlorine anions in water vapor at the temperature of 400 K have been calculated on the molecular level by the Monte Carlo method. It has been shown that, when the detailed interaction model is used, the effect of pulling the ion from microdroplets is kept at relatively high temperatures, accelerating the loss of the thermodynamic stability of the center of nucleation and the formation of the typical nucleation barrier, the same as in homogeneous microdroplets. When ion admixtures are present, the free energy barrier to the growth of centers of nucleation in strongly supersaturated vapors is considerably lowered. The nucleation velocity and minimum relative contents of the ion admixtures, able to accelerate nucleation, have been estimated in the region of strong supersaturation.

The formation and removal characteristics of aerosols in ammonia-based wet flue gas desulfurization

The characteristics of aerosol generation were studied experimentally in an ammonia-based wet flue gas desulfurization process. Particle size distributions and concentrations, morphologies and compositions before and after desulfurization were measured using an electrical low pressure impactor and scanning electron microscopy, respectively. The results show that aerosols can be generated between ammonia and sulfur dioxide resulting in gas-phase reaction and the aerosol concentration at the outlet of scrubber is significantly higher than at the inlet. Before desulfurization the particles are primarily silica-alumina minerals including O, Al, Si and C, while after ammonia-based desulfurization aerosol particles have smooth surfaces with regular structures, such as cubic and prismatic crystals, and contain principally O, S and N. Particle sizes up to 10 μm were measured, but the majority of the aerosol particles are in the submicron range. Separation of submicron particles from flue gas is difficult by conventional desulfurization scrubber; however high removal efficiency can be achieved based on the enlargement of the particles by heterogeneous condensation. The influence of the supersaturation degree and liquid–gas ratio on the removal efficiencies of the particles are demonstrated additionally. It indicates that aerosols generated in ammonia-based desulfurization process can be deliquesced and gradually enlarged in a high humidity flue gas, but the supersaturation degree higher than 1.2 is required for heterogeneous nucleation of water vapor with ash particles from coal combustion.

Numerical simulations of the three‐dimensional distribution of polar mesospheric clouds and comparisons with Cloud Imaging and Particle Size (CIPS) experiment and the Solar Occultation For Ice Experiment (SOFIE) observations

Polar mesospheric clouds (PMC) routinely form in the cold summer mesopause region when water vapor condenses to form ice. We use a three‐dimensional chemistry‐climate model based on the Whole‐Atmosphere Community Climate Model (WACCM) with sectional microphysics from the Community Aerosol and Radiation Model for Atmospheres (CARMA) to study the distribution and characteristics of PMCs formed by heterogeneous nucleation of water vapor onto meteoric smoke particles. We find good agreement between these simulations and cloud properties for the Northern Hemisphere in 2007 retrieved from the Solar Occultation for Ice Experiment (SOFIE) and the Cloud Imaging and Particle Size (CIPS) experiment from the Aeronomy of Ice in the Mesosphere (AIM) mission. The main discrepancy is that simulated ice number densities are less than those retrieved by SOFIE. This discrepancy may indicate an underprediction of nucleation rates in the model, the lack of small‐scale gravity waves in the model, or a bias in the SOFIE results. The WACCM/CARMA simulations are not very sensitive to large changes in the barrier to heterogeneous nucleation, which suggests that large supersaturations in the model nucleate smaller meteoric smoke particles than are traditionally assumed. Our simulations are very sensitive to the temperature structure of the summer mesopause, which in the model is largely dependent upon vertically propagating gravity waves that reach the mesopause region, break, and deposit momentum. We find that cloud radiative heating is important, with heating rates of up to 8 K/d.

Modelling aerosol number distributions from a vehicle exhaust with an aerosol CFD model

Vehicular traffic contributes significantly to the aerosol number concentrations at the local scale by emitting primary soot particles and forming secondary nucleated nanoparticles. Because of their potential health effects, more attention is paid to the traffic induced aerosol number distributions. The aim of this work is to explain the phenomenology leading to the formation and the evolution of the aerosol number distributions in the vicinity of a vehicle exhaust using numerical modelling. The emissions are representative of those of a light-duty diesel truck without a diesel particle filter. The atmospheric flow is modelled with a computational fluid dynamics (CFD) code to describe the dispersion of pollutants at the local scale. The CFD code, coupled to a modal aerosol model (MAM) describing the aerosol dynamics, is used to model the tailpipe plume of a vehicle with emissions corresponding to urban driving conditions. On the basis of available measurements in Schauer et al. (1999), three surrogate species are chosen to treat the semi-volatile organic compounds in the emissions. The model simulates the formation of the aerosol distribution in the exhaust plume of a vehicle as follows. After emission to the atmosphere, particles are formed by nucleation of sulphuric acid and water vapour depending strongly on the thermodynamic state of the atmosphere and on the dilution conditions. The semi-volatile organic compounds are critical for the rapid growth of nanoparticles through condensation. The semi-volatile organic compounds are also important for the evolution of primary soot particles and can contribute substantially to their chemical composition. The most influential parameters for particle formation are the sulphur fuel content, the semi-volatile organic emissions and also the mass and initial diameter of the soot particles emitted. The model is able to take into account the complex competition between nucleation, condensation and dilution, as well as the interactions among the different aerosol modes. This type of model is a useful tool to better understand the dynamics leading to the formation of traffic induced aerosol distributions. However, some key issues such as the turbulence in the exhaust plume and in the wake of the car, the magnitude and chemical composition of semi-volatile organic emissions and the possible nucleation of organic species need to be investigated further to improve our understanding of ultrafine particle formation.

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensedphase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investiga� tions carried out earlier in (1) with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investi� gated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial local� ization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the iceforming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mecha� nism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The lead� ing factor that determines the iceforming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.

A turbulent Eulerian multi-fluid model for homogeneous nucleation of water vapour in transonic flow

A model for homogeneous nucleation in rapid expansion that is applicable to moist gas and wet steam nucleating flow is presented. The governing equations of the two-phase flow are set up using the Eulerian multi-fluid model. Droplet formation is described by the Classical Nucleation Theory, and the turbulence of the two-phase flow is closed by a high-Reynolds k−ε−kp model. In this model, the droplet turbulence and the influences of gas compressibility, as well as droplet generation and influence on gas phase turbulence are all considered. CFD is used to solve the equations governing the gas phase, droplets and turbulence. The model is validated with the experimental data, and good agreement has been obtained. Moreover, this model is applied to simulate the two-dimensional nucleation process of moist gas and wet steam in the symmetrical converging–diverging nozzles with rectangular section, and some new results are obtained.

Numerical simulation of water vapor nucleation on electrically neutral nanoparticles

Atomic-level Monte Carlo simulations are performed to calculate the free energy, entropy, and work of nucleation for clusters of more than 6 × 103 water molecules growing on silver iodide crystalline particles of size up to 4 nm at a temperature of 260 K. The Hamiltonian of the system includes explicit expressions for hydrogen bonding energy and Coulomb, dispersion, exchange, and polarization interactions. The work of nucleation exhibits complex behavior depending on the nucleation-site size. With increasing nanoparticle size, clusters become less stable and the probability of crystallization increases. Mutual polarization enhances the bonding between a cluster and a crystalline particle. Cluster growth on relatively large nanoparticles involves two stages characterized by two critical sizes: monolayer growth on the surface and growth normal to the surface. Spontaneous microdroplet polarization involving domain formation is found to occur at the crystal surface. The dependence of the ice-forming activity of an aerosol on particulate size observed in experiments is explained by combined effects of several competing factors, the dominant ones being the stabilizing and destabilizing effects of the nanoparticle electric field.

Quantum-mechanical solution to fundamental problems of classical theory of water vapor nucleation

The inconsistent temperature dependence of nucleation rates, disagreement of theoretical critical or onset supersaturations with experimental data, and insufficiently accurate predictions of nucleation rates are fundamental problems of the classical nucleation theory (CNT) of water vapors, which is a foundation of various multicomponent nucleation models widely used in the aerosol microphysics, physical chemistry, and chemical technology. In the present study, a correction to the CNT obtained from "first principles" has been derived and significant progress has been made in solving the fundamental problem of predicting nucleation rates of water vapors. The modified model with the quantum-mechanical correction incorporated is in very good agreement with experiments over the full range of temperatures (T=210-290K) , saturation ratios (S=2-100) , and nucleation rates (J= approximately 10{1}-10{17} cm-3).

Stimulation of vapor nucleation on perfect and imperfect hexagonal lattice surfaces

Monte Carlo simulations of water vapor nucleation on a perfect crystal surface and on a surface with defects are performed. Mass exchange with the vapor phase is modeled by using an open ensemble. Cluster-substrate interaction is described in terms of conventional atom-atom potentials. The Hamiltonian of the system includes expressions for electrostatic, polarization, exchange, and dispersion interactions. The Gibbs free energy and work of adsorption are calculated by Monte Carlo simulation in the bicanoĭnical ensemble. The microscopic structure of nuclei is analyzed in terms of pair correlation functions. Periodic boundary conditions are used to simulate an infinite substrate surface. Molecule-substrate and molecule-molecule long-range electrostatic interactions are calculated by summing the Fourier harmonics of the electrostatic potential. Dispersion interactions are calculated by direct summation over layers of unit cells. Nucleation on a surface with matching structure follows a layer-by-layer mechanism. The work of adsorption per molecule of a monolayer on the substrate surface has a maximum as a function of nucleus size. The steady rate of nucleation of islands of supercritical size is evaluated. The work of adsorption per molecule for layer-by-layer film growth is an oscillating function of cluster size. As a function of layer number, it has a minimum depending on the vapor pressure. The electric field generated by a microscopic surface protrusion destroys the layered structure of the condensate and eliminates free-energy nucleation barriers. However, point lattice defects do not stimulate explosive nucleation.

Numerical analysis of water vapor nucleation on PM2.5 from municipal solid waste incineration

Vapor nucleation on PM 2.5 from MSWI (municipal solid waste incineration) was modeled combined with the wetting abilities, compositions and surface characteristics of the particles. Nucleation characteristics of the fine particles were numerically studied. Results show that when the mass fraction of each salt is constant the free energy barrier of embryo formation is not influenced by the total mass of soluble salts in the particle. While the total mass of soluble salts does not change, the free energy barrier changes remarkably with the compositions of the soluble salts in the particle. It is also found that the free energy barrier decreases with roughness of particle surfaces. Simulation results of the nucleation rate and the critical nucleation saturation indicate that both the compositions of the soluble salts and the surface roughness dominate the activation of the PM 2.5 from MSWI.