Supersaturated Vapor Phase Research Articles

Reaction coordinates and rate constants for liquid droplet nucleation: Quantifying the interplay between driving force and memory

In this work, we revisit the classic problem of homogeneous nucleation of a liquid droplet in a supersaturated vapor phase. We consider this at different extents of the driving force, or equivalently the supersaturation, and calculate a reaction coordinate (RC) for nucleation as the driving force is varied. The RC is constructed as a linear combination of three order parameters, where one accounts for the number of liquidlike atoms and the other two for local density fluctuations. The RC is calculated from biased and unbiased molecular dynamics (MD) simulations using the spectral gap optimization approach "SGOOP" [P. Tiwary and B. J. Berne, Proc. Natl. Acad. Sci. U. S. A. 113, 2839 (2016)]. Our key finding is that as the supersaturation decreases, the RC ceases to simply be the number of liquidlike atoms, and instead, it becomes important to explicitly consider local density fluctuations that correlate with shape and density variations in the nucleus. All three order parameters are found to have similar barriers in their respective potentials of mean force; however, as the supersaturation decreases, the density fluctuations decorrelate slower and thus carry longer memory. Thus, at lower supersaturations, density fluctuations are non-Markovian and cannot be simply ignored from the RC by virtue of being noise. Finally, we use this optimized RC to calculate nucleation rates in the infrequent metadynamics framework and show that it leads to a more accurate estimate of the nucleation rate with four orders of magnitude acceleration relative to unbiased MD.

Numerical Investigation on Wet Removal Properties of Superfine Aerosol Particles

Wet removal efficiency of aerosol particles (AP) are investigated in this paper, three main theory models of AP coalescence, water droplets (WD) capture, and supersaturated vapor phase are introduced. Grade removal efficiency (GRE) versus AP diameter is numerical obtained and compared with experimental data only under the WD capture. Then, the influence of WD diameter on GRE is examined. Simulation results show that GRE can be effectively improved with decreasing WD diameter especially for AP in the 0.1-2.5um diameter range. Furthermore, the removal rate under three mechanisms is increased greatly. Therefore, taking advantage of multiple mechanisms and exploring the proper wet removal technical will provide the guide for the future application.

A statistical mechanics investigation about general aspects of wetting transition occurring in nonpolar neutral molecule system with a smooth solid wall

Classical density functional theory is used to study the general aspects of wetting phenomena occurring in nonpolar neutral molecule system near a flat solid wall. Current cognitions of the wetting behavior are looked at, validated, corrected and extended. Several new observations are made: (i) the present theoretical calculations suggest that over a broad range of the surface potential parameters the pre-wetting transition temperature interval significantly reduces with the wetting temperature, and at the same time a continuous and monotonous increase of the wetting temperature is induced by decreasing the surface potential range and/or relative strength of surface atom versus fluid atom interaction. (ii) There exist lower limit values of both the surface potential range and the relative strength, below which the wetting transition is impossible. (iv) The present theoretical calculations confirm an experimentally found no adsorption occurring in weak surface potential substrates all the way to the relevant critical temperature by showing the impossibility of wetting at bulk critical temperature and its surroundings; moreover, the calculations extend the experimental observation to a wider parameter domain by indicating that the impossibility is not only unique feature of the weak surface potential substrates, but also applies to all situations of the surface potential range and relative strength. (v) With progressive increase of the relative strength and/or progressive decrease of the temperature, the interfacial phase behavior becomes increasingly complex: the (pre-)wetting transition and layering transition (which can be thermodynamically stable, metastable or even situated at supersaturated vapor phase) are intertwined closely; this challenges the popular knowledge that there is no (pre-)wetting for strong substrates for which the pre-wetting is replaced by the layering transitions.

Improved coarse-grained model for molecular-dynamics simulations of water nucleation

We developed a new coarse-grained (CG) model for water to study nucleation of droplets from the vapor phase. The resulting potential has a more flexible functional form and a longer range cutoff compared to other CG potentials available for water. This allowed us to extend the range of applicability of coarse-grained techniques to nucleation phenomena. By improving the description of the interactions between water molecules in the gas phase, we obtained CG model that gives similar results than the all-atom (AA) TIP4P model but at a lower computational cost. In this work we present the validation of the potential and its application to the study of nucleation of water droplets from the supersaturated vapor phase via molecular-dynamics simulations. The computed nucleation rates at T = 320 K and 350 K at different supersaturations, ranging from 5 to 15, compare very well with AA TIP4P simulations and show the right dependence on the temperature compared with available experimental data. To help comparison with the experiments, we explored in detail the different ways to control the temperature and the effects on nucleation.

Phase-field crystal model with a vapor phase

Phase-field crystal (PFC) models are able to resolve atomic length scale features of materials during temporal evolution over diffusive time scales. Traditional PFC models contain solid and liquid phases, however many important materials processing phenomena involve a vapor phase as well. In this work, we add a vapor phase to an existing PFC model and show realistic interfacial phenomena near the triple point temperature. For example, the PFC model exhibits density oscillations at liquid-vapor interfaces that compare favorably to data available for interfaces in metallic systems from both experiment and molecular-dynamics simulations. We also quantify the anisotropic solid-vapor surface energy for a two-dimensional PFC hexagonal crystal and find well-defined step energies from measurements on the faceted interfaces. Additionally, the strain field beneath a stepped interface is characterized and shown to qualitatively reproduce predictions from continuum models, simulations, and experimental data. Finally, we examine the dynamic case of step-flow growth of a crystal into a supersaturated vapor phase. The ability to model such a wide range of surface and bulk defects makes this PFC model a useful tool to study processing techniques such as chemical vapor deposition or vapor-liquid-solid growth of nanowires.

Removal of fine particles by heterogeneous condensation in the double-alkali desulfurization process

Removal of fine particles by heterogeneous condensation in the rotating-stream-tray scrubber was investigated experimentally for the double-alkali desulfurization process in this paper. A supersaturated vapor phase, necessary for condensational growth of fine particles, was achieved in the SO 2 absorption zone and at the top of the scrubber by adding steam in the gas inlet, between the pieces of rotating-stream-tray and above the desulfurization liquid inlet of the scrubber, respectively. Fine particles grew in size by vapor heterogeneous condensation with the particles acting as nucleation centers. Then the condensational grown droplets were removed efficiently by the desulfurization liquid and a high-efficiency demister. In order to optimize the removal process, the influences of temperature of inlet flue gas and desulfurization liquid, steam addition method and amount of steam added on the particle removal efficiency were presented. The results show that a few fine particles could be removed in the rotating-stream-tray scrubber. The removal efficiency can be significantly improved for various steam addition cases, and the improve performance is related to the method and the amount of steam addition. Particle removal efficiency with steam added between the pieces of rotating-stream-tray is higher than that of else steam addition cases.

Removal of fine particles in wet flue gas desulfurization system by heterogeneous condensation

A novel process to remove fine particles with high efficiency by heterogeneous condensation in a wet flue gas desulfurization (WFGD) system is presented. A supersaturated vapor phase, necessary for condensational growth of fine particles, was achieved in the SO2 absorption zone and at the top of the wet FGD scrubber by adding steam in the gas inlet and above the scrubbing liquid inlet of the scrubber, respectively. The condensational grown droplets were then removed by the scrubbing liquid and a high-efficiency demister. The results show that the effectiveness of the WFGD system for removal of fine particles is related to the SO2 absorbent employed. When using CaCO3 and NH3·H2O to remove SO2 from flue gas, the fine particle removal efficiencies are lower than those for Na2CO3 and water, and the morphology and elemental composition of fine particles are changed. This effect can be attributed to the formation of aerosol particles in the limestone and ammonia-based FGD processes. The performance of the WFGD system for removal of fine particles can be significantly improved for both steam addition cases, for which the removal efficiency increases with increasing amount of added steam. A high liquid to gas ratio is beneficial for efficient removal of fine particles by heterogeneous condensation of water vapor.

Experimental investigation on removal of coal-fired fine particles by a condensation scrubber

The removal of coal-fired fine particles by heterogeneous condensation of water vapor in a scrubber was investigated. Supersaturated vapor phase necessary for the condensation growth of fine particles was achieved in the scrubbing zone as well as in the top of the scrubber by means of steam addition in the gas inlet and above the scrubbing liquid inlet, respectively. The condensation grown droplets were then removed by the scrubbing liquid and high-efficient demister. Influences of the steam addition amount, the temperature difference between the inlet gas and scrubbing liquid, and the liquid-to-gas ratio, etc. on removal efficiency were examined. The results show that with a steam addition amount of 0 .04 kg/ m STP 3 flue gas, number removal efficiencies of more than 60% and 70% can be attained for the two cases of steam addition, respectively. For the cases with steam addition in the gas inlet of scrubber, higher temperature difference between the inlet gas and scrubbing liquid enhances the removal of fine particles. Under the conditions of steam addition above the scrubbing liquid inlet, the removal efficiency decreases with increasing temperature difference. The removal efficiency can be effectively improved by increasing the liquid-to-gas ratio for both steam addition modes.

Elucidating the Mechanism of Nucleation near the Gas-Liquid Spinodal

We have constructed a multidimensional free energy surface of nucleation of the liquid phase from the parent supercooled and supersaturated vapor phase near the gas-liquid spinodal. In particular, we remove the Becker-Doring constraint of having only one growing cluster in the system. Close to the spinodal, the free energy, as a function of the size of the largest cluster, develops surprisingly a minimum at a subcritical cluster size. It is this minimum at intermediate size that is found to be responsible for the barrier towards further growth of the nucleus at large supersaturation. An alternative free energy pathway involving the participation of many subcritical clusters is found near the spinodal where the growth of the nucleus is promoted by a coalescence mechanism. The growth of the stable phase becomes collective and spatially diffuse, and the significance of a "critical nucleus" is lost for deeper quenches.

Molecular-dynamics simulation of argon nucleation from supersaturated vapor in the NVE ensemble

The possibility to conduct simulations of homogeneous nucleation of argon from a supersaturated vapor phase using a microcanonical or NVE ensemble is evaluated (NVE: number of particles N, volume V, and energy E are constant). In order to initiate a phase separation kinetic energy is removed from the system in one step which transfers the system into a supersaturated state. After this temperature jump the simulation is continued in a NVE ensemble. The simulations are performed for different initial-state points and different temperature jumps. The cluster formation and growth over the course of the adiabatic simulations are analyzed. The progression of the temperature being related to the cluster size in NVE systems is traced. Also the influence of the size of the simulation system is investigated. For a certain range of low supersaturation a dynamic coexistence between two states has been found. Furthermore, the obtained nucleation rates are correlated with two simple functions. By applying the nucleation theorems to these functions the size and excess energy of the critical cluster are estimated. The results are consistent with other theoretical data and experimental data available in the literature.

Equation of State for Free Energy of Homogeneous Nucleation in Supersaturated Lennard-Jones Vapor Phase Derived by Monte Carlo Simulations

Abstract We estimated the free energy of homogeneous nucleation in supersaturated Lennard-Jones vapor phase by using Monte Carlo simulations. The nucleation free energy of an N-particle cluster is obtained as the difference between the free energies of the cluster phase and the monomer phase in an N-particle system. In the previous papers, we estimated the Helmholtz free energy of the nucleation by MC calculations under a fixed volume per particle, V/N = 43.2 σ3, and the equation of state of the nucleation energy as a function of cluster size and temperature. In this study, we carried out some additional MC calculations under different volumes per particle and took into consideration the pressure to estimate the Gibbs free energy of the nucleation. Then we established the equations of state of the Helmholtz and Gibbs free energies of nucleation as a function of cluster size, temperature, and volume or pressure.

Dynamics of atoms in a condensing cluster.

The dynamics of single particles in a cluster on condensation from the supersaturated vapor phase is studied by a kinetic approach. An insight into the distinctive flow field in the vicinity of a cluster is obtained for initial and late stage evolution. Inside the core the single atoms diffuse freely and the initial velocity decays rapidly with time. In the interfacial region between the cluster core and the vapor, the surface pressure produces a directed radial motion and a long time radial drift into the cluster core. Far from the cluster, the atoms move in the vapor state of low density and high diffusion constant. The mean square displacement and the velocity correlation lend support to the results and are compared with recent molecular dynamics simulations on a nucleating argon cluster.

Homogeneous nucleation of droplets from a supersaturated vapor phase

An approximate semianalytic theory is developed to describe the homogeneous nucleation of droplets from a supersaturated vapor, beginning with a partition function and including rigorously the translation and surface tension contributions. The liquid and vapor phases are treated as uniform (step density profile) and may be described by any accurate equation of state. It is shown that the classical approximation for the free energy of droplet formation may be derived from the present theory by making additional approximations (ideal gas, incompressible liquid), and the two are compared for the case where the vapor phase forms a reservoir (constant supersaturation). In the case of a finite-sized vapor phase, where the supersaturation decreases as the droplet grows, a free energy minimum exists beyond the critical radius, and this stable droplet equilibrium is examined in detail. Comparison with computer simulations proves the quantitative accuracy of the present theory for a Lennard-Jones system. Also derived is a new, formally exact expression for the surface tension that is useful for computer simulations.

Molecular dynamics simulation of the homogeneous nucleation of UF6 molecules: Configurations and infrared spectra of the excited hot clusters

The temperature, potential energy, and configurations of the clusters produced in the homogeneous nucleation of UF6 molecules from the supercooled (supersaturated) vapor phase were determined by classical molecular dynamics (MD) simulations. We observed two phenomena which demonstrate that the nucleation process occurs in the state far from thermal equilibrium. First, the excited hot clusters, the temperature of which is much higher than that of the monomer, were produced and continued to exist during the nucleation process. Second, the relationship between the potential energy and temperature of the clusters depends on the monomer temperature, that is, the potential energy at a temperature decreases with the increase in monomer temperature. In the simulations, various types of cluster configurations were observed: prolate, oblate, spherelike, and confeitolike. The confeitolike cluster is composed of one core and a few horns, and it was found predominantly in the hotter clusters. The infrared spectra of the UF6 clusters were calculated using a dipole–dipole and dipole-induced dipole interaction model and compared with those measured in a supercooled state by Tanimura et al. [J. Chem. Phys. 107, 7096 (1997)]. As a result, we found that the spectra measured in the supercooled state can be attributed to the excited hot clusters, the configuration of which are confeitolike.

Saturation vapor pressures over supercooled water

In analyses of certain nucleation problems in cloud physics, estimates of the saturation vapor pressure over water at very large degrees of supercooling are needed. For example, since nucleation of a liquid phase from a supersaturated vapor phase is thermodynamically more probable than nucleation of a solid phase from the same vapor, prediction of nucleation rates in clouds at very low temperatures calls for such information on saturation vapor pressures over highly supercooled water. Formation of nacreous clouds [Hesstvedt, 1960] provides a particular instance of that type, and homogeneous nucleation processes in rapid adiabatic expansions constitute another case of theoretical interest. Since the degree of ice-supersaturation of a vapor saturated with respect to liquid water is an important parameter in the theory of ice nucleation [Fletcher, 1962] the same data are of interest in that connection also.