Nucleation Of Supersaturated Vapor Research Articles

Homogeneous Nucleation of Supersaturated Vapors. A Comparison of Experimental Results with Theory

Some theories of homogeneous nucleation based on liquid droplet model were tested by comparing with experimental supersaturations of naphthalene, phthalic anhydride, stearic acid, bis(2-ethylhexyl) phthalate, and bis(2-ethylhexyl) sebacate required to bring about the rate of homogeneous nucleation of about 5 drops/cm3 s-1. It was found that except for highly polar substance phthalic anhydride, the classical (Becker-Doring) theory of homogeneous nucleation, if suitably scaled, yielded fairly good predictions of the nucleation behaviour.

A kinetic approach to the theory of nucleation in gases

A theory of nucleation in gases is developed in which the rate of evaporation of the molecules from the cluster is calculated on the basis of a diffusion equation in the energy space. In contrast to the classical theory of nucleation, the theory does not employ macroscopic thermodynamics. Numerical calculations were performed by assuming an intermolecular potential which combines the dispersive attraction with the rigid core repulsion. The predictions of the theory are consistent with the classical theory in the limit of small supersaturations, i.e., for large critical clusters. For small critical clusters the present theory provides much higher rates of nucleation than the classical one. This difference can be attributed to the use of the macroscopic surface tension in the classical theory, which overpredicts the surface energy of small clusters, and consequently provides lower values for the nucleation rate. The theoretical results are compared with experimental data for nucleation of supersaturated vapors of lower alcohols carried in argon.

Homogeneous nucleation of supersaturated vapors

A theory of homogeneous nucleation from the vapor phase is presented. The classical Becker-Döring-Zel'dovich theory is contained as a limiting case. The theory is based on a semiphenomenological droplet model for the Gibbs free energy of formation of the cluster. Since this droplet model also allows the construction of a thermal equation of state for real gases below the critical point, all free parameters of the model can be determined from well-known macroscopic quantities. A new formulation of the surface free energy for small droplets is crucial. The theory gives a much better prediction of observed nucleation rates than the classical theory.

Homogeneous nucleation in supersaturated vapor containing foreign seed aerosol

The formation of aerosol particles by homogeneous nucleation in a supersaturated vapor containing seed aerosol has been studied experimentally and theoretically. In the laboratory, a room-temperature gas optionally containing zinc chloride particles is continuously mixed with a high-temperature gas saturated with dibutyl phthalate (DBP) vapor in a previously discussed device for the study of aerosol nucleation known as a particle size magnifier (PSM). A highly supersaturated vapor is rapidly formed in the mixing zone of the PSM, and gas-to-particle conversion ensues. The vapor may be converted to the aerosol phase by condensation onto the preexisting particles or by homogeneous nucleation to form new particles which then serve as condensation sites themselves. The split between these alternate pathways for gas-to-particle conversion may be deduced from measurements of the resulting aerosol concentrations for different initial supersaturations, seed aerosol concentrations, and seed aerosol sizes. The measured final aerosol concentrations are compared with those predicted by a dynamic model that combines expressions for classical nucleation theory and for steady-state particle growth, and agreement is found to within experimental uncertainties. Suppression of homogeneous nucleation by seed aerosol is not predicted to be strong unless seed aerosol number concentrations are larger than the number concentrations which would result from homogeneous nucleation alone.

Homogeneous nucleation in supersaturated vapors

Homogeneous nucleation in supersaturated vapors

Experimental results of homogeneous nucleation of supersaturated vapors

An r0 structure is determined for dimethylamine from earlier microwave data. With the constraint δr0(CH–CD)=δr0(NH–ND), CH bond lengths of 1.0958, 1.0981, 1.1060 A≡ are obtained, in excellent agreement with earlier predictions from ‘‘isolated’’ CH stretching frequencies. The new value of r0NH of 1.011 A≡ is significantly shorter than the earlier rs value of 1.022. The angular asymmetry of the methyl group is greater than previously supposed.

Dynamics of Cluster Growth During Homogeneous Nucleation of Supersaturated Vapors

An analysis of cluster growth and dynamics leading to phase transformation through homogeneous nucleation is developed to account for thermal nonaccommodation within the period of one vibration of the newly formed cluster. The approach, which is an extension of studies by Volmer and Feder et al., considers the following different mechanisms of interaction between clusters: (1) condensation of single molecules, (2) evaporation of molecules from clusters, and (3) coagulation between clusters. This approach is evaluated by comparing computer simulations of the population balance equations with classical nucleation theory and with experimental results reported in the literature. The interaction of the impinging molecule with the surface of the condensed phase is central to the development. The probability that such an impinging molecule will condense, be thermally equilibrated, and be incorporated into the cluster is included in the thermal nonaccommodation term in the basic population balance equations. Simu...

Energy of cluster formation for homogeneous nucleation in supersaturated vapors

Energy of cluster formation for homogeneous nucleation in supersaturated vapors

Photoinduced nucleation of supersaturated vapors in the presence of carbon disulfide

With illumination by ultraviolet light in the 280-390-nm wavelength region, copious nucleation is observed in supersaturated vapors containing carbon disulfide. This photoinduced nucleation cannot be explained by conventional nucleation theories nor by the photochemistry of C${\mathrm{S}}_{2}$. We suggest that the nucleation is initiated by the clustering of photoaffected C${\mathrm{S}}_{2}$ molecules, and that these molecules may be stable complexes formed by an interaction of the photoexcited C${\mathrm{S}}_{2}$ and host vapor molecules.

Condensation of a supersaturated vapor. V. The nucleating effects of ultraviolet light on vapors containing very low concentrations of o-tolualdehyde

The photoinduced nucleation of supersaturated vapors containing low concentrations of o-tolualdehyde has been studied. We have made measurements of the photonucleation spectrum using various host liquids, the steady-state rate of nucleation, the delay time before the start of nucleation, the initial slope of the nucleation rate, the decay of the rate after the light is turned off, the o-tolualdehyde concentration dependence, and the dependence on total pressure and the effect of temperature changes. We show that these measurements are in quantitative agreement with a phenomenological model in which the photoinduced nucleation is initiated by clusters accumulating a small number of photoaffected molecules. Using the vapor concentration, the photon cross section of o-tolualdehyde, and the photon flux, we calculate that a photoaffected o-tolualdehyde concentration as low as 3×107 molecules cm−3 sec−1 (i.e., about 1 part per trillion) is sufficient to cause photoinduced nucleation. This concentration is too small to cause either ordinary heterogeneous nucleation or binary homogeneous nucleation. However, a collision frequency calculation shows that this low concentration of photoaffected molecules is in good agreement with the phenomenological model, since a much smaller number of collisions of photoaffected molecules are then required.

Nucleation on Photoexcited Molecules

On irradiation with light of suitable wavelength and intensity, certain organic compounds, even at very low concentrations, cause very efficient nucleation of supersaturated vapors. A mechanism is suggested to account for this phenomenon. Nuclei containing only a few photoexcited molecules are responsible for the nucleation.

Monte Carlo study of the surface area of liquid droplets

Computer simulation of droplets containingl molecules (l ⩽ 1000) in a lattice gas shows that the average surface area is proportional tolσ′;σ′≃ 0.6 in two and σ′= 0.825 in three dimensions for small droplets. These exponents agree approximately with those in Kadanoff's modification of Fisher's droplet model near critical points [σ′= (1 + β)/βδ; ourT/Tc is 0.4, 0.7, and 0.9]. For larger droplets, these exponents change to 1/2 (d = 2) and 2/3 (d = 3), the transition occurring for droplet diameters larger than the coherence length and smaller than the critical diameter in the nucleation of supersaturated vapors. This latter result rises some doubts on a recent nucleation theory of Eggingtonet al.

Droplet Model and Nucleation of Supersaturated Vapors Near the Critical Point

Fisher's droplet model is applied to the calculation of the nucleation rate in a metastable supersaturated vapor near ${T}_{c}$. At a constant scaled supersaturation $A\ensuremath{\propto}\frac{(\ensuremath{\mu}\ensuremath{-}{\ensuremath{\mu}}_{c})}{{({T}_{c}\ensuremath{-}T)}^{\ensuremath{\beta}\ensuremath{\delta}}}$ we find a strong critical slowing down for the nucleation rate. Experiments to check this theory are proposed.

Nucleation of Supersaturated Vapors in Nozzles. I. H2O and NH3

Measurements have been made of the results of homogeneous nucleation of NH3 and H2O vapors expanding in steady flow in a supersonic air nozzle. From the effect of the nucleation and growth of liquid from the vapor phase on the nozzle pressure distribution, the point of incidence of condensation is readily detected. Experimental results have been compared with the predictions of the classical nucleation rate equation and with the quantum-statistical treatment of Lothe and Pound. While H2O nucleation rates are in agreement with the classical equation, NH3 nucleation follows the very much higher rates (12 to 18 orders of magnitude) of the Lothe–Pound equation. A mass accommodation coefficient of 0.01 < ξ < 0.1 for H2O was inferred from the experimental data.

Some remarks about nucleation theory

The due applicability of equilibrium thermodynamics to be the theory of supersaturated vapor nucleation depends on the ratio of the vapor and carrier gas concentrations. The contribution of cluster collective rotation and translation to its free energy of formation cannot be determined at present with certainty.