Homogeneous nucleation rates of water vapor measured in a two-piston expansion chamber

Homogeneous water nucleation in carbon dioxide-nitrogen mixtures: Experimental study on pressure and carrier gas effects

New homogeneous nucleation experiments are presented at 240 K for water in carrier gas mixtures of nitrogen with carbon dioxide molar fractions of 5%, 15%, and 25%. The pulse expansion wave tube is used to test three different pressure conditions, namely, 0.1, 1, and 2 MPa. In addition, a restricted series of nucleation experiments is presented for 25% carbon dioxide mixtures at temperatures of 234 and 236 K at 0.1 MPa. As pressure and carbon dioxide content are increased, the nucleation rate increases accordingly. This behavior is attributed to the reduction in the water surface tension by the adsorption of carrier gas molecules. The new data are compared with theoretical predictions based on the classical nucleation theory and on extrapolations of empirical surface tension data to the supercooled conditions at 240 K. The extrapolation is carried out on the basis of a theoretical adsorption/surface tension model, extended to multi-component mixtures. The theoretical model appears to strongly overestimate the pressure and composition dependence. At relatively low pressures of 0.1 MPa, a reduction in the nucleation rates is found due to an incomplete thermalization of colliding clusters and carrier gas molecules. The observed decrease in the nucleation rate is supported by the theoretical model of Barrett, generalized here for water in multi-component carrier gas mixtures. The temperature dependence of the nucleation rate at 0.1 MPa follows the scaling model proposed by Hale [J. Chem. Phys. 122, 204509 (2005)].

Homogeneous water nucleation: Experimental study on pressure and carrier gas effects

Homogeneous nucleation of water is investigated in argon and in nitrogen at about 240 K and 0.1 MPa, 1 MPa, and 2 MPa by means of a pulse expansion wave tube. The surface tension reduction at high pressure qualitatively explains the observed enhancement of the nucleation rate of water in argon as well as in nitrogen. The differences in nucleation rates for the two mixtures at high pressure are consistent with the differences in adsorption behavior of the different carrier gas molecules. At low pressure, there is not enough carrier gas available to ensure the growing clusters are adequately thermalized by collisions with carrier gas molecules so that the nucleation rate is lower than under isothermal conditions. This reduction depends on the carrier gas, pressure, and temperature. A qualitative agreement between experiments and theory is found for argon and nitrogen as carrier gases. As expected, the reduction in the nucleation rates is more pronounced at higher temperatures. For helium as the carrier gas, non-isothermal effects appear to be substantially stronger than predicted by theory. The critical cluster sizes are determined experimentally and theoretically according to the Gibbs-Thomson equation, showing a reasonable agreement as documented in the literature. Finally, we propose an empirical correction of the classical nucleation theory for the nucleation rate calculation. The empirical expression is in agreement with the experimental data for the analyzed mixtures (water-helium, water-argon, and water-nitrogen) and thermodynamic conditions (0.06 MPa-2 MPa and 220 K-260 K).

Data evaluation of laminar flow diffusion chamber nucleation experiments with different computational methods

In order to evaluate the experimental data from laminar flow diffusion chamber (LFDC) experiments on homogeneous nucleation, an extensive postmeasurement computational analysis is required. The present work investigates the influence of the used computational methodology on the derived nucleation curves. To this end a reanalysis is made of previous LFDC experiments of 1-butanol nucleation in helium [D. Brus et al., J. Chem. Phys. 122, 214506 (2005)] using two different methods. The first method is based on single fluid heat and vapor transport in the carrier gas ignoring the aerosol processes, as commonly made in LFDC data evaluations. The second method is more comprehensive as is based on multidimensional computational fluid-particle dynamics. The calculations are made under the usual simplification of one-way coupling between fluid flow and particles, which is a valid approximation in most practical aerosols, while full aerosol dynamical effects are accommodated. Similar results were produced by the two methods. This finding corroborates the usual practice of omitting aerosol calculations in LFDC experimental data evaluation.

Microscopic simulations of molecular cluster decay: Does the carrier gas affect evaporation?

We develop a kinetic theory of cluster decay by considering the stochastic motion of molecules within an effective potential of mean force (PMF) due to the cluster. We perform molecular dynamics simulations on a 50-atom argon cluster to determine the mean radial force on a component atom and hence the confining potential of mean force. Comparisons between isolated clusters and clusters thermostatted through the presence of a 100-atom helium carrier gas show that the heat bath has only a slight effect upon the PMF. This confirms the validity of calculations of cluster properties using isolated cluster simulations. The PMF is used to calculate the atomic evaporation rate from these clusters, and results are compared with the predictions of the capillarity approximation together with detailed balance, both components of the classical theory of aerosol nucleation.

Homogeneous nucleation and droplet growth in supersaturated argon vapor: The cryogenic nucleation pulse chamber

We built a cryogenic nucleation pulse chamber for measuring homogeneous nucleation rates of argon. First measurements show that the growth rate of argon droplets at nucleation conditions is rather high so that nucleation and growth could not yet be decoupled. Nevertheless, the experiments permit an estimate of the onset of nucleation corresponding to a nucleation rate of J=10(7(+/-2)) cm(-3) s(-1) at temperatures 52<TK<59 and supersaturations around S approximately 10. Despite their preliminary nature these experiments indicate a severe failure of the classical nucleation theory, which predicts nucleation rates on the order of 10(-28)-10(-13) cm(-3) s(-1) for the quoted conditions. Recent calculations based on density functional theory can only partially explain the discrepancy. In addition to the first nucleation experiments, we obtained and analyzed growth curves for argon droplets from constant angle Mie-light scattering. The good agreement of the experimental growth curves with model calculations according to Fuchs and Sutugin [Highly Dispersed Aerosols (Ann Arbor Science, Ann Arbor, MI, 1970)] permits a near-quantitative description of the experimental light-scattering signal. The procedure provides an estimate for the number density of the droplets along with a measure of their polydispersity.

Homogeneous nucleation rate measurements of 1-butanol in helium: A comparative study of a thermal diffusion cloud chamber and a laminar flow diffusion chamber

Isothermal homogeneous nucleation rates of 1-butanol were measured both in a thermal diffusion cloud chamber and in a laminar flow diffusion chamber built recently at the Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague, Czech Republic. The chosen system 1-butanol-helium can be studied reasonably well in both devices, in the overlapping range of temperatures. The results were compared with those found in the literature and those measured by Lihavainen in a laminar flow diffusion chamber of a similar design. The same isotherms measured with the thermal diffusion cloud chamber occur at highest saturation ratios of the three devices. Isotherms measured with the two laminar flow diffusion chambers are reasonably close together; the measurements by Lihavainen occur at lowest saturation ratios. The temperature dependences observed were similar in all three devices. The molecular content of critical clusters was calculated using the nucleation theorem and compared with the Kelvin equation. Both laminar flow diffusion chambers provided very similar sizes slightly above the Kelvin equation, whereas the thermal diffusion cloud chamber suggests critical cluster sizes significantly smaller. The results found elsewhere in the literature were in reasonable agreement with our results.

Simple correction to the classical theory of homogeneous nucleation

Formation of the new disperse phase via homogeneous nucleation plays a fundamental role wherever the first-order phase transitions occur. Inconsistent temperature dependence of the nucleation rates and poor agreement of theoretical critical supersaturations with experimental data for a number of substances are fundamental problems of the classical nucleation theory (CNT). Here we show that these problems can be solved with a simple empirical correction to CNT. Despite its simplicity, the corrected CNT (CCNT) accurately predicts temperature dependences and absolute values of the critical supersaturations for both organic and inorganic substances with widely varying properties at different ambient conditions and it works surprisingly well in a wide size range down to few molecules. The difference in predictions of CCNT and other versions of the classical nucleation theory commonly used in analyzing experimental data is discussed. It has been found that CCNT consistently gives better agreement with experimental data than other versions of classical nucleation theory.

Phase transition dynamics of liquid phase precipitation from a supersaturated gas mixture

This work presents a self-consistent description of phase transition dynamics of disperse liquid phase precipitating from a supersaturated gas mixture. The unified approach integrates the macroscale transport phenomena of cloud dynamics with the essential microphysical kinetic processes of droplet condensation, evaporation, and droplet collisions simultaneously taking place in stochastic population of liquid droplets. A complete set of governing equations with well-defined dissipative fluxes and kinetic rates is derived for phase transition dynamics from nucleation to postnucleation to coarsening stages. The local thermodynamics of precipitating system, which is considered as ternary mixture of disperse liquid phase and water vapor with dry air, is redefined to explicitly include on equal basis both the vapor content and liquid content into the fundamental thermodynamic relations and equation of state. The molecular kinetic flux regularization method for growth of submicron droplets is reexamined to include, among others, significant contribution of vapor molecular energy flux into total heat flux, resulting in new expressions for the droplet temperature, growth rate, and effective diffusion coefficients. The local kinetic rates are determined on the basis of microscale kinetic equation for the droplet distribution function. This is in contrast to commonly used semiempirical parametrization schemes for kinetic rates with adjustable parameters, wherein the probabilistic aspects of microphysical processes are not rigorously addressed. Stochastic diffusion interactions among droplets competing for the available water vapor and modifications in the kinetic equation for droplets growing in stochastic population with direct long-range diffusion interactions amongst them are discussed and formulated as well.

Nucleation rates and induction times during colloidal crystallization: links between models and experiments

A kinetic model for the evolution of the cluster size distribution during crystal nucleation and growth is presented. The model allows one to establish precise links between model parameters and experimental measures of nucleation kinetics. This approach demonstrates the significance of several processes not accounted for in classical nucleation theories. Chief among these is that the driving force for crystal nucleation decreases rapidly due to a reduction of the background monomer concentration as crystallization progresses, resulting in a reduction of nucleation rates. This, coupled with the disparities in the definitions of measured and predicted quantities, leads to significant discrepancies between predictions of extant models and experimental estimates of nucleation rates. Accounting for these effects, calculations of the kinetic model are shown to be in good agreement with experimental estimates of nucleation rates, crystal growth velocities, and induction times during the crystallization of hard sphere colloidal suspensions.