The nucleation behavior of supercooled water vapor in helium

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).

Curvature Dependence of the Liquid-Vapor Surface Tension beyond the Tolman Approximation

Surface tension is a macroscopic manifestation of the cohesion of matter, and its value σ_{∞} is readily measured for a flat liquid-vapor interface. For interfaces with a small radius of curvature R, the surface tension might differ from σ_{∞}. The Tolman equation, σ(R)=σ_{∞}/(1+2δ/R), with δ a constant length, is commonly used to describe nanoscale phenomena such as nucleation. Here we report experiments on nucleation of bubbles in ethanol and n-heptane, and their analysis in combination with their counterparts for the nucleation of droplets in supersaturated vapors, and with water data. We show that neither a constant surface tension nor the Tolman equation can consistently describe the data. We also investigate a model including 1/R and 1/R^{2} terms in σ(R). We describe a general procedure to obtain the coefficients of these terms from detailed nucleation experiments. This work explains the conflicting values obtained for the Tolman length in previous analyses, and suggests directions for future work.

Observation of a transition in the water-nanoparticle formation process at 167K

Rapid-scan Fourier transform infrared spectroscopy of the vapor/solid formation process of water nanoparticles in the 180-140 K temperature range at thermal-equilibrium conditions is reported. At 167 K a transition in the formation process was observed: the particle volume quintuples and the particle formation time triples within a temperature interval of +/-0.4 K caused by the temperature control. The authors interpret this behavior by an abrupt change in the nucleation rate of the H2O monomers in He buffer gas kept at 167 K and 200 mbar. A size and shape analysis of the particles during the formation process was carried out by application of the discrete dipole approximation method which delivers excellent accordance between experimental and calculated mid-IR spectra. Compared to other compact shapes (cube, prolate ellipsoid, and hexagonal prism) the ideal spherical shape fits the experimental spectra best. A distinct change in shape by particle conversion or agglomeration could be excluded to be involved in the formation process. As a possible explanation of the observed phenomenon, a transition from vapor/liquid/solid to vapor/solid nucleation with decreasing temperature is considered which was recently theoretically predicted by van Dongen and co-workers [J. Chem. Phys. 117, 5647 (2002); private communication; J. Chem. Phys. 120, 6314 (2004)].

Homogeneous nucleation of water between 200 and 240 K: New wave tube data and estimation of the Tolman length

We have measured homogeneous nucleation rates of water at 200-240 K in the carrier gas helium, in the range of 10(13) - 10(17) m(-3) s(-1) using an expansion wave tube. The rates agree well with the results of Wolk and Strey [J. Phys. Chem. B 105, 11683 (2001)] in the range of overlap (220-240 K), and are summarized by the empirical fit J = S exp[4.6 + 0.244T-(906.8 - 2.914T)(ln S)(2)], with J the nucleation rate in m(-3) s(-1), S the supersaturation, and T the temperature in K. We find that the supersaturation dependence of both our rates and those of Wolk and Strey is lower than classical theory predicts, and that the critical cluster is smaller than the classical critical size. These deviations are explained in the framework of the Tolman theory for surface tension, and the "Tolman length" is estimated from our experimental results. We find a positive Tolman length that increases with decreasing temperature, from about 0.1 Angstrom at 260 K to (0.6 +/- 0.4) Angstroms at 200 K. We present a nucleation rate expression that takes the Tolman length into account and show that both the supersaturation and temperature dependence are improved, compared to the classical theory.

Simulating Vapor−Liquid Nucleation of Water: A Combined Histogram-Reweighting and Aggregation-Volume-Bias Monte Carlo Investigation for Fixed-Charge and Polarizable Models

The method of histogram-reweighting was integrated with a recently developed approach using aggregation-volume-bias Monte Carlo and self-adaptive umbrella sampling to develop the AVUS-HR algorithm that allows for exceedingly efficient calculations of nucleation properties over a wide range of thermodynamic conditions. Simulations were carried out for water using both fixed-charge and polarizable force fields belonging to the TIP4P family (namely, TIP4P, TIP4P-FQ, TIP4P-pol2, and TIP4P-pol3) to investigate the nucleation of water over a temperature range from 200 to 300 K and the concentration of water clusters in the atmosphere at elevations up to 15 km. It was found that the nucleation free energy barriers and atmospheric concentrations are extremely sensitive to the force field, albeit all of the models investigated in this study support the following general conclusions: (i) ice nucleation is not present under the thermodynamic conditions and cluster-size range investigated here, i.e., the critical nuclei possess liquidlike structures, and (ii) the atmospheric concentrations of water clusters under homogeneous conditions are very low with the mole fraction of hexamers being about 10(-10), a number probably too low to influence the solar radiation balance. Compared to the experimental data, the TIP4P-pol3 model yields the most accurate nucleation results, consistent with its excellent performance for the second virial coefficient and the minimum cluster energies.

Comment on “The nucleation behavior of supercooled water vapor in helium” [J. Chem. Phys. 117, 5647 (2002)]

In a recent paper Peeters et al. published new experimental data on nucleation rates of water in the temperature range of 200-235 K. They reported about a drastic change in the nucleation rate at 207 K. An error in their experimental procedure has been found. The data of Peeters et al. have been reinterpreted. The jump in nucleation rate disappears and the corrected nucleation rate data are in good agreement with data found by Wolk and Strey with a different experimental facility.