Christopher H. Heath

Empirical function for homogeneous water nucleation rates

Very recently, Wolk and Strey [J. Phys. Chem. B 105, 11683 (2001)] presented empirical temperature correction functions for calculating homogeneous nucleation rates J of H2O and D2Ou2002(1<J/cm−3u200as−1<1020) from classical nucleation theory over an extended range of temperature T (200<T/K<310) and supersaturations S (5<S<200). Here, we critically test the correction functions to the Becker–Doring nucleation rate equation JBD against an extensive set of experimental data, and find that the equations distinctly improve the agreement between theory and experiment for very little extra work. The success of the corrected nucleation rate functions is surprising, given that they were developed based on experimental nucleation rates measured in a nucleation pulse chamber over a limited nucleation rate range 105<J/cm−3u200as−1<1010, supersaturation range 6<S<22, and temperature range 220<T/K<260.

Binary condensation in a supersonic nozzle

We present data from the first systematic studies of binary condensation in supersonic nozzles. The apparatus used to conduct the experiments is described in detail, and the important issues of stability and reproducibility of the experiments are discussed. Experiments were conducted with water, ethanol, propanol, and binary mixtures of these compounds. Onset was determined in the temperature range of 190–215 K, and for each mixture composition the pressures of the condensible species at an onset temperature of 207 K were determined. For the ideal ethanol–propanol mixtures, the onset pressures at constant temperature vary almost linearly between those of the pure components. In contrast the isothermal onset pressures for the nonideal water–ethanol and water–propanol mixtures lie below the straight line joining the pure component values. This large reduction in the total pressure of condensible at onset for the aqueous alcohol mixtures is indicative of a strong mutual enhancement in the particle formation process.

Homogeneous nucleation rates for D2O in a supersonic Laval nozzle

We measured the homogeneous nucleation rates of D2O in a supersonic nozzle. Small angle neutron scattering (SANS) experiments characterized the nanodroplet aerosols and yielded the number density N of particles formed. In these new SANS measurements the number densities were in the range of 4×1011<N/cm−3<2×1012. We then determined the characteristic time interval corresponding to the peak nucleation rate ΔtJmax from conventional pressure profile measurements in the nozzle. The sensitive time is typically (11±3) μs. Combining these two pieces of information we obtained the nucleation rate J=NNZ/ΔtJmax, where NNZ is the number density of the aerosol in the nucleation zone. In this nozzle, the peak nucleation rate ranges between 6×1016 and 1.2×1017u200acm−3u200as−1 and is quite insensitive to the initial conditions of the expansion.

Experimental evidence for internal structure in aqueous–organic nanodroplets

The spatial distribution of species within an aerosol droplet influences how it interacts with its environment. Despite the ubiquity of multicomponent nanodroplets in natural and technological aerosols, there are no published measurements of their internal structure. Here, we report the first experimental results for structure in aqueous organic nanodroplets based on small angle neutron scattering by high number density aerosols. For H(2)O-n-butanol droplets, fitting of the diffraction patterns confirms the picture of an aqueous core containing approximately 3 mol% alcohol covered by a shell of densely packed alcohol molecules.

Small angle neutron scattering from D2O–H2O nanodroplets and binary nucleation rates in a supersonic nozzle

Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets rg was rg=13 (±1) nm, the polydispersity lnu200aσr was lnu200aσr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011u2009cm−3. The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering spectra as a function of the isotopic composition gave further evidence that the binary droplets exhibit ideal mixing behavior. Because both the stagnation temperature T0 and the location of onset were fixed, the temperature corresponding to the maximum nucleation rate was constant at TJu200amax=229 (±1) K. Thus, the experiments let us estimate the isothermal peak nucleation rates as a function of the isotopic composition. The nucleation rates were found to be essentially constant with Jmax equal to (3.6±0.5)⋅1016u2009cm−3u200as−1 at a mean supersaturation of 44 (±3).Small angle neutron scattering (SANS) experiments were used to characterize binary nanodroplets composed of D2O and H2O. The droplets were formed by expanding dilute mixtures of condensible vapor in a N2 carrier gas through a supersonic nozzle, while maintaining the onset of condensation at a fixed position in the nozzle. It is remarkable, given the small coherent scattering length density of light water, that even the pure H2O aerosol gave a scattering signal above background. The scattering spectra were analyzed assuming a log-normal distribution of droplets. On average, the geometric radius of the nanodroplets rg was rg=13 (±1) nm, the polydispersity lnu200aσr was lnu200aσr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011u2009cm−3. The aerosol volume fractions derived from the SANS measurements are consistent with those derived from the pressure trace experiments, suggesting that the composition of the droplets was close to that of the initial condensible mixture. A quantitative analysis of the scattering...

H2O–D2O condensation in a supersonic nozzle

We examined the condensation of H2O, D2O, and four intermediate mixtures (20, 40, 60, and 80 molu200a% D2O) in a supersonic nozzle. Because the physical and chemical properties of protonated and deuterated water are so similar, this system is ideal for studying the change in condensation behavior as a function of condensible composition. In our experiments dilute mixtures of condensible vapor in N2 are expanded from three different stagnation temperatures resulting in a broad range of onset temperatures (190–238 K) and pressures (27–787 kPa). For a fixed stagnation temperature, the partial pressure required to maintain the onset of condensation at a given location or temperature in the nozzle is consistently higher for H2O than for D2O. In contrast, the supersaturation at fixed onset temperature is usually higher for D2O than for H2O and this difference increases toward lower temperature. The partial pressure at onset for the intermediate mixtures varied linearly between the values observed for the pure compo...

Effect of supersaturation, temperature and total pressure on the homogeneous nucleation of n-pentanol

Publisher Summary Homogeneous nucleation of droplets from supersaturated vapors depends on vapor supersaturation, temperature, and carrier gas pressure. Recent nucleation experiments confirm the dependence on supersaturation, reveal slight deviations of the temperature dependence, but heavily disagree on the pressure dependence. While with expansion techniques, only marginal pressure dependencies were detected. Diffusion cloud chambers yield enormous pressure effects. The magnitude of the pressure effect is reported by Katz and Heist to increase with the molecular mass of both the carrier gas and the condensable vapor and decreasing temperature. To examine this pressure effect most sensitively in an expansion chamber, argon as the carrier gas, n-pentanol as the vapor, and a rather low nucleation temperature (T = 250K) for investigating the pressure effect are chosen. The supersaturation and temperature dependencies of n-pentanol nucleation rates are found to be in close agreement with the so-called “self-consistent classical theory.” The pressure dependence is found to be much weaker and opposite in sign than the pressure effects found in diffusion cloud chambers.

D2O-H2O condensation in supersonic nozzles: I. Experiments

Pressure trace measurements and small angle neutron scattering (SANS) were used to probe the binary condensation of D2O-H2O mixtures in a supersonic nozzle. Each expansion started from the same initial pressure (carrier gas and condensible vapor) and temperature. The partial pressures of D2O and H2O were adjusted so that the onset of condensation always occurred at the same position in the nozzle. Under these conditions, the total pressure of condensible at onset varied linearly between the pressures of the pure components. Furthermore, the partial pressure at onset for pure H2O was 29–34% higher than that for pure D2O. The SANS scattering signals also varied systematically with the mixture composition. As the mixtures became progressively richer in H2O, the scattering intensity dropped rapidly because the scattering length of H2O is much smaller in magnitude and of opposite sign to that of D2O. Further analysis of the scattering spectra showed that the particle size of the aerosol was increasing as the m...

D2O-H2O condensation in supersonic nozzles. II. Modeling

An integral steady state model of nucleation and condensation was used to examine the formation and growth of D2O droplets in a supersonic nozzle. The classical nucleation rate expression was used together with isothermal and nonisothermal droplet growth laws. For each experiment, the nucleation rate expression was multiplied by a temperature independent parameter in order to match the experimentally observed onset of condensation. In all cases, the predicted pressure traces lie above the measured ones. For one of the condensation experiments, the corresponding neutron scattering spectrum was also available. Modeling showed that once the rate expression was adjusted to match onset, the predicted scattering spectrum was a strong function of the growth law. Furthermore, the match between the measured and predicted scattering spectra was much better for the isothermal growth law than for the nonisothermal growth law.