CH₃CH₂OD/D₂O Binary Condensation in a Supersonic Laval Nozzle: Presence of Small Clusters Inferred from a Macroscopic Energy Balance

We determined the heat released in the condensing flow of a CH3 CH2 OD/ D2 O /carrier gas mixture (EtOD/ D2 O for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and D2 O monomers [S. Tanimura, B. E. Wyslouzil, M. S. Zahniser, J. Chem. Phys. 127, 034305 (2007)] as inputs. by considering the depletion of the monomer species, the deviation of the pressure from the isentropic value, and the heat released, we estimated that ∼10% of the EtOD molecules are present as pure clusters (dimer to tetramer) upstream of the onset point of condensation. In contrast, clustering was not detected when only pure EtOD was present under the same conditions (temperature and the partial pressure of EtOD) for which clustering was observed in the EtOD/ D 2 O flow. This suggests that the formation of EtOD clusters is facilitated by D2 O in the EtOD/ D2 O flow. A comparison of the heat released to the flow and the expected heat of dissociation of the EtOD/ D2 O droplets suggests that small EtOD clusters persist downstream of the onset point. Both upstream and downstream of the onset point of condensation, the concentration of these clusters in the nozzle is higher than that expected at equilibrium. A possible mechanism for the overabundance of pure EtOD clusters is that they form in the mixed EtOD/ D2 O particles (droplets or clusters) and evaporate from them

Controlling Nucleation and Growth of Nanodroplets in Supersonic Nozzles

We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3×1015⩽J/cm−3 s−⩽6.0×1015 at 230.1⩽T/K⩽230.4 and 29.2⩽S⩽32.4

Controlling Nucleation and Growth of Nanodroplets in Supersonic Nozzles

We present the first results for a new supersonic nozzle that decouples nucleation and droplet growth, and closely controls the supersaturation and temperature during nucleation. We characterize the expansions using pressure trace measurements, and the aerosol properties using light scattering and small angle neutron scattering. We show that when nucleation and droplet growth are separated, the aerosol number density decreases, the average particle size increases, and the aerosol can be more monodisperse than that formed in a conventional nozzle. Under these conditions, we can estimate the nucleation rate J as a function of supersaturation S and temperature T directly from the experimental data. For D2O we find that the nucleation rate is 4.3×1015⩽J/cm−3 s−⩽6.0×1015 at 230.1⩽T/K⩽230.4 and 29.2⩽S⩽32.4

Small-Angle Neutron Scattering of Soot Formed in Laminar Premixed Ethylene Flames

We used small-angle neutron scattering (SANS) to measure soot development in one-dimensional, laminarpremixed flames. Scattering spectra were collected for two sooting ethylene/oxygen/argon flames at six positions above the burner surface. A detailed analysis of the scattering signals yielded the basic properties of the soot size distributions as a function of position. The experiments demonstrate that SANS can be used to extract spatially resolved, quantitative information about incipient soot formation and growth with better particle size resolution than light scattering and extinction

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 ln σr was ln σr=0.19 (±0.07), and the number density N was N=(2±0.2)⋅1011 cm−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 TJ max=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)⋅1016 cm−3 s−1 at a mean supersaturation of 44 (±3)

H2O–D2O Condensation in A Supersonic Nozzle

We examined the condensation of H2O, D2O, and four intermediate mixtures (20, 40, 60, and 80 mol % 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 components in this ideal system

New Particle Formation from the Vapor Phase : From Barrier-Controlled Nucleation to the Collisional Limit

Studies of vapor phase nucleation have largely been restricted to one of two limiting cases—nucleation controlled by a substantial free energy barrier or the collisional limit where the barrier is negligible. For weakly bound systems, exploring the transition between these regimes has been an experimental challenge, and how nucleation evolves in this transition remains an open question. We overcome these limitations by combining complementary Laval expansion experiments, providing new particle formation data for carbon dioxide over a uniquely broad range of conditions. Our experimental data together with a kinetic model using rate constants from high-level quantum chemical calculations provide a comprehensive picture of new particle formation as nucleation transitions from a barrier-dominated process to the collisional limit.Peer reviewe

EMSL Science Theme Advisory Panel Workshop - Atmospheric Aerosol Chemistry, Climate Change, and Air Quality

This report contains the workshop scope and recommendations from the workshop attendees in identifying scientific gaps in new particle formation, growth and properties of particles and reactions in and on particles as well as the laboratory-focused capabilities, field-deployable capabilities and modeling/theory tools along with linking of models to fundamental data

Overview: Homogeneous nucleation from the vapor phase-The experimental science

Homogeneous nucleation from the vapor phase has been a well-defined area of research for similar to 120 yr. In this paper, we present an overview of the key experimental and theoretical developments that have made it possible to address some of the fundamental questions first delineated and investigated in C. T. R. Wilson's pioneering paper of 1897 [C. T. R. Wilson, Philos. Trans. R. Soc., A 189, 265-307 (1897)]. We review the principles behind the standard experimental techniques currently used to measure isothermal nucleation rates, and discuss the molecular level information that can be extracted from these measurements. We then highlight recent approaches that interrogate the vapor and intermediate clusters leading to particle formation, more directly. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)