Nucleation Of Vapor Research Articles

Surface tension of water droplets upon homogeneous droplet nucleation in water vapor

A method has been proposed for determining interfacial free energy from the data of molecular dynamics simulation. The method is based on the thermodynamic integration procedure and is distinguished by applicability to both planar interfaces and those characterized by a high curvature. The workability of the method has been demonstrated by the example of determining the surface tension for critical nuclei of water droplets upon condensation of water vapor. The calculation has been performed at temperatures of 273–373 K and a pressure of 1 atm, thus making it possible to determine the temperature dependence of the surface tension for water droplets and compare the results obtained with experimental data and the simulation results for a “planar” vapor–liquid interface.

Organic Nanoaerosol in Coal Mines: Formation Mechanism and Explosibility

ABSTRACTThe mechanism of aerosol formation in coal mines during the operation of a longwall shearer was studied using a diffusion battery, optical counter, and by means of Transmission Electron Microscopy (TEM). The aerosol number concentration was measured to be (2–5) × 105 cm–3. The aerosol size spectrum contained three modes, at about 10, 100, and 1000 nm. The first mode relates to single (primary) particles formed by the homogeneous nucleation of supersaturated organic vapor. This vapor is formed by the evaporation of organic matter from coal due to the release of frictional heat at the interface between the cutting pick and coal. The second mode relates to the particles that are aggregates formed by coagulation of primary particles. The third mode relates to the particles formed by direct grinding of coal by the cutting picks. The laboratory studies of organic aerosol formation in a flow coal grinding machine showed that the size spectrum and morphology of aerosol from the grinding machine were close to those in the coal mine. This fact confirms the thesis that the release of frictional heat is the driving force for the formation of organic aerosol. The analysis of gas-phase products in the outflow of the grinding machine showed that along with aerosol formation, gas products such as CO, CO2, CH4, C2H6, H2O are released from coal due to the frictional heat. Methane and ethane concentrations in the flow reached 10 and 5 vol. %, respectively. To demonstrate the explosibility of organic aerosol, the combustion of organic aerosol in the air was studied. It was shown that the lower explosive limit for organic aerosol is less than 50 g m–3. A conclusion is made that the formation of organic aerosol is to be taken into account when estimating the safety limits in coal mines.

Experimental study of multi-scale heat transfer characteristics at pool boiling

This study presents the results of the experimental investigation of local and integral characteristics of heat transfer at liquid pool boiling. Saturated ethanol and water were used as the working fluids. Thin, resistively heated indium-tin oxide films deposited onto the sapphire substrates were used as the heaters. The synchronized measurements of the heater surface temperature field and dynamics of vapor bubbles were performed by high-speed infrared thermography with the frame rate of 1000 fps and resolution of up to 0.13 μm/px and high-speed video recording. In this paper new data on major local boiling characteristics, such as nucleation site density, dynamics of vapor bubbles, temporal characteristics and nucleation frequency at different heat fluxes and superheating and their comparison with correlations are presented.

Laser-induced damage thresholds of gold, silver and their alloys in air and water

The nanosecond-laser-induced damage thresholds of gold, silver and gold-silver alloys of various compositions in air and water have been measured for single-shot irradiation conditions. The experimental results are analyzed theoretically by solving the heat flow equation for the samples irradiated in air and in water taking into account vapor nucleation at the solid-water interface. The damage thresholds of Au-Ag alloys are systematically lower than those for pure metals, both in air and water that is explained by lower thermal conductivities of the alloys. The thresholds measured in air agree well with the calculated melting thresholds for all samples. The damage thresholds in water are found to be considerably higher, by a factor of ∼1.5, than the corresponding thresholds in air. This cannot be explained, in the framework of the used model, neither by the conductive heat transfer to water nor by the vapor pressure effect. Possible reasons for the high damage thresholds in water such as scattering of the incident laser light by the vapor-liquid interface and the critical opalescence in the superheated water are suggested. Optical pump-probe measurements have been performed to study the reflectance dynamics of the surface irradiated in air and water. Comparison of the transient reflectance signal with the calculated nucleation dynamics provides evidence that the both suggested scattering mechanisms are likely to occur during metal ablation in water.

Theoretical assessment of particle generation from sodium pool fires

Potential sodium discharge in the containment during postulated Beyond Design Basis Accidents (BDBAs) in Sodium-cooled Fast Reactors (SFRs) would have major consequences for accident development in terms of energetics and source term. In the containment, sodium vaporization and subsequent oxidation would result in supersaturated oxide vapours that would undergo rapid nucleation creating toxic aerosols. Therefore, modelling this vapour nucleation is essential to proper source term assessment in SFRs. In the frame of the EU-JASMIN project, a particle generation model to calculate the particle generation rate and their primary size during an in-containment sodium pool fire has been developed. Based on a suite of individual models for sodium vaporization, oxygen natural circulation (3D modelling), sodium-oxygen chemical reactions, sodium-oxides-vapour nucleation and condensation, its consistency has been partially validated by comparing with available experimental data. As an outcome, large temperature and vapour concentration gradients set over the sodium pool have been found which result in large particle concentrations in the close vicinity of the pool.

Focus Article: Theoretical aspects of vapor/gas nucleation at structured surfaces.

Heterogeneous nucleation is the preferential means of formation of a new phase. Gas and vapor nucleation in fluids under confinement or at textured surfaces is central for many phenomena of technological relevance, such as bubble release, cavitation, and biological growth. Understanding and developing quantitative models for nucleation is the key to control how bubbles are formed and to exploit them in technological applications. An example is the in silico design of textured surfaces or particles with tailored nucleation properties. However, despite the fact that gas/vapor nucleation has been investigated for more than one century, many aspects still remain unclear and a quantitative theory is still lacking; this is especially true for heterogeneous systems with nanoscale corrugations, for which experiments are difficult. The objective of this focus article is analyzing the main results of the last 10-20 years in the field, selecting few representative works out of this impressive body of the literature, and highlighting the open theoretical questions. We start by introducing classical theories of nucleation in homogeneous and in simple heterogeneous systems and then discuss their extension to complex heterogeneous cases. Then we describe results from recent theories and computer simulations aimed at overcoming the limitations of the simpler theories by considering explicitly the diffuse nature of the interfaces, atomistic, kinetic, and inertial effects.

Application of the characteristics-based sectional method to spatially varying aerosol formation and transport

The characteristics-based ssolution. It is easy to verify thatectional method (CBSM) offers an Eulerian description of an internally mixed aerosol. It was shown to be robust and capable of exact preservation of lower order moments, allowing for highly skewed sectional droplet size distributions. In this paper we apply CBSM to a spatially varying flow, by incorporating the fractional step method. In this way an accurate time integration of the spatial terms in the transport equations for the velocity, mass fractions and sectional droplet concentrations is achieved. Integrating CBSM into the compressible PISO (Pressure-Implicit with Splitting of Operators) algorithm allows for phase change and corresponding changes in pressure. We apply CBSM to a lid-driven cavity flow. First, the steady state isothermal flow solution is validated against published data. Next, by releasing a saturated vapor into the cavity while cooling the walls, we simulate the formation of aerosol. The accuracy of the solution is studied, as well as the performance of the CBSM scheme in the spatially varying context. The solution of the velocity is shown to be accurate, even at CFL (Courant–Friedrichs–Lewy) numbers of unity. The feasibility of the developed method is demonstrated in a 3D complex geometry studying the aerosol generation via nucleation of hot vapors cooled by a dilution stream of cold air in a double-mixing tee system. The sectional approach delivers detailed information about the aerosol formation and size distribution of the droplets in the domain.

Targeted Nanoparticle Thermometry: A Method to Measure Local Temperature at the Nanoscale Point Where Water Vapor Nucleation Occurs.

An optical nanothermometer technique based on laser trapping, moving and targeted attaching an erbium oxide nanoparticle cluster is developed to measure the local temperature. The authors apply this new nanoscale temperature measuring technique (limited by the size of the nanoparticles) to measure the temperature of vapor nucleation in water. Vapor nucleation is observed after superheating water above the boiling point for degassed and nondegassed water. The average nucleation temperature for water without gas is 560 K but this temperature is lowered by 100 K when gas is introduced into the water. The authors are able to measure the temperature inside the bubble during bubble formation and find that the temperature inside the bubble spikes to over 1000 K because the heat source (optically-heated nanorods) is no longer connected to liquid water and heat dissipation is greatly reduced.

Enhancement of the droplet nucleation in a dense supersaturated Lennard-Jones vapor.

The vapor-liquid nucleation in a dense Lennard-Jones system is studied analytically and numerically. A solution of the nucleation kinetic equations, which includes the elementary processes of condensation/evaporation involving the lightest clusters, is obtained, and the nucleation rate is calculated. Based on the equation of state for the cluster vapor, the pre-exponential factor is obtained. The latter diverges as a spinodal is reached, which results in the nucleation enhancement. The work of critical cluster formation is calculated using the previously developed two-parameter model (TPM) of small clusters. A simple expression for the nucleation rate is deduced and it is shown that the work of cluster formation is reduced for a dense vapor. This results in the nucleation enhancement as well. To verify the TPM, a simulation is performed that mimics a steady-state nucleation experiments in the thermal diffusion cloud chamber. The nucleating vapor with and without a carrier gas is simulated using two different thermostats for the monomers and clusters. The TPM proves to match the simulation results of this work and of other studies.

Modeling and measurements of urban aerosol processes on the neighborhood scale in Rotterdam, Oslo and Helsinki

Abstract. This study evaluates the influence of aerosol processes on the particle number (PN) concentrations in three major European cities on the temporal scale of 1 h, i.e., on the neighborhood and city scales. We have used selected measured data of particle size distributions from previous campaigns in the cities of Helsinki, Oslo and Rotterdam. The aerosol transformation processes were evaluated using the aerosol dynamics model MAFOR, combined with a simplified treatment of roadside and urban atmospheric dispersion. We have compared the model predictions of particle number size distributions with the measured data, and conducted sensitivity analyses regarding the influence of various model input variables. We also present a simplified parameterization for aerosol processes, which is based on the more complex aerosol process computations; this simple model can easily be implemented to both Gaussian and Eulerian urban dispersion models. Aerosol processes considered in this study were (i) the coagulation of particles, (ii) the condensation and evaporation of two organic vapors, and (iii) dry deposition. The chemical transformation of gas-phase compounds was not taken into account. By choosing concentrations and particle size distributions at roadside as starting point of the computations, nucleation of gas-phase vapors from the exhaust has been regarded as post tail-pipe emission, avoiding the need to include nucleation in the process analysis. Dry deposition and coagulation of particles were identified to be the most important aerosol dynamic processes that control the evolution and removal of particles. The error of the contribution from dry deposition to PN losses due to the uncertainty of measured deposition velocities ranges from −76 to +64 %. The removal of nanoparticles by coagulation enhanced considerably when considering the fractal nature of soot aggregates and the combined effect of van der Waals and viscous interactions. The effect of condensation and evaporation of organic vapors emitted by vehicles on particle numbers and on particle size distributions was examined. Under inefficient dispersion conditions, the model predicts that condensational growth contributes to the evolution of PN from roadside to the neighborhood scale. The simplified parameterization of aerosol processes predicts the change in particle number concentrations between roadside and urban background within 10 % of that predicted by the fully size-resolved MAFOR model.

Heterogeneous Nucleation onto Ions and Neutralized Ions: Insights into Sign-Preference

Heterogeneous nucleation of vapor on a seed particle surface is dependent on the seed properties such as size, chemical composition, and electric charging state, of which the significance of the charging state has not been uncovered unambiguously. The underlying problem is that, on the molecular level, the charging state and the chemical composition of the seed are connected and cannot be well separated without a direct mass spectrometric measurement of the ion. By generating sub-3 nm size selected seeds of different size, chemical composition, electric charging state, and letting three different vapors nucleate onto the seeds, we show that heterogeneous nucleation does not clearly prefer either positive or negative seeds. Rather, the most important parameter determining the nucleation probability in the sub-3 nm size range was the seed chemical composition. Our findings help to understand the dynamics in various nanoparticle systems, such as nucleation chambers, industrial processes, or atmospheric aerosols.

Nanothermometry using optically trapped erbium oxide nanoparticle

A new optical probe technique using a laser-trapped erbium oxide nanoparticle (size ~150 nm) is introduced that can measure absolute temperature with a spatial resolution on the size of the trapped nanoparticle. This technique (scanning optical probe thermometry) is used to collect a thermal image of a gold nanodot prepared with hole-mask colloidal lithography. A convolution analysis of the thermal profile shows that the point spread function of our measurement is a Gaussian with a FWHM of 165 nm. We attribute the width of this function to clustering of Er2O3 nanoparticles in solution. The scanning optical probe thermometer is used to measure the temperature where vapor nucleation occurs in degassed water (555 K), confirming that a nanoscale object heated in water will superheat the surrounding water to the spinodal decomposition temperature. Subsequently, the temperature inside the vapor bubble rises to the melting point of the gold nanostructure (~1300) where a temperature plateau is observed. The rise in temperature is attributed to inhibition of thermal transfer to the surrounding liquid by the thermal insulating vapor cocoon.

Hollow Mesoporous Plasmonic Nanoshells for Enhanced Solar Vapor Generation

In the past decade, nanomaterials have made their way into a variety of technologies in solar energy, enhancing the performance by taking advantage of the phenomena inherent to the nanoscale. Recent examples exploit plasmonic core/shell nanoparticles to achieve efficient direct steam generation, showing great promise of such nanoparticles as a useful material for solar applications. In this paper, we demonstrate a novel technique for fabricating bimetallic hollow mesoporous plasmonic nanoshells that yield a higher solar vapor generation rate compared with their solid-core counterparts. On the basis of a combination of nanomasking and incomplete galvanic replacement, the hollow plasmonic nanoshells can be fabricated with tunable absorption and minimized scattering. When exposed to sun light, each hollow nanoshell generates vapor bubbles simultaneously from the interior and exterior. The vapor nucleating from the interior expands and diffuses through the pores and combines with the bubbles formed on the outer wall. The lack of a solid core significantly accelerates the initial vapor nucleation and the overall steam generation dynamics. More importantly, because the density of the hollow porous nanoshells is essentially equal to the surrounding host medium these particles are much less prone to sedimentation, a problem that greatly limits the performance and implementation of standard nanoparticle dispersions.

Pool boiling heat transfer of deionized and degassed water in packed-perforated copper beads

Nucleate pool boiling with porous media made of perforated copper beads as the enhanced structure is conducted in saturated, deionized and degassed water. Data are taken at an atmospheric pressure (saturation temperature of 100 °C) and at heat fluxes from 4500 to 72,300 W/m2 while increasing the heat flux. The bead-packed structure is heated on the bottom. The layer of loose particles on the heated surface is free to move under the action of bulk liquid convection and vapor nucleation. The effects of the weight (number), size and layers of the free particles are experimentally explored using copper particles for different copper bead diameters which were 2, 3, 4 and 5 mm. The boiling enhancement is closely related to the particle weight, size and layers, and the heat flux applied. The results show that free particles are presented to have a distinct advantage in boiling heat transfer, resulting in an average increase in the heat transfer coefficient of 126 % relative to the flat plate without particles. In order to obtain insight into the fluid boiling phenomena, flow visualization is also made to observe the detailed fluid boiling characteristics of the copper particles present. The visualizations show that bubble nucleation preferentially occurs at the narrow corner cavities formed between the free particles and the heated surface.

An adsorption theory of heterogeneous nucleation of water vapour on nanoparticles

Abstract. Heterogeneous nucleation of water vapour on insoluble nuclei is a phenomenon that can induce atmospheric water and ice cloud formation. However, modelling of the phenomenon is hampered by the fact that the predictive capability of the classical heterogeneous nucleation theory is rather poor. A reliable theoretical description of the influence of different types of water-insoluble nuclei in triggering the water condensation or ice deposition would help to decrease uncertainty in large-scale model simulations. In this paper we extend a recently formulated adsorption theory of heterogeneous nucleation to be applicable to highly curved surfaces, and test the theory against laboratory data for water vapour nucleation on silica, titanium dioxide and silver oxide nanoparticles. We show that unlike the classical heterogeneous nucleation theory, the new theory is able to quantitatively predict the experimental results.

A variational approach to nucleation simulation

We study by computer simulation the nucleation of a supersaturated Lennard-Jones vapor into the liquid phase. The large free energy barriers to transition make the time scale of this process impossible to study by ordinary molecular dynamics simulations. Therefore we use a recently developed enhanced sampling method [Valsson and Parrinello, Phys. Rev. Lett.113, 090601 (2014)] based on the variational determination of a bias potential. We differ from previous applications of this method in that the bias is constructed on the basis of the physical model provided by the classical theory of nucleation. We examine the technical problems associated with this approach. Our results are very satisfactory and will pave the way for calculating the nucleation rates in many systems.

Homogeneous vapor nucleation of water in 3 M NaCl solution within a nanopore

Homogeneous vapor nucleation of water in the electrolyte solution within a nanopore at its superheat limit was studied using the bubble nucleation model based on molecular interaction. The wall motion of the bubble that evolved from the evaporated water was obtained using the Keller–Miksis equation and the distribution of temperature inside the bubble was obtained by solving the continuity, momentum and energy equations for the vapor inside the bubble. Heat transfer at the interface was also considered in this study. The nucleation rate of the 3M NaCl solution at 571K is estimated to be approximately 0.15×1028clusters/(m3s). With this value of the nucleation rate, the complete evaporation time of the 50nm radius of the electrolyte solution is approximately 0.60ns. The calculated life time of the bubble that evolved from the evaporated solution, or the time duration for the growth and subsequent collapse of the bubble, is approximately 32ns, which is close agreement with the observed result of 28ns. The bubble reaches its maximum radius of 301nm at 13.2ns after the bubble evolution.

Atmospheric-pressure dielectric barrier discharge with capillary injection for gas-phase nanoparticle synthesis

We present an atmospheric-pressure dielectric barrier discharge (DBD) reactor for gas-phase nanoparticle synthesis. Nickel nanoparticles are synthesized by homogenous nucleation from nickelocene vapor and characterized online by aerosol mobility measurements. The effects of residence time and precursor concentration on particle growth are studied. We find that narrower distributions of smaller particles are produced by decreasing the precursor concentration, in agreement with vapor nucleation theory, but larger particles and aggregates form at higher gas flow rates where the mean residence time should be reduced, suggesting a cooling effect that leads to enhanced particle nucleation. In comparison, incorporating a capillary gas injector to alter the velocity profile is found to significantly reduce particle size and agglomeration. These results suggest that capillary gas injection is a better approach to decreasing the mean residence time and narrowing the residence time distribution for nanoparticle growth by producing a sharp and narrow velocity profile.

Roles of Organic Emissions in the Formation of Near Field Aircraft-Emitted Volatile Particulate Matter: A Kinetic Microphysical Modeling Study

A kinetic microphysical modeling approach that describes the formation of volatile aerosols in the presence of organic emissions in near field aircraft emitted plumes at ground level is presented. Our model suggests that self nucleation of organic species, binary nucleation of water-soluble organic vapors with water, and multicomponent nucleation of water-soluble organic vapors with sulfuric acid and water all have very slow nucleation rates. The formation of new homogeneous particles in near field aircraft plumes is thus considered to be driven by binary nucleation of sulfuric acid and water. Volatile organic vapors emitted from aircraft engines primarily contribute to the nucleation process by condensing on existing homogeneous aerosols and only affect the size and the composition (not the number) of the homogeneous aerosols. Our model also shows that under low ambient relative humidity levels or high ambient temperatures, nucleation mode particles are more organic-rich than soot coatings. Organic mass fraction of nucleation mode particles is more sensitive to organic emissions levels compared to that of soot coatings. Ambient temperature and relative humidity were also predicted to affect the nucleation of sulfuric acid–water cores, where higher ambient relative humidity level and lower ambient temperature strongly favor binary sulfuric acid–water nucleation. The effect of ambient conditions on organic fractions was predicted to be relatively insignificant.

A molecular dynamics study of water vapor nucleation in the presence of ions

Ions make a significant contribution to the nucleation dynamics of aqueous aerosol particles, but the understanding about microscopic mechanism is insufficient. In this paper, molecular dynamics simulations are conducted to investigate the condensation of supersaturated water vapor in the presence and absence of ions. The Yasuoka–Matsumoto (YM) method and the kinetic analysis are extended to investigate the influence of ions on the nucleation dynamics. It is observed that the presence of ions has a dramatic influence on the water vapor nucleation, which makes the clustering of water molecules easily. The ion concentration and the supersaturation are key factors that control the phase transition in the simulation system. For a certain supersaturation, the generation rate of clusters is proportional to the ion density. Water molecules tend to condense around ions with negative charge and high electric quantity. In addition, details of the cluster change are explored by the kinetic analysis when the system is added with cations and anions simultaneously. It is found that the change rate of the cluster increases with the rise of ions and decreases with the rise of water molecules. This phenomenon can be related to the microscopic structure of clusters that water molecules wrap around the ions to form a shell. The coalescence effect plays an important role in the nucleation process, and the proportion of the coalescence effect increases as the increase of supersaturation.