Thermal Diffusion Cloud Chamber Research Articles

Vapor-phase nucleation of n-pentane, n-hexane, and n-heptane: Critical cluster properties.

The first and second nucleation theorems provide a way to determine the molecular content and excess internal energies of critical clusters, which rely solely on experimental nucleation rates measured at constant temperatures and supersaturations, respectively. Here, we report the size n* and excess internal energy Ex(n*) of n-pentane, n-hexane, and n-heptane critical clusters when particles form under the highly supersaturated conditions present in supersonic expansions. In summary, critical clusters contain from ∼2 to ∼11 molecules and exhibit the expected increase in the critical cluster size with increasing temperature and decreasing supersaturation. Surprisingly, the n* values for all three alkanes appear to lie along a single line when plotted as a function of supersaturation. Within the framework of the capillarity approximation, the excess internal energies determined for the n-heptane critical clusters formed under the low temperature (∼150 K) conditions in our supersonic nozzle are reasonably consistent with those determined under higher temperature (∼250 K) conditions in the thermal diffusion cloud chamber by Rudek et al. [J. Chem. Phys. 105, 4707 (1996)].

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.

Improvement of Cooling Capacity of A Educational Thermal Diffusion Cloud Chamber Operated by The Peltier Cooler Device

Improvement of Cooling Capacity of A Educational Thermal Diffusion Cloud Chamber Operated by The Peltier Cooler Device

The influence of buoyant convection on the nucleation of n-propanol in thermal diffusion cloud chambers

A two-dimensional numerical model has been applied to three thermal diffusion cloud chamber (TDCC) investigations of n-propanol in helium taken by two different research groups to provide a quantitative example of how the results in these chambers can be affected by buoyant convection. In the first set of TDCC data, corrections for buoyancy resolve an apparent discontinuity in critical supersaturation data and also yield nucleation rate data that tend to agree better with higher rate, expansion-based studies at the same temperature. In the second TDCC study, the nucleation of propanol was studied over an extended pressure range. When the model was applied to these data, the possible variation in supersaturation values due to convection induced by conditions at the chamber sidewall was found to be comparable in magnitude to the experimentally observed range and may be responsible for some of this observed pressure dependence. In the third TDCC study, the combination of an error in a transport property and buoyant convection appear responsible for a perceived pressure effect in the experimental data. After correcting for this transport property and for buoyancy, the results at higher temperatures agree quite closely with the predictions of classical nucleation theory.

Homogeneous nucleation rate measurements in supersaturated water vapor II

The homogeneous nucleation of water was studied experimentally in this work using a thermal diffusion cloud chamber; droplets were counted by the photomultiplier method and helium was used as a carrier gas. The nucleation rates range from 3x10(-2) to 3x10(1) cm(-3) s(-1) and six isotherms from 295 to 320 K with step of 5 K are measured. The experimental setup and obtained data are mutually compared to our previous publication [Brus et al., J. Chem. Phys. 129, 174501 (2008)], where the droplets were counted using digital photography and image processing. The molecular content of the critical clusters was estimated from the slopes of experimental data. The measured isothermal dependencies of the nucleation rate of water on the saturation ratio were compared with previously published data of others, several theoretical predictions, and the former nucleation onset data. The aim of the present investigation was to show for the first time that nucleation results can be quantitatively reproduced with two different experimental setups operated in different ways.

Buoyancy-driven convective motion in a thermal diffusion cloud chamber using a water/helium mixture

This work is focused on a 2D numerical simulation of a thermal diffusion cloud chamber (TDCC) operating with water–helium mixture. We particularly address the impact of the stability of the vapor–gas mixture with respect to buoyancy-driven convective motion on homogeneous water nucleation rates. A comparison of our model results with Heist and Reiss results of nucleation of water in helium is first proposed. So, no convective fluxes are assumed within the TDCC and the critical supersaturation of water vapor is in agreement as obtained from numerical predictions. However, the influence of wall heating on the critical supersaturation results is found to be significant as obtained when convective transfers is accounted for within the TDCC and the results deviate significantly from those provided by Heist and Reiss. Their approach may lead large differences in terms of temperature, saturation ratio and nucleation profiles as it oversimplifies heat and mass transfer in TDCC compared to a 2D mass, heat and momentum model.

Homogeneous nucleation rate measurements in supersaturated water vapor

The rate of homogeneous nucleation in supersaturated vapors of water was studied experimentally using a thermal diffusion cloud chamber. Helium was used as a carrier gas. Our study covers a range of nucleation rates from 3x10(-1) to 3x10(2) cm(-3) s(-1) at four isotherms: 290, 300, 310, and 320 K. The molecular content of critical clusters was estimated from the slopes of experimental data. The measured isothermal dependencies of nucleation rate of water on saturation ratio were compared with the prediction of the classical theory of homogeneous nucleation, the empirical prediction of Wolk et al. [J. Chem. Phys. 117, 10 (2002)], the scaled model of Hale [Phys. Rev. A 33, 4156 (1986)], and the former nucleation onset data.

Binary Homogeneous Nucleation in Selected Aqueous Vapor Mixtures

The objective of this article is to give an overview of available experimental techniques and theoretical fundaments of the classical theories of homogeneous binary nucleation. The principles of the experimental setups are reviewed, ranging from low-nucleation-rate (≤103 nuclei/(cm3⋅s)) devices, i.e., thermal-diffusion cloud chamber, expansion chamber, over-nucleation pulse technique, and nozzle flow, to the condensation wave technique which is applied for the fastest nucleation rates (∼1015 nuclei⋅(cm3⋅s)−1). The theoretical description is based on the capillary approximation and takes into account the real properties of fluids (liquid mixtures) including the effects of non-equilibrium processes. A very important effect is associated with the surface tension dependence on the concentration of admixtures that can have a serious impact on the value of the nucleus formation energy. In the case of surfactants (i.e., molecules that can transiently bond with water through hydrogen bonding, e.g., alcohols) the nucleation work is decreased and for inorganic salts (hydrophobic molecules tend to be non-polar) the nucleation work is increased. The theoretical results are compared with the available experimental data. Also, the role of salts in power plant chemistry is discussed.

Numerical study of the droplet behavior in a thermal diffusion cloud chamber operating with saline water

The droplet behavior in a thermal diffusion cloud chamber (TDCC) using a saline water/helium mixture was studied numerically. A sub-saturated zone was created in the chamber by depositing some salt on the lower plate. In this zone droplets could evaporate and the droplet cloud behavior is then modified. Although the effect of wall heating has little influence on the droplets growth, it strongly affects the droplet trajectories as obtained when convective transfers are accounted for within the TDCC.

The carrier gas pressure effect in a laminar flow diffusion chamber, homogeneous nucleation of n-butanol in helium

Homogeneous nucleation rate isotherms of n-butanol+helium were measured in a laminar flow diffusion chamber at total pressures ranging from 50 to 210 kPa to investigate the effect of carrier gas pressure on nucleation. Nucleation temperatures ranged from 265 to 280 K and the measured nucleation rates were between 10(2) and 10(6) cm(-3) s(-1). The measured nucleation rates decreased as a function of increasing pressure. The pressure effect was strongest at pressures below 100 kPa. This negative carrier gas effect was also temperature dependent. At nucleation temperature of 280 K and at the same saturation ratio, the maximum deviation between nucleation rates measured at 50 and 210 kPa was about three orders of magnitude. At nucleation temperature of 265 K, the effect was negligible. Qualitatively the results resemble those measured in a thermal diffusion cloud chamber. Also the slopes of the isothermal nucleation rates as a function of saturation ratio were different as a function of total pressure, 50 kPa isotherms yielded the steepest slopes, and 210 kPa isotherms the shallowest slopes. Several sources of inaccuracies were considered in the interpretation of the results: uncertainties in the transport properties, nonideal behavior of the vapor-carrier gas mixture, and shortcomings of the used mathematical model. Operation characteristics of the laminar flow diffusion chamber at both under-and over-pressure were determined to verify a correct and stable operation of the device. We conclude that a negative carrier gas pressure effect is seen in the laminar flow diffusion chamber and it cannot be totally explained with the aforementioned reasons.

Cloud condensation nuclei measurements at Mace Head, Ireland, over the period 1994–2002

Analyses of cloud condensation nuclei (CCN) number concentrations (cm − 3 ) measured at the Mace Head Atmospheric Research Station, near Carna, County Galway, Ireland, using a DH Associates Model M1 static thermal diffusion cloud chamber over the period from March 1994 to September 2002 are presented in this work. Air masses are defined as being ‘marine’ if they originate from a wind direction of 180–300° and ‘continental’ air masses are defined as originating from a wind direction of 45–135°. Air masses without such filtering were classified as ‘undefined’ air masses. Air masses were found to be dominated by marine sector air, re-affirming Mace Head as a baseline atmospheric research station. CCN levels for specific air masses at Mace Head were found to be comparable with earlier studies both at Mace Head and elsewhere. Monthly averaged clean marine (wind direction of 180–300° and black carbon absorption coefficient < 1.425 Mm − 1 ) CCN and marine CCN varied between 15–247 cm − 3 and 54–670 cm − 3 , respectively. As expected, significant increases in number concentration were found in continentally sourced CCN over that of marine CCN and were found to follow a log-normal distribution significantly tighter than that of clean marine air masses. No significant trend was found for CCN over the 9-year period. While polluted continental air masses showed a slight increase in CCN concentrations over the winter months, most likely due to increased fuel usage and a lower mixed boundary layer, the dominance of marine sector air arriving at Mace Head, which generally consists of background CCN concentrations, reduced seasonal differences for polluted air. Marine air showed a distinct seasonal pattern, with elevated values occurring over the spring and summer seasons. This is thought to be due to enhanced biogenic aerosol production as a result of phytoplankton bloom activity in the North Atlantic.

Homogeneous nucleation rate measurements of 1-propanol in helium: The effect of carrier gas pressure

Kinetics of homogeneous nucleation in supersaturated vapor of 1-propanol was studied using an upward thermal diffusion cloud chamber. Helium was used as a noncondensable carrier gas and the influence of its pressure on observed nucleation rates was investigated. The isothermal nucleation rates were determined by a photographic method that is independent on any nucleation theory. In this method, the trajectories of growing droplets are recorded using a charge coupled device camera and the distribution of local nucleation rates is determined by image analysis. The nucleation rate measurements of 1-propanol were carried out at four isotherms 260, 270, 280, and 290 K. In addition, the pressure dependence was investigated on the isotherms 290 K (50, 120, and 180 kPa) and 280 K (50 and 120 kPa). The isotherm 270 K was measured at 25 kPa and the isotherm 260 K at 20 kPa. The experiments confirm the earlier observations from several thermal diffusion chamber investigations that the homogeneous nucleation rate of 1-propanol tends to increase with decreasing total pressure in the chamber. In order to reduce the possibility that the observed phenomenon is an experimental artifact, connected with the generally used one-dimensional description of transfer processes in the chamber, a recently developed two-dimensional model of coupled heat, mass, and momentum transfer inside the chamber was used and results of both models were compared. It can be concluded that the implementation of the two-dimensional model does not explain the observed effect. Furthermore the obtained results were compared both to the predictions of the classical theory and to the results of other investigators using different experimental devices. Plotting the experimental data on the so-called Hale plot shows that our data seem to be consistent both internally and also with the data of others. Using the nucleation theorem the critical cluster sizes were obtained from the slopes of the individual isotherms and compared with the Kelvin prediction. The influence of total pressure on the observed isothermal nucleation rate was studied in another experiment, where not only temperature but also supersaturation was kept constant as the total pressure was changed. It was shown that the dependence of the nucleation rate on pressure gets stronger as pressure decreases.

Experimental studies of the vapor phase nucleation of refractory compounds. VI. The condensation of sodium

In this paper we discuss the condensation of sodium vapor and the formation of a sodium aerosol as it occurs in a gas evaporation condensation chamber. A one-dimensional model describing the vapor transport to the vapor/aerosol interface was employed to determine the onset supersaturation, in which we assume the observed location of the interface is coincident with a nucleation rate maximum. We then present and discuss the resulting nucleation onset supersaturation data within the context of nucleation theory based on the liquid droplet model. Nucleation results appear to be consistent with a cesium vapor-to-liquid nucleation study performed in a thermal diffusion cloud chamber.

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.

Numerical investigation of the motion of a growing droplet in a thermal diffusion cloud chamber

The evolution of spherical droplets resulting from the homogeneous condensation of vapor phase diluted in a background gas has been studied numerically. The model was compared with experimental data of dioctylphthalate (DOP) droplets in binary gaseous mixture of DOP-helium and of DOP-hydrogen in a thermal diffusion cloud chamber (TDCC). The motion of droplets due to thermodiffusiophoresis as obtained from numerical predictions is in agreement with the experimental results. Two-dimensional droplet dynamics involving the effect of convection, thermodiffusiophoresis, gravity and condensation has been examined in details for two flow configurations depending on the cloud chamber geometry. When the height of the TDCC is increased, the isothermal patterns and streamlines are similar to those obtained in the case of Rayleigh–Bénard instabilities, leading a notable alteration of the droplet behavior. While the thermodiffusiophoresis forces govern the drop motion at low thermal Rayleigh numbers (in conductive regime), the behavior of the droplet is mainly driven by the drag force when the TDCC operates in convective regime (at high Rayleigh numbers).

Simulating vapor–liquid nucleation of n-alkanes

A combination of the aggregation-volume-bias Monte Carlo algorithm, the configurational-bias Monte Carlo algorithm, and the umbrella sampling technique was applied to investigate homogeneous vapor–liquid nucleation in ethane, n-butane, and n-heptane. The simple transferable potentials for phase equilibria-united atom (TraPPE-UA) force field was used in this investigation. It was found that for the n-heptane case, the TraPPE-UA force field predicted a nucleation rate that is about three to four orders of magnitude higher than that measured by an upward thermal diffusion cloud chamber experiment. Comparison of the simulation results to the classical nucleation theory (CNT) shows that CNT consistently overestimates the barrier heights for all chain lengths investigated. The offset on the barrier heights was found nearly independent of the supersaturation for both ethane and n-butane, similar to a Lennard-Jones system previously studied. This also directly leads to a good agreement on the cluster sizes between the simulation and the CNT calculated from the nucleation theorem. For n-heptane, however, the offset was found to depend on the supersaturation. It appears that CNT predicts a slightly weaker dependence of the nucleation rate on supersaturation, which agrees with both density functional calculations and the experiments. Structural analysis demonstrates that the orientational order near the surface differs significantly between the critical nucleus and the bulk planar liquid–vapor interface for n-heptane systems, whereas the density in the interior of the critical nucleus is in good agreement with the bulk liquid density. The different surface ordering offers a microscopic explanation for the differences observed for n-heptane between the CNT on one side and experimental observations and simulations on the other side.

The effect of carrier gas pressure and wall heating on the operation of the thermal diffusion cloud chamber

Experimental observations indicate that the nucleation behavior within the thermal diffusion cloud chamber (TDCC) changes with increasing carrier gas pressure and applied sidewall heating, even though such an effect is not predicted by typical nucleation theories and it is not seen in typical expansion-based nucleation studies. In this work we present a model of the chamber which shows that both of these effects are likely due to buoyancy-induced convection within the TDCC. As the chamber pressure is increased, the calculated critical supersaturation within the chamber decreases. Results from a simple model of the chamber wall heating are also presented. Previously, it was argued that unheated chamber walls result in a significant, radial concentration gradient which lowers the vapor concentration and condensation flux within the chamber center. In contrast, we show that this reduction is due primarily to a convective flow induced by the sidewall concentration gradient. The model has been applied to recent experimental data for n-pentanol. Results indicate that, with respect to buoyancy-induced convection, the typical 1D model should be regarded as an upper limit to the maximum attainable supersaturation within the chamber.

Vapor Phase Homogeneous Nucleation of Higher Alkanes: Dodecane, Hexadecane, and Octadecane. 1. Critical Supersaturation and Nucleation Rate Measurements

The critical supersaturations and isothermal homogeneous nucleation rates of dodecane, hexadecane, and octadecane vapors have been measured over wide temperature ranges (e.g., 285−340 K) using an upward thermal diffusion cloud chamber. Dodecane shows the best agreement between the experiment and the classical nucleation theory. Hexadecane and octadecane exhibit critical supersaturations lower than those predicted by the theory. All three compounds show increasing deviation from theory with decreasing temperature. This trend is also observed in the measured nucleation rates where the predictions of the theory at lower temperatures are shifted to much lower values compared to the experimental data. Temperature dependence correction factors to the calculated rates have been evaluated based on the experimental rates of the studied higher alkanes. The classical theory predicts the correct supersaturation dependence of the nucleation rates of these compounds. Analysis of the supersaturation dependence of the nu...

The Impact of Convective Flow on Thermal Diffusion Cloud Chamber Operation

We report here new results that further elucidate the role of buoyancy-driven convective flows in determining the distribution of vapor mole fraction and temperature in the thermal diffusion cloud chamber. A two-dimensional model describing both diffusion and convection in the presence of chamber sidewalls is presented. The model equations are used to analyze 1-butanol and 1-pentanol nucleation data obtained earlier. The results of the investigation show that convective flow is nearly always present and, in the cases considered here, result in a reduction in the value of the calculated critical supersaturation (as compared to the one-dimensional case). The dependence of critical supersaturation with total pressure reported earlier is smaller but is not eliminated.

2-D Model for the Description of Thermal Diffusion Cloud Chambers: Description and First Results

In thermal diffusion cloud chambers (TDCCs), a decrease of nucleation rate with increasing total pressure was observed repeatedly, both near atmospheric pressures and at elevated pressures. Because this pressure effect was observed almost only using TDCCs, there have been some concerns inside the nucleation community whether it is not an experimental artifact. One questionable point in this context is that, to calculate the profiles of temperature and saturation ratio in TDCCs it is assumed that if the chamber is sufficiently “flat”, a 1-d model can be used for the description of the transport processes in the central part of the chamber. The major concern in this context is that the heated chamber wall may cause a buoyancy driven convection that can propagate toward the center of the chamber and cause a slow motion of the gas mixture. To investigate this effect, a 2-d model of coupled mass, heat, and momentum transport in TDCCs has been developed. In the paper this model is described, and its predictive force is shown in comparison with a state-of-the-art 1-d model and with experimental data from the system n-propanol−He. Furthermore, results concerning the effects of wall heating, total chamber pressure, carrier gas, and chamber geometry on nucleation rates are presented. The influence of wall heating on the nucleation rate was found to be significant when transport processes inside TDCC are dominated by buoyant convection. Using the 2-d model made it possible to partly (but not completely) explain the pressure effect observed in TDDCs. It was also shown that including the process of thermal diffusion in the model may have a significant influence on the resulting profiles of nucleation rate. It was confirmed that the choice of the carrier gas in the chamber can strongly influence stability of chamber operation. With respect to the optimal chamber geometry it was shown that TDCCs should have a diameter as small and a diameter-to-height ratio as large as possible to minimize the 2-d effects. All in all, it was proven that application of 1-d models to describe the heat and mass transfer in TDCC may lead to an erroneous determination of temperature, saturation ratio, and nucleation rate profiles.