Estimation Of Nucleation Rates Research Articles

Characterization of shapes and volumes of droplets generated in PDMS T-junctions to study nucleation

We characterize shapes and volumes of droplets generated in PDMS T-junctions and assess the use of this type of microfluidic device to generate droplets suitable for the study of nucleation. Water droplets were formed in oil in a PDMS T-junction and subsequently stored. Droplet volume reproducibility and stability were investigated from acquired micrographs. By theoretically analyzing the influence of the mean volume of a population of droplets on the estimation of nucleation rates, we have shown that deviations in mean volumes can seriously affect the estimates, unless such deviation is smaller than 10%. This condition is fulfilled if experiments are repeated using the same microdevice. Measured droplet polydispersity remained low enough to treat the droplets as monodisperse. Immersing the microdevice in a water bath mitigates solvent evaporation, and allows for very accurate temperature control. Finally, a screening procedure was used to select the ideal operating conditions to obtain droplets with the desired sizes. Applying this method in devices with increasing T-junction cross sectional area, we have demonstrated a scaling-up of droplet volumes close to an order of magnitude while tuning the droplet shape, i.e., the average length to width ratio, at values between 1 and 1.2.

Diffusion-driven nucleation from surface nuclei in hydrodynamic cavitation

Investigations about the role of nuclei and nucleation for the inception and formation of cavitation have been part of cavitation research since Harvey et al. (J. Cell. Physiol., vol. 24 (1), 1944, pp. 1–22) postulated the existence of gas filled crevices on surfaces and particles in liquids. In a supersaturated liquid, surface nuclei produce small gas bubbles due to mass transfer of gas or themselves work as weak spots in the liquid that are necessary for a phase change under technically relevant static pressures. Although various theories and models about nuclei and nucleation have found their way into standard literature, there is a lack of experimentally validated theories that describe the process of diffusion-driven nucleation in hydrodynamic cavitation. In order to close this gap we give new theoretical insights into the physics of this nucleation mechanism at technically relevant low supersaturations validated with extensive experimental results. The nucleation rate, the number of produced bubbles per second, is proportional to the supersaturation of the liquid and shows a nonlinear dependence on the shear rate at the surface nucleus. A model for the Strouhal number as dimensionless nucleation rate is derived allowing the estimation of nucleation rates from surface nuclei in hydrodynamic cavitation. The model provides three asymptotes, being a function of Péclet number, Weber number, the supersaturation of the liquid$\unicode[STIX]{x1D701}$and gas solubility$\unicode[STIX]{x1D6EC}$for three different detachment mechanisms,$Sr\propto \unicode[STIX]{x1D701}\unicode[STIX]{x1D6EC}We^{n}Pe^{1/3}$with$n=1/3,3/4,1$. The theoretical findings are in good agreement with experimental results, leading to a new assessment of the role of diffusion in cavitating flows.

α-Pinene Oxidation in the Presence of Seed Aerosol: Estimates of Nucleation Rates, Growth Rates, and Yield

A recently developed inverse-modeling procedure has been applied to a case study of particle nucleation and growth following alpha-pinene and SO2 oxidation in a smog chamber. With the use of only the measured aerosol size distributions as input, the condensational growth rate is obtained by regression analysis of the general dynamic equation, taking into account coagulation and wall losses. The growth rate provides an indirect measure of the concentration of the condensing species, offset by their vapor pressures. Assuming a particle density of 1.0 g cm(-3), an aerosol yield of 7 +/- 1% is obtained for an initial alpha-pinene concentration of 14 ppbv and a final organic aerosol mass of 4 microg m3. Using the estimated vapor concentration, we show that the time-dependence of the yield is at least partly due to the time needed for condensation. Such a kinetic limitation to secondary organic aerosol formation may have implications for our understanding of gas-particle partitioning. The measured size distributions are also used to determine the empirical nucleation rate; it appears to be enhanced by the presence of organics.

Extracting nucleation rates from current–time transients: Part I: the choice of growth models

The behaviour of the transients due to the model of the growth of right-circular cones is fully explored and compared to that of hemispheroids. The conditions that justify the use of the limiting forms of the transient equations for the so-called ‘instantaneous’ and ‘progressive’ nucleation processes are evaluated. Nucleation rates obtained from the fit of the recorded transients to the generalised equations derived for both growth models are compared. It is shown that even in those cases where electrocrystallisation proceeds via nucleation and growth of hemispheroids, analysis of the transients according to the model of the growth of cones results in estimates of nucleation rates which differ by at most a factor of three, and in most cases are as close as half the actual value.

Estimation of nucleation rate and growth rate from time dependence of global microstructural properties during phase transformations

An approach has been developed for calculating nucleation and growth rates from the time variation of the volume fraction, surface area, and integral mean curvature of the product phase during a phase transformation. The local growth rate of the product phase can be estimated without any assumption or knowledge regarding the nucleation behavior. The approach is applicable over the complete range of volume fractions(i.e., from zero to one). Practical feasibility of the approach has been demonstrated by deducing the nucleation and growth rates of austenite during austenitization of pearlite in an Fe-0.83 wt pct C alloy at 730 ‡C, 740 ‡C, and 750 ‡C. It is concluded that the local growth rate and nucleation rate of austenite remain constant during an isothermal austenitization of pearlite.

Nucleation of epitaxial films at chemical growth

A theoretical analysis and an experimental investigation of nucleating epitaxial Ge and GaAs films under chemical transport conditions at homoepitaxy were made. Estimation of nucleation rate was made taking into account the differences of temperature between the source and deposition zones, substrate orientation and its surface state. Investigation of crystallite nucleation and growth was carried out by electron-microscope replica method for deposition time from 5 sec up to 1 hour. The nucleus distribution curves of size and changes of nucleus density and size with time were obtained. The initial stage of deposition of GaAs occurs by forming three-dimensional nuclei and their growth occurs at constant rate. Nucleation rate defines the film perfection and its minimal thickness. The change of the nucleus form with time is observed and the role of the diffusion process is discussed.