Thermodynamics of supersaturated vapor nucleation on molecular condensation nuclei

A key problem in the theory of a supersaturated vapor nucleation on molecular condensation nuclei (namely, the work of formation and the equilibrium concentrations of clusters) is considered. To calculate these quantities using the structural models of clusters, which are better suited for this purpose than the classical droplet model, we derive the equation connecting the work of transfer of a molecular condensation nucleus from the gas phase to a homogeneous cluster with a change in the number of contacts between molecules, occurring in the course of this transfer, and with the work of rupture of individual contacts.     

Specifics of nucleation in superheated water and supersaturated vapor

This work is dedicated to the experimental studying of the nucleation kinetics in superheated water and supersaturated water vapor. A percolation model for the liquid water structure that explains a number of anomalous thermophysical properties of water and water vapor in the metastable region is proposed.     

Sound amplification in a supersaturated vapor

A sound wave in supersaturated water vapor can produce the process of heat release caused by condensation, which occures to be in phase with sound. The resonance interaction of sound with modulated heat release provides the sound amplification and the spectra of raindrops changes.     

Alpha-particle condensation in nuclei

A round-up of the present status of the conjecture that nα nuclei form an α-particle condensate in excited states close to the nα threshold is given. Experiments which could demonstrate the condensate character are proposed. Possible lines of further theoretical developments are discussed.     

Oscillatory regimes of vapor condensation

The problem of the condensation of supersaturated vapor in an open system at a constant rate of production of a monomer and a continuous flow of a carrier gas that removes the products of condensation from the system is considered. It is shown both analytically and by numerical experiment that with a decreasing rate of the carrier gas to below a critical magnitude, the condensation regime becomes oscillatory; namely, time oscillations of the cluster-size distribution in the vapor to be condensed are set in. The cause of the phenomenon is in the suppression of the rate of nucleation and the presence of large clusters.     

Metastability and nucleation in capillary condensation

This paper is devoted to thermally activated dynamics of capillary condensation. On the basis of a simple model we identify the critical nucleus involved in the transition mechanism and calculate the nucleation barrier from which we obtain information on the nucleation time. Close to the condensation point, the theory predicts extremely large energy barriers leading to strong metastabilities, long time dependencies, and large hysteresis in agreement with experimental observations in mesoporous media. The validity of the model is assessed using a numerical simulation of a time-dependent Ginzburg-Landau model for the confined system.     

Optoelectronic system of the aerosol photometer in the detector of molecular condensation nuclei

We study the light scattering by aerosol particles and air in the photometer of the molecular condensation nuclei, as well as the sensitivity of the photodector of the photometer. The interference nature of light scattering by aerosol particles is established and is found to be comparable (in order of magnitude) with the scattering of light by air in the photometer. The sensitivity of the photometer can be increased by more than an order of magnitude due to optimization of the optoelectronic part of the photometer. The detection threshold for the target component of the gas analyzer is attained at the spontaneous ionization background level and not at the limiting sensitivity level of the photodetector.     

Model of filmwise vapor condensation on curved surfaces

A three-dimensional unsteady model of filmwise vapor condensation on a curved surface that takes into account the mass forces, the surface tension, and the friction on the surface of the condensate film is developed. This model generalizes the previous models of the filmwise vapor condensation. An evolution equation is derived for a condensate-layer thickness. Numerical algorithms are developed for solving the evolution equation. The condensation of ethanol vapor moving in a round tube is calculated. The effect of the tube diameter on the intensity of condensation is analyzed.     

Meson condensation and anomalous nuclei

The possibility of the existence of 1) superdense, 2) neutron and 3) supercharged nuclei, is discussed. The stability of such nuclei can be provided by the π-condensation.     

Computer simulation of water vapor nucleation on charged nanoparticles

The Monte Carlo method is applied to the study of the formation of condensed-phase nuclei from water vapor on electrically charged silver iodide nanocrystals. This study is a continuation of the investigations carried out earlier in [1] with electrically neutral nucleation centers. Nanoparticles with a size of up to 4 nm and flat nanoparticles with a size of up to 10 nm are investigated. The free energy, entropy, and the work of formation of nuclei with a size of up to 6729 molecules are calculated at the atomic level by the bicanonical statistical ensemble (BSE) method at a temperature of 260 K. Thermodynamic stability of nuclei is investigated depending on the size, shape, and charge of nanocrystal nucleation centers, as well as depending on the presence of crystal defects and the degree of spatial localization of charge on the surface of nanoparticles. The excess charge has a crucial effect on the work of formation of a nucleus only in the case of strong spatial localization of the latter near a point crystal defect; however, this effect is restricted to a relatively small size of the nuclei and therefore cannot substantially enhance the ice-forming activity of nanoparticles. A nucleus that grows on the surface of a nanoparticle evolves through three stages that differ in molecule retention mechanism and thermodynamic stability. The charge of a nanoparticle has a small effect on these factors. The leading factor that determines the ice-forming activity of ion nanocrystals is their intrinsic electric field due to the nonuniform distribution of charge within a unit cell of the crystal lattice.     

On pion condensation in finite nuclei

The critical pionic state leading to pion condensation is investigated for finite nuclear systems. The threshold density for light nuclei, including⁴He, is found to be close to that of nuclear matter. For¹⁶O and⁴⁰Ca, the angular momenta of the most critical states areL=0 andL=2.     

Numerical simulation of water vapor nucleation on electrically neutral nanoparticles

Atomic-level Monte Carlo simulations are performed to calculate the free energy, entropy, and work of nucleation for clusters of more than 6 × 10³ water molecules growing on silver iodide crystalline particles of size up to 4 nm at a temperature of 260 K. The Hamiltonian of the system includes explicit expressions for hydrogen bonding energy and Coulomb, dispersion, exchange, and polarization interactions. The work of nucleation exhibits complex behavior depending on the nucleation-site size. With increasing nanoparticle size, clusters become less stable and the probability of crystallization increases. Mutual polarization enhances the bonding between a cluster and a crystalline particle. Cluster growth on relatively large nanoparticles involves two stages characterized by two critical sizes: monolayer growth on the surface and growth normal to the surface. Spontaneous microdroplet polarization involving domain formation is found to occur at the crystal surface. The dependence of the ice-forming activity of an aerosol on particulate size observed in experiments is explained by combined effects of several competing factors, the dominant ones being the stabilizing and destabilizing effects of the nanoparticle electric field.     

Effect of diffusion on nucleation of 2D and 3D nanoclusters in supersaturated solutions

The effect of diffusion on the steady-state nucleation of 2D and 3D nanoclusters is described analytically. Proposed approach takes self-consistently into account coupling between the kinetics of monomers near the cluster boundary, their diffusion and annihilation at other clusters. It has been shown that due to this coupling the nucleation barriers can considerably differ from those predicted by the thermodynamic approach.     

Surface pion condensation in finite nuclei

We discuss here the possible occurrence of surface pion condensation in finite nuclei. We show the argument for this possibility by the recent findings in few-body calculations and the spin-flip experiments. We show the calculated results on various N = Z nuclei using the relativistic mean-field theory with a pion.Received: 30 September 2002, Published online: 22 October 2003PACS: 21.60.-n Nuclear-structure models and methods     

Simulation of the free molecular flow of vapor for laser isotope separation

Vapor flows from a single channel (tube) and from a set of tubes are simulated in the free molecular motion approximation. The integrated characteristics (the fractions of transmitted, sticking, and reflected particles) and the angular distributions of the flux and flux density of the vapor are obtained. Calculations of the flux of atoms scattered as a result of single collisions in an atomic beam are performed. Zh. Tekh. Fiz. 68, 77–81 (May 1998)     

Stimulation of vapor nucleation on perfect and imperfect hexagonal lattice surfaces

Monte Carlo simulations of water vapor nucleation on a perfect crystal surface and on a surface with defects are performed. Mass exchange with the vapor phase is modeled by using an open ensemble. Cluster-substrate interaction is described in terms of conventional atom-atom potentials. The Hamiltonian of the system includes expressions for electrostatic, polarization, exchange, and dispersion interactions. The Gibbs free energy and work of adsorption are calculated by Monte Carlo simulation in the bicano?nical ensemble. The microscopic structure of nuclei is analyzed in terms of pair correlation functions. Periodic boundary conditions are used to simulate an infinite substrate surface. Molecule-substrate and molecule-molecule long-range electrostatic interactions are calculated by summing the Fourier harmonics of the electrostatic potential. Dispersion interactions are calculated by direct summation over layers of unit cells. Nucleation on a surface with matching structure follows a layer-by-layer mechanism. The work of adsorption per molecule of a monolayer on the substrate surface has a maximum as a function of nucleus size. The steady rate of nucleation of islands of supercritical size is evaluated. The work of adsorption per molecule for layer-by-layer film growth is an oscillating function of cluster size. As a function of layer number, it has a minimum depending on the vapor pressure. The electric field generated by a microscopic surface protrusion destroys the layered structure of the condensate and eliminates free-energy nucleation barriers. However, point lattice defects do not stimulate explosive nucleation.