Effect of the Surface-Stimulated Mode on the Kinetics of Homogeneous Crystal Nucleation in Droplets

Crystallization of highly supercooled glass-forming alloys induced by anomalous surface wetting

Crystallization in highly supercooled Cu50Zr50 films close to the glass transition is studied by using molecular dynamics simulations. Spontaneous nucleation is observed at the simulation timescale in contrast to the bulk counterpart. We find that nucleation occurs at free surfaces owing to the partial wetting of the nucleus by melt. The anomalous wetting phenomenon is closely related to strong density layering arising from the surface: the high density associated with surface layering increases surface energy of supercooled melts, resulting in that one facet of the crystalline embryo is preferentially formed on the film surface. The surface-based embryo is then developed into a stable nucleus by bridging two surfaces of thin films. The kinetics and thermodynamics analyses based on the mean first-passage time method show that the nucleation process still follows the description of the classical nucleation theory despite extremely high supercoolings. In nucleating, the slow interface dynamics becomes dominant and induces a low nucleation rate although the nucleation barrier is very low. The subsequent crystal growth is found to proceed in a quasi-two-dimensional manner with a ramified interface morphology, which is analogous to percolative crystals predicted in glass-forming liquids.

Formation and evolution of aqueous organic aerosols via concurrent condensation and chemical aging

We review recent results on the formation and evolution of aqueous organic aerosols via concurrent nucleation/condensation and chemical aging processes obtained mostly using the formalism of classical nucleation theory In this framework, an aqueous organic aerosol was modeled as a spherical particle of liquid solution of water and hydrophilic and hydrophobic condensable organic compounds; besides these compounds, the surrounding air contained some chemically reactive, non-condensable species. Hydrophobic organic molecules on the aerosol surface can be processed by chemical reactions with some atmospheric species; this affects the hygroscopicity of the aerosol and hence its ability to become a cloud droplet. Such processing is most probably triggered by atmospheric hydroxyl radicals that abstract hydrogen atoms from surfactant molecules located on the aerosol surface (first step), resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). These two reactions play a crucial role in the enhancement of the Köhler activation of the aerosol. Taking them and a third reaction (next in the multistep chain of relevant heterogeneous reactions) into account, one can derive an explicit expression for the free energy of formation of a four-component aqueous droplet on a ternary aqueous organic aerosol as a function of four independent variables of state of a droplet. This approach was also applied to study a large subset of primary marine aerosols which can be initially treated using an "inverted micelle" model whereof the core consists of aqueous "salt" solution. Numerical evaluations suggest that the formation of cloud droplets on such (both aqueous hydrophilic/hydrophobic organic and marine) aerosols is most likely to occur via Köhler activation rather than via nucleation. The models allow one to determine the threshold parameters necessary for the Köhler activation of such aerosols. Furthermore, heterogeneous chemical reactions involved in the chemical aging of aerosols are most likely exothermic. Due to the release of the enthalpy of these reactions, the temperature of an aerosol particle during its chemical aging may become greater than the ambient (air) temperature. The analysis of the characteristic timescales of four most important processes involved suggests that this effect may play a significant impeding role in the formation of an ensemble of aqueous secondary organic aerosols via nucleation and, hence, must be taken into account in atmospheric aerosol and global climate models.

Effect of Heterogeneous Chemical Reactions on the Köhler Activation of Aqueous Organic Aerosols

We study some thermodynamic aspects of the activation of aqueous organic aerosols into cloud droplets considering the aerosols to consist of liquid solution of water and hydrophilic and hydrophobic organic compounds, taking into account the presence of reactive species in the air. The hydrophobic (surfactant) organic molecules on the surface of such an aerosol can be processed by chemical reactions with some atmospheric species; this affects the hygroscopicity of the aerosol and hence its ability to become a cloud droplet either via nucleation or via Köhler activation. The most probable pathway of such processing involves atmospheric hydroxyl radicals that abstract hydrogen atoms from hydrophobic organic molecules located on the aerosol surface (first step), the resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). These two reactions play a crucial role in the enhancement of the Köhler activation of the aerosol and its evolution into a cloud droplet. Taking them and a third reaction (next in the multistep chain of relevant heterogeneous reactions) into account, one can derive an explicit expression for the free energy of formation of a four-component aqueous droplet on a ternary aqueous organic aerosol as a function of four independent variables of state of a droplet. The results of numerical calculations suggest that the formation of cloud droplets on such (aqueous hydrophilic/hydrophobic organic) aerosols is most likely to occur as a Köhler activation-like process rather than via nucleation. The model allows one to determine the threshold parameters of the system necessary for the Köhler activation of such aerosols, which are predicted to be very sensitive to the equilibrium constant of the chain of three heterogeneous reactions involved in the chemical aging of aerosols.

Dependence of homogeneous crystal nucleation in water droplets on their radii and its implication for modeling the formation of ice particles in cirrus clouds

Dependence of the ice-nucleation-rate in water droplets on their radii and temperature is determined by taking into account volume-based and surface-stimulated modes.

Critical Radius of Supercooled Water Droplets: On the Transition toward Dendritic Freezing

The freezing of freely suspended supercooled water droplets with a diameter of bigger than a few micrometers splits into two rather different freezing stages. Within the first very fast dendritic freezing stage a spongy network ice with an ice portion of less than one-third forms and more than two-thirds of liquid water remain. In the present work the distribution of the ice portion in the droplet directly after the dendritic freezing phase as well as the evolution of the ice and temperature distribution has been investigated in dependence of the most relevant parameters as droplet diameter, dendritic freezing velocity (which correlates with the supercooling) and heat transfer coefficient to the surroundings (which correlates with the relative droplet velocity compared to the ambient air and with the droplet size). For this purpose on the experimental side acoustically levitated droplets in climate chambers have been investigated in combination with high-speed cameras. The obtained results have been used for finite element method (FEM) simulations of the dendritic freezing phase under consideration of the beginning second, much slower heat-transfer dominated freezing phase. A theoretical model covering 30 layers and 5 shells of the droplet has been developed which allows one to describe the evolution of both freezing phases at the same time. The simulated results are in good agreement with experimental as well as with calculated results exploiting the heat balance equation. The most striking result of this work is the critical radius of the droplet which describes the transition of one-stage freezing of the supercooled water droplet toward the thermodynamically forced dendritical two-stage freezing in which the droplet cannot sufficiently get rid of the formation heat anymore. Depending on the parameters named above this critical radius was found to be in the range of 0.1 to 1 μm by FEM simulation.

Thermodynamics of water condensation on a primary marine aerosol coated by surfactant organic molecules

A large subset of primary marine aerosols can be initially (immediately upon formation) treated using an "inverted micelle" model. We study the thermodynamics of heterogeneous water condensation on such a marine aerosol. Its hydrophobic organic coating can be processed by chemical reactions with atmospheric species; this enables the marine aerosol to serve as a nucleating center for water condensation. The most probable pathway of such "aging" involves atmospheric hydroxyl radicals that abstract hydrogen atoms from organic molecules coating the aerosol (first step), the resulting radicals being quickly oxidized by ubiquitous atmospheric oxygen molecules to produce surface-bound peroxyl radicals (second step). Taking these two reactions into account, we derive an expression for the free energy of formation of an aqueous droplet on a marine aerosol. The model is illustrated by numerical calculations. The results suggest that the formation of aqueous droplets on marine aerosols is most likely to occur via Köhler activation rather than via nucleation. The model allows one to determine the threshold parameters necessary for the Köhler activation of such aerosols. Numerical results also corroborate previous suggestions that one can omit some chemical species of aerosols (and other details of their chemical composition) in investigating aerosol effects on climate.