No abstract

We carry out molecular Monte Carlo simulations of clusters in an imperfect vapor. We show that down to very small cluster sizes, classical nucleation theory built on the liquid drop model can be used very accurately to describe the work required to add a monomer to the cluster. However, the error made in modeling the smallest of clusters as liquid drops results in an erroneous absolute value for the cluster work of formation throughout the size range. We calculate factors needed to correct the cluster formation work given by the liquid drop model. The corrected work of formation results in nucleation rates in good agreement with recent nucleation experiments on argon and water.

Using the classical nucleation theory corrected with line tension and experimental data of heterogeneous nucleation of n-nonane, n-propanol, and their mixture on silver particles of three different sizes, the authors were able to estimate the line tensions and the microscopic contact angles for the above mentioned systems. To do this they applied generalized Young's equation for the line tension and calculated the interfacial tensions using Li and Neumann's equation [Adv. Colloid Interface Sci. 39, 299 (1992)]. It has been found that, for both unary and binary systems, the line tension is negative and the resulting microscopic contact angle derived from experimental nucleation data is most of the time larger than the macroscopic one. This is in contrast to earlier studies where the influence of line tension has not been accounted for. The values of the three phase contact line tension obtained in this way are of the same order of magnitude as the estimations for other systems reported in literature. The line tension effect also decreases considerably the nucleation barrier.

[1] We developed models for unary nucleation of water and carbon dioxide in the Martian atmosphere. Both homogeneous and heterogeneous nucleation on dust particles were studied. Our models are based on classical theory. We compare results of different adsorption approaches. Heterogeneous nucleation on the abundant dust particles seems to be the primary mechanism of both H 2 O and CO 2 cloud formation in the Martian atmosphere. Heterogeneous nucleation is obtained at a saturation ratio of about 1.18 for H 2 O and 1.32 for CO 2 . Homogeneous nucleation is not likely to occur since it would require high supersaturations. We use our models to study nucleation as a function of height at different locations on Mars where ice fog or clouds have been observed. H 2 O ice nucleation results are in good agreement with surface fog observations and previous model studies. CO 2 ice nucleation simulations in the polar hood cloud areas suggest that negative temperature perturbations caused by, e.g., adiabatic cooling in orographic waves or in convective plumes are required for the formation of CO 2 clouds.

Nucleation probabilities for binary heterogeneous nucleation have been measured quantitatively. Heterogeneous nucleation of binary n-propanol-water vapor mixtures on partially soluble 6.9-nm NaCl particles and on nonsoluble oxidized and nonoxidized 8-nm Ag particles at a constant nucleation temperature of 288 K (NaCl) or 285 K (Ag) has been investigated experimentally and theoretically. An expansion chamber was applied to generate supersaturated vapor mixtures. Number concentrations of particles activated to condensational growth were determined optically for various vapor phase activities at constant temperature. Nucleation probabilities have been measured as functions of the vapor phase activities for several n-propanol-water vapor mixing ratios. Beyond certain vapor phase activities a sharp onset of heterogeneous nucleation was observed both for NaCl and for Ag particles. For insoluble particle surfaces the theoretical slopes of the nucleation probability versus vapor phase activity curves were found to be somewhat steeper as compared to the experiment. On the other hand, for soluble particles the experimental slopes were found to be in satisfactory agreement with theory. The onset of the nucleation process is defined at an activity value where 50% of the particles were activated to condensational growth. Onset activities were obtained for various mixing ratios of the binary vapor mixtures. In the case of NaCl nuclei for n-propanol rich mixtures, the heterogeneous nucleation calculations based on a thermodynamically consistent version of Fletcher theory and an experimentally determined contact angle provide a reasonable approximation of the experimental data. However, it appears that the Fletcher theory is not applicable in the region of transition from n-propanol rich to water rich mixtures. Based on the Köhler theory of activation of soluble particles, a theory was formulated accounting for the presence of two condensable vapors and limited solubility of NaCl in water-propanol liquid mixtures. This approach provides a satisfactory description of the transition from activation of soluble particles to nucleation of vapors on insoluble particles. In the case of silver particles Fletcher theory with macroscopic experimental contact angles as well as with contact angle zero results in strong deviations from experimental data. A significant improvement was achieved by introducing fit contact angles, which are substantially lower than the macroscopic experimental values and may be considered as approximate values of the microscopic contact angles.