(494e) The Competition between Surface and Capillary Condensation of Vapors on Soot Aggregates

Soot is major pollutant and climate forcer [1]. The optical properties of soot that determine its impact on the climate strongly depend on its morphology [2]. When initially generated, soot nanoparticles are branched fractal aggregates of primary carbon spheres. During aging in the atmosphere, soot aggregates often collapse into compact globules, with restructuring driven by a condensate of atmospheric vapors on the aggregate surface. Although there have been several studies of this restructuring effect [3-5], a model which could predict whether the agglomerates will collapse or not is still lacking.

Analysis of the recent laboratory experiments suggested that the spatial distribution of condensate on the soot agglomerates may determine its fate with respect to restructuring [6]. When a vapor condenses in the junction between the spheres, the capillary and disjoining forces are likely to generate mechanical stresses sufficient for plastic deformation of an agglomerate. Therefore, the agglomerate restructuring is directly related to the domination of one of the two scenarios of vapor condensation: if uniform condensation on the surface of primary carbon spheres dominates, the aggregate backbone will remain fractal; if condensation in the junctions between the primary spheres ("capillary condensation") dominates the aggregate will likely collapse.

Here we propose a model describing the vapor condensation on a soot aggregate, consisting of two spherical carbon particles, to elucidate the conditions at which one of the two condensation scenarios is realized. Since the rate of capillary condensation of vapor strongly depends on the curvature of the liquid-vapor interface, we explore three different approximations for the geometry of the droplet forming in the junction, assuming that it has the interface of a cylinder, a globoid, and a catenoid. The former two give the closed-form solutions for the amount of vapor condensed as a function of time, but the latter, which is the most rigorous, does not. However, we show that the difference between the three interfaces is relatively small, and thus, simpler analytical models can be used for semi-quantitative predictions of the vapor condensation on soot and subsequent soot aggregate restructuring.

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2. Xue, H., Khalizov, A. F., Wang, L., Zheng, J., and Zhang, R. (2009). Effects of dicarboxylic acid coating on the optical properties of soot. Phys. Chem. Chem. Phys., 11(36):7869–7875
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5. Schnitzler, E. G.; Gac, J. M. and Jäger, W. (2017) Coating surface tension dependence of soot aggregate restructuring J. Aerosol Sci., 106:43-55
6. Chen, C., Fan, X., Shaltout, T., Qiu, C., Ma, Y., Goldman, A., and Khalizov, A. F. (2016). An unexpected restructuring of combustion soot aggregates by subnanometer coatings of polycyclic aromatic hydrocarbons. Geophys. Res. Lett., 43(20).