On the Problem of Condensation-Induced Restructuring of Soot Agglomerates

G. Y. Gor [1], C. Chen [2,3], and A. F. Khalizov [1,2]

1 Department of Chemical, Biological, and Pharmaceutical Engineering,
New Jersey Institute of Technology, Newark, New Jersey 07102, USA

2 Department of Chemistry and Environmental Science,
New Jersey Institute of Technology, Newark, New Jersey 07102, USA

3 Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control,
Nanjing University of Information Science & Technology, Nanjing 210044, China


Soot is a major environmental pollutant which deteriorates air quality and affects human health. Moreover, soot nanoparticles serve
as condensation nuclei for atmospheric aerosols, light absorbers and scatterers, impacting the climate [1,2]. Soot nanoparticles are
fractal agglomerates of graphitic spheres mixed with organic or inorganic products of combustion. Both the optical properties of soot
in the atmosphere and the transport properties of soot in human respiratory tract strongly depend on the morphology of soot
agglomerates [3,4]. Therefore, in order to assess the negative impacts of soot on both climate and human health one has to know the
microstructure of soot nanoparticles.

After their formation, the soot agglomerates are exposed to other vapors that condense on the agglomerates surface. In some cases vapor
condensation does not affect the soot morphology, however, often it induces restructuring of soot agglomerates, so that they collapse
into globules [5,6]. While some studies suggest that the tendency to restructure depends on the surface tension of condensing liquid [7],
other experiments show that even solid coatings can cause restructuring [8]. A theory of this process is still lacking.

Restructuring of soot agglomerates involves two aspects: mechanics of the agglomerates and kinetics of vapor condensation on agglomerates
surface. Here we focus on the latter. When considering vapor condensation on agglomerate of spheres, there are two distinct regimes of
condensation: liquid can form a film on a surface of the spheres or it can fill the gaps between the spheres ("capillary condensation").
We hypothesize that two regimes of condensation correspond to the two scenarios in mechanical evolution: when vapor condenses uniformly,
it is unlikely to induce significant mechanical stresses and cause restructuring; but when it condenses in the gaps, the capillary forces
at the joints of the spheres can collapse of the agglomerate.

The kinetics of vapor condensation on a spherical surface is well-known [9]. Here we describe the condensation of a vapor into a pendular
ring in the gap between the adjacent spheres. We found an analytical expression for the amount of condensed liquid as a function of time.
This result allowed us to find a non-dimensional parameter which value determines realization of one of the two condensation/restructuring
scenarios. Our model not only supports well our experimental observations, but also can be used to rationalize most of the published experimental
results from other groups [10-12].

References:

1. Bond, T. C., Doherty, S. J., Fahey, D., Forster, P., Berntsen, T., DeAngelo, B. J., Flanner, M. G., Ghan, S., Karcher, B., Koch, D., et al. (2013).
Bounding the role of black carbon in the climate system: A scientific assessment. J. Geophys. Res. Atmos., 118(11):5380–5552

2. Shiraiwa, M., Selzle, K., and Poschl, U. (2012). Hazardous components and health effects of atmospheric aerosol particles: reactive oxygen species,
soot, polycyclic aromatic compounds and allergenic proteins. Free Radical Res., 46(8):927–939.

3. 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

4. Broday, D. M. and Rosenzweig, R. (2011). Deposition of fractal-like soot aggregates in the human respiratory tract. J. Aerosol Sci., 42(6):372–386.

5. Weingartner, E., Baltensperger, U., and Burtscher, H. (1995). Growth and structural change of combustion aerosols at high relative humidity. Environ.
Sci. Technol., 29(12):2982–2986.

6. Mikhailov, E. F., Vlasenko, S. S., Kiselev, A. A., and Ryshkevich, T. I. (1998). Restructuring factors of soot particles. Ivz. Atmos. Ocean. Phys.,
34:307–317. 7. Schnitzler, E. G.; Gac, J. M. and Jäger, W. (2017) Coating surface tension dependence of soot aggregate restructuring J. Aerosol Sci.,
106:43-55

8. 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).

9. Seinfeld, J. H. and Pandis, S. N. (2006). Atmospheric chemistry and physics: From air pollution to climate change. John Wiley & Sons, 2nd edition.

10. Ma, X.; Zangmeister, C. D.; Gigault, J.; Mulholland, G. W. and Zachariah, M. R., Soot aggregate restructuring during water processing. (2013)
J. Aerosol Sci. 66:209-219. 11. Lu, Z. F.; Hao, J. M.; Hu, L. H. and Takekawa, H., The compaction of soot particles generated by spark discharge
in the propene ozonolysis system. (2008) J. Aerosol Sci., 39 (10):897-903.

12. Miljevic, B.; Surawski, N. C.; Bostrom, T. and Ristovski, Z. D., Restructuring of carbonaceous particles upon exposure to organic and water
vapours. (2012) J. Aerosol Sci., 47:48-57.