(6di) Flame-Made Nanoparticles: Morphology, Optical Properties and Climate Impact

Combustion-generated nanoparticles during material-synthesis, energy generation (e.g. for transportation and power generation) as well as open fires form fractal-like structures. Yet, their optical properties are calculated typically by Rayleigh or Mie theories for spheres impeding accurate estimation of their environmental impact, characterization by optical diagnostics and selective sensing by fire detectors. For example, the direct radiative forcing of soot given by global climate models (90 % uncertainty) is 0.44 - 1.32 W/m². As soot constitutes 19 - 58 % of the total anthropogenic forcing, there is a clear need for an accurate estimation of its contribution. Similarly, fire detectors are not selective enough to soot light scattering and thus produce false alarms costing up to 1 billion £/y in United Kingdom alone. This motivates research for understanding the evolution of nanoparticle morphology and optical properties during flame synthesis and processing.

Discrete element modeling (DEM) for soot dynamics by agglomeration and surface growth¹ describes a realistic pathway for the evolution from nascent spheres to mature soot agglomerates. So, right after inception by reactive dimerization,² soot nuclei coagulate into small clusters of primary particles in point contact (i.e. agglomerates). However, surface growth by acetylene reactions forms covalent bonds between soot primary particles, compacting them into hard aggregates.¹ The size distribution and morphology of these nascent soot aggregates found excellent agreement with data from different combustion groups, explaining several experimental observations made in the last 15 years. This DEM is also extended to account for coagulation of nascent single and aggregated primary particles into mature soot agglomerates after the complete conversion of gaseous hydrocarbons.³ This allowed the distinction of nascent and mature soot morphology with simple and easy-to-use scaling laws, finding good agreement with both premixed and diffusion flame data.

The DEM is then coupled with discrete dipole approximation (DDA) to simulate the evolution from nascent to mature soot light absorption by agglomeration and surface growth.⁴ These robust DEM-DDA simulations found excellent agreement with laser induced incandescence⁴ and light scattering data.⁵ In contrast, using Rayleigh Debye Gans (RDG) theory for soot agglomerates consisting of monodisperse and non-aggregated primary particles underestimates soot light scattering up to 60 %.⁵ Coupling RDG theory with a newly-derived scaling law accounting for primary particle aggregation and polydispersity³ results in good agreement with the soot scattering cross sections estimated by more accurate but laborious models. Thus, the revised RDG theory can be used to estimate climate forcing and sharpen optical diagnostics for soot characterization. For example, accounting for the detailed soot morphology yields an average direct radiative forcing of 0.63 ± 0.05 W/m² in a cloud-free sky.⁶ This is in between the predictions of current global climate models for cloudy skies (0.44 - 1.32 W/m²) using Mie theory for spheres. Such results indicate that the soot contribution on global warming might be overpredicted substantially by these models that need to account for soot structure and polydispersity.

The nanoparticle structure is also essential for the performance of inorganic materials in different applications, including biosensors.⁷ The morphology of flame-made nanoparticle agglomerates changes drastically during atmospheric aging, storage or industrial processing by granulation, spray drying or fluidization in the presence of humidity. So, the impact of humidity on the structure of inorganic nanoparticle agglomerates is quantified experimentally for the first time.⁸ This research showed that ramified agglomerates of flame-made silica primary particles and aggregates restructure to smaller and compact structures by water condensation-evaporation, similarly to what had been observed for decades only for soot. Most importantly, it was found that agglomerate compaction by condensation-evaporation depends on the size of their constituent aggregates that is determined in the early stages of flame synthesis.⁸

In sum, the development of detailed models for nanoparticle dynamics and their thorough validation with experiments provides insight into the morphology and optical properties of soot and other nanomaterials during flame synthesis, atmospheric aging and industrial processing. This facilitates the mitigation of particulate emissions, control of their environmental impact and detection by robust fire sensors, as well as the design and optimization of nanomaterial performance in different applications.

References

[1] Kelesidis, G.A., Goudeli, E., and Pratsinis, S.E. “Flame synthesis of functional nanostructured materials and devices: Surface growth and aggregation” (2017) Proc Combust Inst 36, 29-50.

[2] Kholghy, M.R., Kelesidis, G.A., and Pratsinis, S.E. “Reactive polycyclic aromatic hydrocarbon dimerization drives soot nucleation” (2018) Phys Chem Chem Phys 20, 10926-10938.

[3] Kelesidis, G.A., Goudeli, E., and Pratsinis, S.E. “Morphology and mobility diameter of carbonaceous aerosols during agglomeration and surface growth” (2017) Carbon 121, 527-535.

[4] Kelesidis, G.A., and Pratsinis, S.E. “Soot light absorption during agglomeration and surface growth” (2018) Proc Combust Inst 37, under review.

[5] Kelesidis, G.A., Kholghy, M.R., Zurcher, J., Robertz, J., Allemann, M., Duric, A., and Pratsinis, S.E. “Light scattering from nanoparticle agglomerates” (2018) Powder Technol, submitted.

[6] Kelesidis, G.A., Kholghy, M.R., and Pratsinis, S.E. “Soot morphology and composition dominate radiative forcing” (2018) Environ Sci Technol, in preparation.

[7] Pratsinis, A., Kelesidis, G.A., Zuercher, S., Krumeich, F., Bolisetty, S., Mezzenga, R., Leroux, J.C., and Sotiriou, G.A. “Enzyme-Mimetic Antioxidant Luminescent Nanoparticles for Highly Sensitive Hydrogen Peroxide Biosensing” (2017) ACS Nano 11, 12210-12218.

[8] Kelesidis, G.A., Furrer, F.M., Wegner, K., and Pratsinis, S.E. “Impact of humidity on silica nanoparticle agglomerate morphology and size distribution” (2018) Langmuir, accepted.

Research Interests:

Accurate estimation of nanoparticle climate impact using detailed models for their morphology and optical properties.

Design of robust fire detectors with high selectivity to smoke and limited false alarms.

Control of flame-made nanoparticle structure, light scattering and absorption for optimization of their performance in different applications, including biomedicine.

Teaching Interests:

• Engineering courses, including "Heat and Mass Transfer", "Fluid Mechanics", "Thermodynamics" and "Numerical Analysis".
• MSc level classes, such as "Aerosol Science and Technology" and "Computational Chemistry and Physics".