Date of Award
Doctor of Philosophy (PhD)
Aggregation of nanoparticles in aerosols is a fundamental phenomenon with important implications to diverse fields ranging from material synthesis to pollutant control. The past few decades have witnessed extensive research on investigating the structure and growth mechanism of aerosol aggregates with sizes spanning across several orders of magnitude. This dissertation focuses on some contemporary problems that remain unaddressed in this topical area. Aerosol aggregates in sub-micron regimes, which are formed via the irreversible collision and aggregation of solid nanoparticle monomers, are fractal-like in their morphology. A mathematical description of this seemingly random structure dates to the seminal works by Forest and Witten (1979). In their work, the aggregate mass and characteristic length were related with a power-law relationship parameterized with a fractal dimension (Df), which quantifies aggregates’ dimensionality, and a prefactor (kf), which is recently shown to be related to their shape anisotropy. With the advent of mass-based aerosol particle classifiers, aggregates morphology can be alternatively characterized with a power-law relationship connecting their mass and mobility diameter, which is parameterized with a pair of mass-mobility exponent (Dfm) and prefactor (kfm). Knowledge of the exact empirical relationships between these pairs of parameters (Df - Dfm and kf - kfm) is essential for accurate characterization of aggregate physical properties. In this dissertation, comprehensive empirical relationships were established between these parameters for aggregates produced with a diffusion-limited cluster-cluster aggregation (DLCA) mechanism. The influence of aggregates’ shape anisotropy on their mass-mobility relationship was evaluated using the concept of apparent monomer screening.Prolonged aggregation leads to the phenomenon of gelation at a micrometer level, in which the submicron DLCA aggregates with a characteristic Df ≈ 1.8 jam together to form volume spanning gels with a characteristic Df ≈ 2.5. These aerosol gel particles, sometimes called superaggregates, have been observed in laboratory-scale diffusion flames, as well as in the naturally occurring large-scale combustions such as wildfires. Toward explaining the morphology and growth mechanism of superaggregates, Sorensen and Chakrabarti (2011) established the theoretical framework of aerosol gelation, which details the dynamic process by which gels are produced from their precursor sols. Part of this dissertation focuses on investigating the kinetics of aerosol gelation with emphasis placed on the previously understudied late-stage regimes in which the mean-field Smoluchowski Equation fails. This late stage kinetics of gelation was probed using a high temporal resolution Monte Carlo DLCA simulation, and system independent kinetic formulations were established along with improved parametrization on the characteristic gelation times.The morphology and growth mechanism of aerosol gels in the super-micron regime can be largely system dependent and poorly understudied. Part of this dissertation studies the growth of soot gel particles toward millimeter size in a novel buoyancy-opposed flame (BOF) aerosol reactor. Characterizations on the packing density of these particles revealed an inflection in their fractal scaling law within the super-micron regime, parameterized with a decrease of Df from 2.5 to 1.7. A late-stage growth mechanism, which involves the cluster-cluster aggregation of monomeric gel particles, was introduced to account for the reappearance of the small Df values in super-micron regime.Lastly, our BOF reactor could be harnessed as an enabling technology for scalable production of gel materials. As a proof-of-concept of this technology, we performed flame synthesis of titanium dioxide (TiO2) aerosol gels using a methane-oxygen BOF reactor with titanium tetraisopropoxide precursor. The in-flame aerosol trapping effect was reproduced in the BOF reactor at a variety of operating conditions. Control of flame temperature was established in the range between c.a. 600 and 1300 °C with the application of nitrogen dilution at variable flow rates. Control of the morphology and crystal phase of the TiO2 was achieved by exploiting the dependencies of monomer sintering and crystal phase transformation on temperature.
Rajan K. Chakrabarty
Richard L. Axelbaum, Pratim Biswas, Milorad P. Dudukovic, Fangqiong Ling,