EECE Seminar - Rajan Chakrabarty

Sep 27, 2019
11:00 AM
12:00 PM
Brauer Hall, room 12

Dr. Rajan Chakrabarty, Assistant Professor
Complex Aerosol Systems Research Laboratory
Department of Energy, Environmental & Chemical Engineering
Washington University in St. Louis

Non-Equilibrium Aerosol Dynamics across Length Scales: Addressing a Few Contemporary Challenges

Abstract: Non-equilibrium systems in nature, such as aerosol and colloids, are subject to the fundamental kinetic processes of diffusion, collision, aggregation and fragmentation. These processes give rise to complex structures and growth mechanisms that impact diverse research fields ranging from climate change to material synthesis. From a statistical perspective, the large size of such a system often plays an advantageous role in exhibiting emergent behaviors leading to simple, scale-invariant collective properties. Unlike the well-developed tools of equilibrium statistical physics, the statistical kinetics description of systems that are out of equilibrium is less mature. This talk will elucidate recent findings on improving our understanding of growth mechanisms and resulting microphysical properties of aerosol spanning across several orders of magnitude in dimensional space. In the sub-micron size range, I will present a scaling-based approach to solving two complex problems involving black and brown carbon aerosol that dynamically change their light-matter interaction behavior with variations in morphology and composition due to atmospheric processing. Resulting power-law expressions hold promise for accurately and inexpensively parameterizing the non-equilibrium process of carbonaceous aerosol light absorption in climate models and satellite retrieval algorithms.
Beyond the sub-micron length scale, aerosol has been observed to form gel-like structures. Recent studies have shown percolated networks of sub-micron aggregates to be emitted into the atmosphere from large-scale wild fires and oil spill burns. Toward explaining the formation of these structures, we systematically studied the sol-to-gel dynamics of a gelling system using (i) a buoyancy-opposed flame (BOF) aerosol reactor, which facilitates the trapping of nanoparticles for substantial amount of time, leading to their cross-over from sub-micron-size aggregates to millimeter-size gels, and (ii) off-lattice diffusion limited cluster-cluster aggregation under high monomer-volume-fraction conditions. We provided a quantitative description of the kinetics of aerosol gelation, that is, time evolution of the decrease in the total number of sol phase particles and the increase in average mass of the gel phase particles. Next, using directional statistics and scaling law relation between the mean-squared displacement and time, we proposed a generalized analytical solution to interpret the rate of transition from ballistic to diffusive motion during sol-to-gel formation. I will conclude this talk by demonstrating our BOF reactor as an enabling technology for scalable production of aerogel materials. As a proof-of-concept of this technology, we synthesized TiO2 gels with tunable crystal phase and monomer size.