10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Experimental Verification of Transition Regime Aggregation Theories
Xiaoshuang Chen, Souvik Ghosh, David Buckley, R. Mohan Sankaran, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 255 Working Group: Aerosol Physics
Abstract Aggregation in the gas phase is frequently described via either diffusion limited cluster aggregation theory (for which the diffusive Knudsen number is 0) or ballistic cluster aggregation theory(for which the diffusive Knudsen number is infinite). However, in most nanomaterial synthesis systems, the Knudsen number is not a fixed parameter; instead, is there is an evolving Knudsen number distribution function for an evolving distribution of aggregates. We have used a combination of TEM based image analysis and differential mobility analysis-aerosol particle mass analysis (with correct inversion of the two dimensional size-mass distribution function) to examine the relationship between scaling hydrodynamic radius and projected area with the number of primary particles per aggregates for aggregates synthesized via the decomposition of nickelocene in a microplasma reactor. The aggregates formed have mobility equivalent sizes in the 50-250 nm range. Scaling relationships for the hydrodynamic radius and projected area are compared to predictions based upon the result of Langevin dynamics simulations, in which the evolution of Knudsen number distribution is appropriately accounted for. Measurement inferred hydrodynamic radii and projected areas are in excellent agreement (typically within 5%) with scaling relationship predictions, suggesting these relationships can be used to infer the transport properties of aggregates in a wide range of gas phase systems. Interestingly, the scaling relationships appear to hold despite the inferred fractal dimensions of individual aggregates varying considerably (from 1.6-2.2). Furthermore, in both experiments and simulations, for aggregates of a given mass, the variations in hydrodynamic radii and projected area equivalent radii are smaller than the variation in radius of gyration. Results hence suggest caution should be exercised when employing mobility/drag based measurements to infer aggregate geometric properties (e.g. fractal dimension, radius of gyration), and that commonly inferred parameters such as the mass-mobility exponent are not strong indicators of aggregate morphology.