AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Scaling Laws for Packing Density of Fractal Aggregates
PAI LIU, William Heinson, Rajan Chakrabarty, Washington University in St Louis
Abstract Number: 66 Working Group: Aerosol Physics
Abstract Fractal aggregates in nature grow with a scaling dimensionality less than the spatial dimension. This results in their packing density (theta$_f)–defined as actual volume occupied by solid subunits constituting an aggregate relative to total aggregate volume–decreasing with increasing size R$_g/a (aggregate radius of gyration normalized by average radius of monomers (repeating subunits)). Fundamental questions remain regarding the scaling laws and physical mechanisms controlling the evolution of theta$_f, especially after the onset of gelation. This is in part because of experimental challenges owing to the effects of gravity and thermal fluctuations on the formation and structural stability of aggregates in the large R$_g/a limit. Here, we experimentally map the scaling of theta$_f for aggregates–made of non-repulsive monomers–across five orders of magnitude of R$_g/a. Our experiments reveal three successive growth regimes, namely diffusion-limited cluster aggregation (DLCA) of monomers, percolation of aggregates, and DLCA of percolates, with distinct cross-over points occurring at R$_g/a ≈ 4 and 1×10$^3. Corresponding to these regimes, we show theta$_f to decrease in distinct power-law exponents of -1.3, -0.5, and -1.3, respectively. Our work, besides demonstrating the experimental realization of stable aggregation in very large R$_g/a limit, redefines the currently held scaling law for theta$_f of rigid aggregates, and has implications for synthesis of materials with superlative properties and accurate estimation of climate forcing by carbonaceous aerosols.