(206f) Sedimentation of Agglomerates Consisting of Polydisperse Nanoparticles

Anastasia Spyrogianni,¹ Katerina S. Karadima,^2,3 Eirini Goudeli,¹ Vlasis G. Mavrantzas^1-3 and Sotiris E. Pratsinis¹

¹Department of Mechanical and Process Engineering, ETH Zurich, Zurich, 8092, Switzerland

²Department of Chemical Engineering, University of Patras, Patras, 26504, Greece

³Institute of Chemical Engineering Sciences, FORTH/ICE-HT, Patras, 26504, Greece

The precipitation of agglomerates is of interest in water cleaning, pharmaceutical processing, nanotoxicology¹ and nanomedicine as well as for the stability of engineered nanofluids. Here, the mobility of nanosized fractal-like SiO₂ agglomerates in water is investigated and their settling rate in infinitely dilute suspensions is calculated by thoroughly validated Brownian dynamics (BD) tracking the agglomerate translational and rotational dynamics by a Cartesian vector formalism. Agglomerates are generated by an event-driven method² and have constant mass fractal dimension of 1.80 ± 0.01 but varying primary particle (PP) size distribution, mass and relative shape anisotropy. The calculated diffusion coefficient tensor is used to obtain their mobility diameter d_m, which is in excellent agreement with the d_m from scaling laws for fractal-like agglomerates. The agglomerate mobility to gyration diameter ratio, d_m/d_g, decreases with increasing relative shape anisotropy κ². For constant d_m, the agglomerate settling rate, u_s, increases with increasing PP diameter and geometric standard deviation σ_p,g (polydispersity). A linear relationship between u_s and agglomerate mass to mobility diameter ratio, m/d_m, is revealed leading to an analytical expression for calculation of u_s for agglomerates consisting of polydisperse PPs. The latter shows that the commonly-made assumption of monodisperse PPs underestimates u_s by a fraction depending on σ_p,g and the agglomerate mass mobility exponent. The simulations are in excellent agreement with precipitation rate measurements of commercial fumed SiO₂ agglomerates in water¹.

1. Spyrogianni, A.; Sotiriou, G. A.; Brambilla, D.; Leroux, J.-C.; Pratsinis, S. E. The effect of settling on cytotoxicity evaluation of SiO₂ nanoparticles. J. Aerosol Sci.108, 56–66 (2017).

2. Goudeli, E.; Eggersdorfer, M.L.; Pratsinis, S.E. Coagulation – Agglomeration of Fractal-like Particles: Structure and Self-Preserving Size Distribution, Langmuir, 31, 1320-1327 (2015).