(77f) Coagulation of Agglomerates with Polydisperse Primary Particles

Agglomeration of nanoparticles is encountered in both atmospheric and industrial processes as in volcanic plumes and aerosol manufacture of carbon black or fumed silica. Even though the dynamics of coagulating spherical particles, such as self-preserving size distribution (SPSD) and coagulation rate are reasonably well-understood, there is significant uncertainty for fractal-like agglomerates (Friedlander, 2000). For the latter, coagulation rates have been proposed (Mountain et al., 1986; Thajudeen et al., 2012), their mobility (Sorensen, 2011) and SPSDs have been determined (Vemury and Pratsinis, 1995) and even the time needed to reach their asymptotic structure has been estimated (Goudeli et al., 2015). All these have been confined to agglomerates with monodisperse PPs. Realistic agglomerates, however, consist of polydisperse primary particles (PPs). So little is known for the effect of constituent PP polydispersity on agglomerate structure and, most importantly, coagulation dynamics.

Here coagulation of nanoparticles of varying PP polydispersity (s_g,PP = 1 â?? 3) in the absence of coalescence, sintering or surface growth is investigated by a discrete element method (DEM) in the free molecular regime (Goudeli et al., 2015). This is an early stage of particle formation especially at high temperatures followed typically by rapid quenching to facilitate collection of particles and retention of their ramified structure as in manufacture of fumed SiO₂, Ni or TiO₂. As a result, particle dynamics at this stage frequently dominate the end product characteristics. The effect of PP polydispersity on agglomerate size (radius of gyration, mobility radius and volume-equivalent radius), morphology (fractal dimension, D_f, mass mobility exponent, D_fm, and their prefactors) as well as on the attainment of the well-known asymptotic fractal-like structure (D_f = 1.91 and D_fm= 2.15) and SPSD is investigated. Increasing the polydispersity of the constituent PPs from s_g,PP = 1 to 3 does not affect but only delays the attainment of the asymptotic D_f, D_fm and SPSD of the resulting agglomerates. The crossover agglomerate size and critical number of PPs per agglomerate that mark the transition between D_f= 3 and 1.91 scaling are obtained by various ways and increase with PP polydispersity. Only clusters larger than the crossover size should be considered for drawing conclusions on the formation mechanism as well as their structure. Furthermore, the effect of PP polydispersity on agglomerate dynamics (coagulation rate and polydispersity) is elucidated quantitatively also.

Such characteristics affect the environmental impact of agglomerates (climate forcing or visibility impairment by soot) or performance of gas sensors, catalysts and even biomaterials and nutritional products.

Keywords:coagulation rate, self-preserving distribution, kinetic gas theory, crossover to fractal regime, event-driven algorithm, log-normal primary particle size distribution.

References:

Friedlander, S. K., Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics. 2nd ed.; Oxford University Press: New York, 2000.

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

Mountain, R. D., Mulholland, G. W. and Baum, H. (1986). Simulation of aerosol agglomeration in the free molecular and continuum flow regimes. J. Colloid Interface Sci.114, (1), 67-81.

Sorensen, C. M. (2011). The mobility of fractal aggregates: a review. Aerosol Science and Technology45: 765-779.

Thajudeen, T., Gopalakrishnan, R., Hogan, C. J., Jr. (2012). The collision rate of non-spherical particles and aggregates for all diffusive Knudsen numbers. Aerosol Sci. Technol., 46: 1174-1186.

Vemury, S., Pratsinis, S.E. (1995). Self-preserving size distributions of agglomerates. J. Aerosol Sci., 26: 175-185.