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Dependence of Aggregate Strength, Structure, and Light Scattering Properties on Primary Particle Size under Turbulent Conditions in Stirred Tank

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journal contribution
posted on 02.04.2020, 16:17 by Lyonel Ehrl, Miroslav Soos, Massimo Morbidelli
The steady-state size and structure of aggregates produced under turbulent conditions in stirred tank, for primary particle diameter, dp, equal to 420 nm and 120 nm, were studied experimentally for various values of the volume average shear rate, 〈G〉, and solid volume fraction, φ, and compared with data for dp = 810 nm. To exclusively investigate the effect of dp, polystyrene latexes with same type and similar density of surface charge groups (sulfate) were used. The mass fractal dimension, df, obtained by image analysis, was found to be invariant of dp and 〈G〉, with a value equal to 2.64 ± 0.18. Small-angle static light scattering was used to characterize the cluster mass distributions by means of the root-mean-square radius of gyration, 〈Rg〉, and the zero-angle intensity of scattered light, I(0), whose steady-state values proved to be fully reversible with respect to 〈G〉. The absolute values of 〈Rg〉 obtained for similar φ and 〈G〉 proved to be independent of dp, and for all studied conditions, 〈Rg〉 was proportional to 〈G-1/2. At very low φ, a critical aggregate size for breakage was obtained and used to evaluate the aggregate cohesive force, as a characteristic for the aggregate strength. The aggregate cohesive force was found to be independent of aggregate size, with similar values for the investigated dp. Due to large dp and high df, the effect of multiple light scattering within the aggregates was found to be present, and by relating the scaling of 〈Rg〉 with I(0) to df, the corresponding correction factors were evaluated. By combination of the independently measured aggregate size and structure, it is possible to experimentally determine the relation between the maximum stable aggregate mass and the hydrodynamic stresses independent of the multiple light scattering present for large dp and compact aggregates.