Filtration behavior of silver nanoparticle agglomerates and effects of the agglomerate model in data analysis

Abstract

In many data evaluation procedures for particle measuring devices and in filtration models, spherical particles are assumed. However, significant fractions of aerosol particles are agglomerates of small primary spheres. The morphology of particles in filtration processes may not be known a priori and if the filtration data are processed with wrong assumption, errors can be induced. In this work, we have quantified such errors for the case of open-structured agglomerates. Filtration efficiency tests with polydisperse silver nanoparticle agglomerates and their sintered spheres were performed. After the sintering process, particles with a compact structure with the shape close to a sphere are obtained, which are referred to as sintered spheres in the present study. The testing method involved generation of particulate forms, passing the particles through the testing section, and measurement of the particle number concentrations and size distributions before and after the filter. Measurements of the aerosols upstream and downstream of the filter were conducted using scanning mobility particle sizers (SMPS, TSI Inc.), which covered the rage from 10 to 480 nm. Particles were additionally characterized from the electron microscopic images and the average primary particle size was determined to be 16.8 nm. The number-size distribution curves were obtained and used for penetration calculation. The penetration was dependent on the particle size and morphology. Silversintered spheres were captured with a lower efficiency than agglomerates with the same mobility diameter because of the stronger interception effect for agglomerates. Data analysis of the number-size distribution for agglomerates was processed based on sphere assumption and using the model for openstructured agglomerates developed by Lall and Friedlander. The efficiencies based on total concentrations of number, surface and volume were affected when the agglomerate model was used. The effect was weakest for the total number efficiency and strongest for the total volume efficiency. Filtration efficiency curves for agglomerates were additionally plotted as a function of the volume equivalent diameter or number of primary particles per agglomerate, because the latter parameters provide more information for the agglomerate structure.

Toward standardized test methods to determine the effectiveness of filtration media against airborne nanoparticles

“…The drag force on the particle can be approximated by that acting on the particle moving in a straight line, and then the movement of the particle can be obtained. Brown (1993) used this perturbation approach and obtained the following expression for the efficiency due to inertial impaction E I at high Stokes' numbers…”

“…When the interception efficiency is finite and the Stokes' number is small, the single-fiber efficiency can be computed based on the assumption that the particle trajectory deviates slightly from the gas streamline. Stechkina et al (1969) gave the following expression for particles with low Stokes' numbers E I ¼ 1 ð2KuÞ 2 ½ð29:6 À 28a 0:62 ÞR 2 À 27:5R 2:8 Stk for R\0:4: ð13Þ Brown (1993) summarized results for inertial impaction from calculations and experiments. The values of the efficiency due to inertial impaction spread out in rather wide ranges at small Stokes' numbers (Figs.…”

“…The electret filters, with intentionally electrically charged fibers, take advantage of the electrostatic attraction to improve the filtration efficiency, without affecting the flow resistance. Brown (1993) discussed the effect of charge amount and configuration on filtration. The greater the amount of charge on the filter media fibers, the greater will be the electrical field, and the higher the filtration efficiency by electrostatic attraction.…”

“…Hence, the electrical charge must have a spatial variation not much smaller than fiber or inter-fiber dimensions. Brown (1993) analyzed the single-fiber efficiency for fibers with uniform charge distributions and twodimensional charge distributions. In the case of a fiber carrying a uniform charge Q per unit length, the nondimensional parameter governing the capture due to coulombic force is…”

“…Filtration has been extensively studied experimentally and theoretically; models for clean fibrous filter media are well developed and systematically documented by Brown (1993), Hinds (1999), and Lee and Mukund (2001). Filtration of nanoparticles, due to its emerging importance and impact on environment and health protection, has attracted voluminous research in recent years.…”