Bipolar Diffusion Charging of Aggregates

Abstract

In this paper, the effect of particle morphology on bipolar diffusion charging is studied. A modified tandem differential mobility analyzer (TDMA) method used to measure the charge distribution of submicron particles in the range of 70-300 nm is described in detail. The method requires an independent measurement of the neutral fraction, followed by the measurement of the size-dependent charge distribution, which requires the knowledge of the neutral fraction. The method was validated experimentally using dioctyl sebacate and ammonium sulfate spherical particles and compared with Fuchs' theory. Diesel particles and silver aggregates were used to evaluate the impact of morphology on charging. The results show that aggregates have a slightly lower (about 7%) neutral fraction than spheres. These results are in agreement with the predictions of Lall and Friedlander's theory and previous studies. However, results from charge distribution indicate that more (about 46%) particles are negatively charged than predicted by Lall and Friedlander's theory, while 32% fewer particles are positively charged. This relatively large asymmetry between the negative and the positive charge fraction is not fully predicted by either Fuchs' or aggregate charging theories. Our results suggest that the current inversion method of scanning mobility particle sizer (SMPS) data, based on Fuchs' or Lall and Friedlander's distribution, would underestimate the total number concentration by about 15% or 27% if applied to diesel aggregates.

Methodology to quantify the ratio of multiple-to single-charged fractions acquired in aerosol neutralizers

“…The experimental data from Maricq (2008) suggested that the regression fit by Wiedensohler (1988) reproduce the ratios of the positively double to single charged particles within §12% for neutralized soot particles produced in a premixed ethylene flame. On the other hand, the experimental results of Xiao et al (2012) for neutralized diesel soot particles, suggested that the ratios of double to single positively charged particles are underestimated by approximately 20% by the regression fit of Wiedensohler (1988). Table 2 compares the results determined for particles produced by the miniCAST (Soot #1 condition) when employing 85 Kr and X-ray sources in the second neutralizer.…”

“…Inaccuracies in the assumed charged fractions is one of the main contributors of the measurement uncertainties of such spectrometers (Liu and Deshler 2003) and can have a strong effect on the recovered number concentrations (Xiao et al 2012). Any progress in the understanding of charging mechanisms can help improve the quality of DMA size distribution measurements (Gopalakrishnan et al 2015), especially since experimental quantification of charging probabilities would be impractical in such applications.…”

The Effect of Particle Morphology on Unipolar Diffusion Charging of Silver Nanowires

“…Xiao et al (2012) found that compact diesel agglomerates with d m D 70-300 nm, which are more compact than the chain aggregates used in Lall and Friedlander's (2006) theory, can show the electrical charging behavior similar to spherical particles.…”

Rationale for Data Evaluation of the Size Distribution Measurements of Agglomerates and Aggregates in Gases with Extended SMPS-Technology

“…For correction the bipolar charge distribution of the agglomerates has to be known. This information is still limited (Rogak and Flagan, 1992;Maricq, 2008;Xiao et al, 2012). Lall et al (2006a, b) used a model for bipolar agglomerate charging developed by Wen et al (1984a, b) approximating agglomerate structure by long prolate spheroids.…”

“…To perform the correction for agglomerates/aggregates similar to what is done with spherical particles one needs the bipolar charge distributions of agglomerates/aggregates as function of mobility diameter for any kind of structure. Unfortunately the information about these distributions in form of measurements and models is limited (Wen et al, 1984a, b;Rogak and Flagan, 1992;Maricq, 2008;Xiao et al, 2012) To demonstrate the procedure and to discuss the importance of the correction we use the limited model for chain like agglomerates by Wen et al (1984a, b). With the known charge distribution Eq.…”