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.

Copyright 2012 American Association for Aerosol Research

Acknowledgments

There is further work to be done because effects of ion mobility, particle density, surface coating, and other properties on the bipolar diffusion charging distribution are unknown. Monte Carlo simulation techniques described by Biskos et al. (2004) may provide an alternative means for theoretically characterizing bipolar diffusion charging of aggregates.

Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. Prof. Chris Hogan provided valuable discussion, which was helpful in interpreting results. Dr. Winthrop Watts gave helpful comments on the manuscript. Zhun Liu and Lin Li, from the Particle Technology Lab at the University of Minnesota, assisted with the silver particle generation experiments.