Differences in Diffusion Charging of Dielectric and Conducting Ultrafine Aerosols

The effect of the dielectric constant on unipolar and bipolar charging rates is investigated. This effect is manifested through the image potential. Bulk dielectric constants of aerosol materials range from 2.5 to 50. The corresponding image potentials range from about 40% to about 96% of the image potential for a perfect conductor. Charging simulations were carried out for this parameter range and typical results are presented. For dielectric constants greater than 2.5 the charging behavior shows a strong image potential effect, and may be approximated by calculations for a perfect conductor. This suggests that charging models that do not allow for image potential effects are likely to be inaccurate except, perhaps, for larger particles, where the Coulomb potential dominates the image potential. The simulations are compared with unipolar charging data from recent experiments, which tend also to support this conclusion.