Bipolar diffusion charging of high-aspect ratio aerosols

Recent studies have raised concerns over applicability of the conventional charging theories to non-spherical particles such as soot aggregates and single-walled carbon nanotube aerosols of complex shape and morphology. It is expected that the role of particle structure and shape on particle diffusion charging characteristics may be significant in the submicron size range for carbon nanotubes (CNTs) and nanofibers (CNFs). In this study, we report experimental data on equilibrium charging characteristics of high-aspect ratio aerosol particles such as CNFs and multi-walled CNTs (MWCNTs) when exposed to a bipolar ion atmosphere. A neutral fraction was measured, i.e., the fraction of particles carrying no electrical charge. A differential mobility analyzer (DMA) was used to classify aerosols, leaving a bipolar radioactive charger to infer the bipolar charging characteristics at different mobility diameters in the submicron size range. The measured neutral fractions for CNF aerosol particles were lower than the corresponding Boltzmann values by 24.4%, 42.0%, and 45.8% for mobility diameters of 400 nm, 600 nm, and 700 nm, respectively, while the neutral fractions for measured aerodynamic diameters of 221 nm, 242 nm, and 254 nm were much lower than those expected by Boltzmann charge distribution, by 43.8%, 63.1%, and 67.3%, respectively. Neutral fractions of spherical particles of polystyrene latex (PSL) and diethylhexyl sebacate (DEHS) particles, measured under identical experimental conditions and procedure, agreed well with the Boltzmann charge distribution. The measured neutral fractions for MWCNT aerosol particles were lower than the corresponding Boltzmann values by 22.3%–25.0% for mobility diameters in the size range from 279 nm to 594 nm. Charging-equivalent diameters of CNF particles correlated well with either mobility diameter or equal-area diameter, which were found to be larger than their mobility or equal-area diameters by up to a factor of 5 in the size range of 400 nm–700 nm, while those of MWCNT particles were larger than the corresponding diameters by a factor of 2 in the size range of 279 nm–594 nm.