Unipolar Field and Diffusion Charging in the Transition Regime—Part I: A 2-D Limiting-Sphere Model

Abstract

This work examines the unipolar charging of aerosol particles smaller than 100 nm by ions (at STP) both theoretically (Part I) and experimentally (Part II). Among numerous well-known models for particle charging by ions a gap has been identified for the case of diffusion charging with superimposed external electric field in the transition regime. A two dimensional extension of the classical Fuchs-model CitationFuchs, 1963) has been developed in order to bridge this gap in the knowledge. For the first time the complete expression for the electrostatic potential between a particle and an ion has been included. By using FEM-simulations, the quantitative influence of an external electric field on the charging process has been predicted. It is shown that an external electric field increases the average particle charge linearly. Compared to pure diffusion charging, an electric field of 10 kV/cm increases the charge on metallic particles up to 28% (30 nm particles), 55% (60 nm), and 95% (100 nm), respectively. When compared to the Fuchs-model, the assumptions of the new model introduce additional uncertainty of less than 3%. Comparison with the simpler, but physically less reasonable continuum model by CitationLawless (1996) shows similar charge predictions (deviations < 20% for n·t-products 1013 s/m3), even for 30 nm particles. So, that continuum charging model can serve as simple approximation of the more accurate charging model for the transition regime.