Development of Unipolar Charger for Nanoparticles Aided by the Numerical Simulation

Abstract

The differential mobility analyzer (DMA) is widely used to measure the aerosol size distribution as well as generate mono-dispersed aerosol. Aerosol particles must be electrically charged before the measuring or classifying by DMA because they are classified according to an electrical mobility. The combination probability of ions to nanometer-sized particles is quite small because of small cross sections of meter-sized particles. Furthermore, the charging fraction in bipolar charger, which is widely used in aerosol research, is limited by the neutralizing as a result of combination of ions and charged particles of opposite polarity.

A numerical simulation code, in which advection-diffusion equations for aerosol particles and ions are combined with computational fluid dynamics code, was developed in this study to optimize the shape of unipolar charger and an electric field in it. The charging frequency of aerosol particles of 5 nm in diameter was predicted to be 7 %, which is improved by a factor of approximately 7 compared with the bipolar charger. We have also experimentally demonstrated the charging efficiency of nanometer-sized particles by the tandem DMA method.