AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Diffusion Charging of Non-Spherical Aerosol Particles from Brownian Dynamics Simulations
RANGANATHAN GOPALAKRISHNAN, Thaseem Thajudeen, Christopher Hogan Jr., University of Minnesota
Abstract Number: 58 Working Group: Aerosol Physics
Abstract To correctly determine the size distribution of aerosol particles from electrical mobility analysis, it is necessary to know precisely the charged fraction of aerosol particles as a function of particle “size”. Much work has thus been done to determine the diffusion charging rate, i.e. the rate at which ions collide with and transfer charge to aerosol particles, for spherical particles, such that upon introduction to an environment of equal positive and negative ion concentrations, the particles arrive at a known diameter-dependent steady state charge distribution. For non-spherical particles, however, the diffusion charging rate remains unknown, leaving ambiguities in all electrostatic based analyses of non-spherical particles.
In this work we examine theoretically the diffusion charging of non-spherical particles (using ensembles of aggregated spheres as test particles) employing a combined approach of dimensional analysis and mean first passage time simulations. We show that although non-spherical particle diffusion charging is a complex process, the dimensionless diffusion charging rate for particles of any shape can nonetheless be expressed as a function of the diffusive Knudsen number, i.e. the ratio of the colliding ion persistence distance to a clearly defined descriptor of the particle size, and a ratio of the surface potential energy of the particle to the thermal energy of the background gas. Particle-shape independent dimensionless functions are found which describe the particle-ion collision rate, enabling calculation of the steady-state charge distribution on any particle, provided its geometry can be mathematically described and the background gas and ion properties are known. These calculations additionally link the non-spherical particle diffusion charging rate to calculations of vapor condensation onto non-spherical particles, non-spherical particle collision rates, and diffusion charging, condensation, and coagulation for spheres in the transition regime.