AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Study of the Unipolar and Bipolar Diffusion Charging of Arbitrary Shaped Aerosol Particles by Brownian Dynamics Simulations
Ranganathan Gopalakrishnan, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 80 Working Group: Aerosol Physics
Abstract With knowledge of the diffusion charging rate, both the evolution of a time-dependent charge distribution on particles in unipolar ion environments, and the steady-state charge distribution on particles in bipolar ion environments can be determined. Discussed in this presentation, we have utilized a combination of Brownian dynamics (BD) simulations to examine (1) the collision rate between particles of arbitrary shape and ions, and (2) the steady state charge distribution on arbitrary shaped particles in the presence of realistic ion populations. In unipolar charging, the collision kernel, which defines the collision rate between particles and ions of known number concentrations, (when expressed in a dimensionless form H) depends upon the diffusive Knudsen number, Kn$_D, the ratio of the ion mean persistence path to a well-defined particle length scale. This particle length scale is a combination of the orientation averaged projected area PA and the Smoluchowski (diffusion) radius R$_s of the particle. In the transition regime (finite Kn$_D), the dimensionless collision kernel is shown to be geometry independent using BD simulations for conducting particles, depending only on Kn$_D whose definition incorporates the potential energy to thermal energy ratios for both the Coulomb and image potentials.
To study bipolar charging, a BD method is also employed, but which circumvents ion-particle collision rate calculation while still enabling direct determination of the steady state charge distribution. Non-spherical particles (aggregates, linear chains and cylinders), with a Projected Area (PA) to diffusion based surface area (pi*R$_s$^2) ratio close to unity have similar bipolar charge distributions to spheres sphere of the same mobility diameter. However, highly non-spherical particles (PA/piR$_s$^2 < 0.5) behave very differently from spheres, and the spatial distribution of charge on the particle surface is seen to have a strong effect in both the nano- and submicrometer size ranges.