AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Modelling and Experimental Evaluation of Aerosol Nanoparticle Photoionization in an Applied Electric Field
Robert Nishida, ADAM M BOIES, Simone Hochgreb, University of Cambridge, University of Minnesota
Abstract Number: 579 Working Group: Aerosol Physics
Abstract Aerosol nanoparticles are more efficiently charged by direct ultraviolet photoionization than by other charging mechanisms such as diffusion charging. The resulting charged nanoparticles can therefore more easily be manipulated for detection, capture, or control in many aspects of aerosol and materials science.
Aerosol particles emit electrons and become positively charged to multiple charge states if they absorb ultraviolet photons of high enough energy. The emitted electrons ionize gas molecules which, in turn, recombine with particles to reduce their charge level. Mechanisms of particle charge recombination with gaseous ions are well understood, but the photoionization process has limited experimental validation. Experimental studies have typically relied on the measurement of current resulting after photocharging via empirical constants, which serve as a fitting parameters for every change in particle size, light intensity, or flow rate when no change should occur [1,2]. Modelling of the photoionization process has not seen significant improvement in over a decade [3,4].
In this work, a generalized model for particle photocharging and transport is proposed, which includes equations for flow, photocharging, recombination, wall losses and electric field transport. The Fowler-Nordheim equation and photocharging theory is evaluated for the applicability of its experimental constants, including a single empirical constant and a particle work function to account for photoionization.
The model is validated using measurements of currents in an experimental photoionization chamber. The device is outfitted with a method for varying the electric field in order to capture ions, reduce recombination, and increase the particle charge state in a continuous flow of aerosol nanoparticles. The variation in electric field allows control the final particle charge state, and thus the total collection of current.
Experimental data is obtained for a range of aerosol particle compositions (e.g. soot, silver), sizes and concentrations. The limitations of the existing theory and models are described in connection with the experimental comparisons. The novel approach of using an electric field to allow control of particle charge states is demonstrated to be useful for a range of photocharging based devices.
[1] Jiang, J. et al., Journal of Electrostatics, 65 (2007), pp. 209-220
[2] Hontanon E. and Kruis F. E., Aerosol Science and Technology, 42,(2008), pp. 310-323
[3] Maisels, A. et al., Journal of Applied Physics, 91 (2002), pp. 3377-3383
[4] Maisels, A. et al., Journal of Aerosol Science, 34 (2003), pp. 117-132