AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Experimental Bipolar Diffusion Charging of Spherical and Cylindrical Aerosol Particles with Detailed Characterization of the Charging Ions
Ranganathan Gopalakrishnan, Peter McMurry, CHRISTOPHER HOGAN JR., University of Minnesota
Abstract Number: 473 Working Group: Aerosol Physics
Abstract The results of bipolar diffusion charging experiments performed using spherical and cylindrical aerosol particles are presented here. Gold nanoparticles (spheres of nominal diameters 50 and 70 nm; cylinders of nominal length to diameter ratios of 2.2 to 14.3) are generated by electrospraying colloidal solutions. The mass and mobility distributions of the charging ions (formed by ionization of gas phase species by a Po-210 source) are measured using a high resolution DMA coupled to a time-of-flight Mass spectrometer. Using a tandem-DMA setup, the fraction of neutral particles and ratios of the singly to doubly charged particles of positive and negative polarities are measured in Ultra-high purity air and Carbon dioxide. Measured charge distributions are compared to predictions of a previously developed Brownian Dynamics based charging model that takes into account the exact geometric size distribution of the particles as inferred from electron microscopy and the mass-mobilities of the ions with minimal simplifying assumptions, while also separately accounting for the operating conditions (transfer function) of the DMAs. From measured charge distributions of cylinders, it is seen that particles that are highly nonspherical (ratio of the orientation averaged Projected Area to the diffusion based surface area of 0.70 and 0.63) attain higher charge levels than mobility equivalent spheres (at 65 nm). The effect of mobility diameter was also studied, and for the particles considered here (aforesaid ratio=0.84) behave akin to mobility equivalent spheres (<65 nm). The measured and simulated charge distribution for spheres show good agreement in air, while in carbon dioxide the presence of high mobility negative charge carriers that escaped detection by our measurement systems are hypothesized to be one of the reasons for differences. The ions generated by radioactive neutralizers were traced to the tubing material based on the measured mass-mobility spectra.