The continuum regime charging of submicrometer particles by bipolar air ions : theory and experiment

Abstract

A study was undertaken to characterize experimentally the continuum regime charging of aerosols by bipolar gaseous ions and to evaluate theoretical models for the charging process. Experiments were conducted on monodisperse particles in the 0.3 to 1 (mu)m size range. The particles were exposed to counter currents by bipolar air ions in a flow-through charging apparatus within which the ratio of positive ion conductivity, (gamma), was varied from zero (unipolar negative) to one (equal bipolar) to infinity (unipolar positive). The charging electric field strength was varied between 100 and 300 Vcm('-1). Mean charge was measured by integral mobility analysis. Unipolar charge levels observed experimentally for a conductivity-charging time product ((tau) = 4(pi)e(mu)('+)N('+)t) of 11.4 and an electric field strength of 250 Vcm('-1) ranged from 42 elementary charges for 1.1 (mu)m particles to approximately 10 charges for 0.31 (mu)m particles. Increasing the field strength to 3000 Vcm('-1) increased these levels to 115 charges for 1.1 (mu)m particles and 17 charges for 0.31 (mu)m particles. Steady state charge levels were attained by particles charged with bipolar ions upon increasing the parameter (tau) sufficiently; however, particles charged under unipolar conditions demonstrated no steady-state charge levels within the experimental range, 0.1 < (tau) < 100. For bipolar conductivity ratios ((gamma)) of 10 and 3, particles attained only about seventy and forty percent respectively of the unipolar value for (tau) = 11.4. Experimental measurements for low field strengths compared favorably with classical continuum diffusion theory. Charge levels acquired in the presence of electric fields exceeding 500 Vcm('-1) in strength, were found to be in excess of the predictions of either continuum diffusion or field charging theories; however, reasonable agreement was obtained with an empirical model based upon the superposition of the continuum field and diffusion theory solutions. Experimental and theoretical trends suggest that classical field theory is not applicable unless the particle diameter exceeds several micrometers. A general continuum model for bipolar charging was developed which incorporates the significant features of the classical diffusion and field charging theories. The general theory was found to accurately predict charging in the presence of an external electric field for the range of particle sizes investigated experimentally.