A Aerosol Beam Study of the Mass Density and Electric Polarizability of Potassium-Iodide Aerosol Particles.

The physical properties of submicroscopic aerosol particles have been measured by adapting and applying methods from molecular beam research. Previous researchers in this laboratory have used this approach to measure the aerosol particles' mass, size, and density. This project extends this work by also measuring the particles' electric polarizability. In addition, the data analysis has been computerized so that a more rigorous analysis is now possible. A potassium iodide/helium aerosol was generated in a single-stage (self-nucleating) condensation generator. A small portion of the aerosol was injected through a capillary nozzle into a high-vacuum chamber, where it was further collimated into a narrow beam. The particles were detected using a hot-wire surface ionization detector, and their velocity was measured using a computer-controlled time -of-flight technique. Some of the particles were charged and then deflected in an homogeneous electric field; this allowed the particle mass to be computed by using a large -shift, strong-field deflection analysis. The particle size was determined by applying free-molecular drag theory and measuring the velocity retardation as the particles flowed through a low-pressure gas. In order to measure the particles' electric polarizability, the aerosol beam was deflected by an inhomogeneous two-wire electric field; this allowed the particles' dielectric constant to be computed by using a small-shift, weak-field deflection analysis. The mean velocity of the particle beam ranged from 189 to 665 m/sec. The velocity distributions tended to be nearly normal, with a relatively small coefficient of variation (average value 7.8 percent). A lognormal mass distribution was assumed, and in each case the aerosol was found to be polydisperse, with a distribution characterized by a geometric standard deviation of about 2.0. The mean particle mass ranged from 0.37 to 6.8 x 10('-19) kg, while the mean particle radius ranged from 3.9 to 9.6 x 10(' -8) m. The mean particle density was only four to nine percent of potassium iodide's bulk density, and it was found to decrease with increasing particle size. This is probably because much of the particles' growth was the result of coagulation. Similarly, the mean polarizability per unit mass was two to six times the value for bulk potassium iodide, and it also decreased with increasing particle size. It is felt that this was probably caused by orientational and interfacial polarization effects within the particles. This low particle density and high polarizability per unit mass combine to produce mean particle dielectric constants (K) between 1.3 and 2.5 (compared to the bulk value of 5.1). In addition, the quantity (K - 1)/(K + 2) was found to be a linear function of particle density, as would be predicted by the molecular Clausius-Mossotti equation, if it could be applied in this case.