Morphological studies of moving aerosol particles by laser microbeam scattering

Abstract

As part of a program of work on the mechanism of aerosol condensation, time-resolved light scattering from single, metallic particles crossing a laser microbeam is used to explore their morphology. In addition to the particle size, which governs the duration of the signal (photon count rate against time), other effects such as photophoresis, evaporation, and Brownian motion can influence the signal shape. Photophoretic effects can be reduced by the use of two horizontally opposed microbeams. When photophoresis is minimal, the signal profile for a spherical particle at short transit times is a smooth Gaussian. The profile is modified in the case of an asymmetrical or faceted particle as the result of Brownian rotation. When using two beams, different wavelengths are necessary to avoid interference. Two detectors may now be used, each tuned to a different wavelength, to observe simultaneously the light scattered on opposite sides of the particle. Differences in the resulting signal profiles are indicative of irregularities in shape or surface condition. Of particular interest are the features observed microscopically in aerosol condensates and which are the result of phase changes occurring in the condensation boundary layer. The possibility is envisaged of observing these changes as they occur by probing the layer with a microbeam. Results are presented for various aerosols obtained by the single-beam and double-beam techniques. These results, which represent preliminary findings, illustrate the sensitivity of time-resolved, single-particle light scattering to certain particle properties. They also establish the potential of the double-beam method in reducing the problem of photophoresis and simplifying the interpretation of signals.