A constant-angle mie scattering method (cams) for investigation of particle formation processes

Abstract

For investigations of particle formation processes in colloidal systems an observation technique, which is based upon light scattering, can be applied. The growing particles are illuminated by a monochromatic, parallel light beam. The light flux transmitted through the considered system and the light flux scattered by the particles under a selectable, constant scattering angle are monitored during the particle growth process (constant-angle Mie scattering, CAMS). If the growing particles are approximately spherical and sufficiently monodisperse, the obtained experimental scattered light flux-vs-time curves show series of maxima and minima in agreement with theoretical scattered light flux-vs-size curves, calculated by means of the Mie theory. After establishing a unique correspondence of experimental and theoretical light scattering extrema, a simultaneous and independent quantitative determination of particle size and number concentration can be performed. From the positions of the experimental light scattering extrema relative to the time axis the particle size can be obtained at various times during particle growth. On the other hand, comparison of the heights of experimental and theoretical light scattering maxima yields the particle number concentration. At comparatively high particle concentrations significant light extinction occurs in the considered system. This effect is taken into account by normalizing the scattered relative to the transmitted light flux. The CAMS method does not depend on external standards or empirical calibrations and thus it can be regarded as an absolute method. CAMS measurements have a negligible response time and generally have no influence on the growing particles. Thus particle growth can be investigated by the CAMS method over wide ranges of growth rates and number concentrations, which are hardly accessible by other techniques. A number of actual applications of the CAMS method are mentioned.