Abstract
We have developed an instrument that is specifically designed to measure cluster size distributions in colloidal dispersions. In this instrument, individual clusters pass single file through an optical flow channel, a portion of which is illuminated by a uniform laser beam. As each cluster passes through the illuminated region, it scatters a pulse of light, which is collected at angles of less than 3° and imaged at the surface of a photomultiplier tube. Multichannel pulse-height analysis yields a histogram of the cluster size distribution. The distinguishing feature of this instrument is its specific use of lowangle light scattering for cluster size analysis. We demonstrate that at sufficiently low angles, pulse height is proportional to the square of the number of monomeric units in a cluster—independent of cluster shape or orientation. Using polystyrene microspheres in water (relative refractive index m = 1.2), we show that this proportionality holds for sphere diameters as large as three times the wavelength of the incident light. Scattering from individual particles and from clusters of spheres is discussed in terms of Rayleigh, Rayleigh-Gans-Debye, and Mie theories.
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