A.W. Willemse

Possibilities of near backward light scattering for characterizing dense particle systems

Abstract A backscattering probe has been tested as an alternative to the more common forward light scattering technique for particle size determination. The backscattering signature of an ensemble of spherical particles in front of the probe window was measured with a charged coupled device (CCD) camera and compared with Lorenz-Mie calculations. The fringe structure allows the measurement of particle size. However, the width of the size distribution must be small to observed the fringes. The phenomenon of enhanced backscattering in concentrated random particle media is investigated and discussed. The width of the coherent backscatter peak, due to multiple scattering serves as an indicator for the mean particle size, independent of the shape of the particles. The total amount of backscattered light is an indication of the total reflecting surface area of the sample.

Photon correlation spectroscopy for analysis of low concentration submicrometer samples

Photon Correlation Spectroscopy is a technique to rapidly measure particle size in the submicrometer region. The use of PCS is, however, limited by concentration. The upper limit is due to multiple scattering of the incident light, the lower limit is determined by the fact that fluctuations of the number of particles in the measuring zone have a significant influence on the apparent diffusion coefficient. In this article a new signal processing method is described which differentiates this influence. With this system the lower limit is no longer limited to about 100 particles in the measuring volume corresponding to a concentration of 109 particles/cm3. The limitation is now the intensity of the scattered light which becomes too weak at a concentration of about 50 particles/cm3. As a consequence of this work, a revision to the basic theory of PCS may be necessary. Moreover, the new processing method enables also the measurement of particle concentration in the sample.