Diffraction in Particle Size Analysis

Abstract

Abstract The diffraction techniques for particle size analysis are based on the measurement of the angular distribution of light diffracted by particles situated in the path of a light beam. These techniques have been used for many years to measure the size of small particulates but the calculation to generate a particle size distribution is so complex that it is only in the last 15 years with the development of small, inexpensive, and powerful computers that these techniques have become so popular. The complete theory describing the diffraction of light by a spherical particle was described by Mie in 1908 and is able to predict the electromagnetic field at any point in space. This theory is adequate for particles with diameters of the same order as the wavelength of the incident light. Some other models have been presented. The most commonly used are based on the Fraunhofer diffraction theory. Light‐scattering techniques have obvious advantages for particle size analysis as compared to other methods. They are faster than mechanical techniques, nonintrusive, nondestructive and do not require large amounts of sample. These techniques can be performed on dry powders or on suspended particles in a liquid or in a gas. The operating range of light diffraction techniques is from 20 µm to several millimeters. This article presents the main theoretical aspects of light diffraction by particles. Also, a brief description of a diffraction particle sizer is given with some practical recommendations. Problems related to particle size analysis by light diffraction are examined. Some other light‐scattering methods (photon correlation spectroscopy, single particle scattering, Doppler techniques) are also presented.