Angular light scattering, particle

Particle Size Determination Using Angular Light Scattering... 

Washington C., The angular distribution of scattered light, in Particle Size Analysis in Pharmaceutics and Other Industries, Ellis Horwood, New York, 1992, 109. 

When a particle is illuminated by a beam of light with specified characteristics, the amount and angular distribution of the light scattered by the particle, as well as the amount absorbed, depends in a detailed way on the nature of the particle, that is, its shape, size, and the materials of which it is composed. This presents us with an almost unlimited number of distinct possibilities. Nevertheless, there are some features common to the phenomena of scattering and absorption by small particles. In this chapter, therefore, our goal is to say as much as possible about such phenomena without invoking any specific particle. This will establish the mathematical and physical framework underlying all the specific problems encountered in later chapters. 

In contrast to traditional dynamic light scattering (which, as we discuss in Section 5.8, probes the continuous random motion of the particles), DWS here probes spatially localized, temporally intermittent events that are characterized by relatively larger length scales. Despite this, the basic quantity measured (g,(trf) in the notation of Section 5.8 there is no angular dependence in DWS because of multiple scattering) is essentially the same, only the method of analysis is different. 

As we shall see, the intensity, polarisation and angular distribution of the light scattered from a colloidal system depend on the size and shape of the scattering particles, the interactions between them, and the difference between the refractive indices of the particles and the dispersion medium. Light-scattering measurements are, therefore, of great value for estimating particle size, shape and interactions, and have found wide application in the study of colloidal dispersions, association colloids, and solutions of natural and synthetic macro-molecules. 

Particles which are too small to show a series of maxima and minima in the angular variation of scattered light are frequently studied by measuring the dissymmetry of scattering (usually defined as the ratio of the light scattered at 45° to that scattered at 135°). The dissymmetry of scattering is a measure of the extent of the particles compared with A. If the molecular or particle size is known, it can be related to the axial ratio of rod-like particles or the coiling of flexible linear macromolecules. 

Figure 1.5. Illustration of the angular light intensity distribution of light scattered from a single particle.

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