Viscous fluids, such as concentrated solutions, polymer melts, or gels, cannot be clarified as they are, and therefore optical purification should carefully be carried out under a different state, such as in dilute solution or before gelation or polymerization. 77 Samples other than dilute solution have to be prepared essentially in the same way. When the sample preparation has to be performed under special conditions, such as at high temperature, a specially designed apparatus for purification of the solution is needed. The sample cell filled with the solution would ideally be sealed to avoid incorporation of dust, moisture, and other impurities from the outside. All the preparation procedures should be performed in a clean environment especially free from dust, for instance, in a clean box. When filtration is not applicable, for example, when the solute molecules could possibly be broken down by shear, or when the solution is very viscous, a centrifuge is usually used to prepare ‘dust-free’ solutions. To make an optically clean solution, the solution is filtered through a filter of an appropriate pore size made of insoluble materials. Less viscous solvents are preferable for handling the solution. In choosing a solvent to prepare a solution, one should remember that the solvent, together with the solute, should have an appropriate refractive index increment for the sample in order to yield a reasonable amount of scattered light intensity, sufficiently strong to detect but not so strong as to make the solution turbid, which is a qualitative sign for multiple scattering. Solutions for light scattering measurements should ideally be colorless and have no absorption of light. Chu, in Polymer Science: A Comprehensive Reference, 2012 2.10.3.5 Sample Preparation and Differential Refractometers Only in those cases, changes in positions give rise to phase changes of the light which lead to appreciable changes in constructive and destructive interference. The scattered intensity contains this structural information only, when the distance between the points is of the order of the wavelength of the scattered light. Information about different Fourier components of the density is obtained, in principle, by measuring the scattered intensity in different directions. A measurement of the electric field strength (or the scattered intensity) thus contains information concerning the relative positions of the points, that is, on the instantaneous realization of the fluctuating microscopic density of the assembly of points. As the two points change their relative position, the phase difference of the electric field strengths scattered by these two points changes, so that the measured total electric field strength changes. Clearly, the phase difference of the scattered light from two points depends on their relative positions, as well as on the direction in which the electric field strength is measured (see fig.3.1). The total electric field strength that is scattered in a certain direction is the sum of the electric fields scattered in the same direction by the individual points. Suppose a plane wave of monochromatic light impinges onto this assembly of points, each of which scatters light without changing its wavelength nor its phase. Consider an assembly of points, fixed in space. Let us first try to understand intuitively why the scattered electric field strength is related to the microscopic density. The same holds for interacting Brownian particles, which are considered in later chapters. For example, the predictions about the dynamics of non-interacting particles, obtained in the previous chapter, can be verified by light scattering in an experimentally straightforward manner. This enables the study of density fluctuations, which are the result of Brownian motion of the colloidal particles. The Fourier component that is probed is set by the direction in which the scattered light is detected. What makes light scattering such an important experimental tool, is that the scattered electric field strength is directly proportional to a certain Fourier component of the instantaneous microscopic density. ![]() In further chapters we present experimental light scattering results, so that knowledge of this important experimental technique is, at least, desirable. ![]() Light scattering by colloidal suspensions is a major experimental tool to study the statistical properties of these systems. In Studies in Interface Science, 1996 3.1 Introduction
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