An optical method for studying the properties of ordered latices

Abstract

A method is described to study the properties of ordered latices, which consists of normally illuminating and observing a latex dispersion covered by a flat glass plate. In this way a hexagonal pattern of diffraction spots, a ring-shaped halo, and a dark ring in the halo region are observed. The size of the hexagonal pattern of diffraction spots depends on the volume fraction of the latex and on the particle size. Observed diffraction angles agree within experimental error with those calculated from the Laue theory. Also, the dependence of the hexagon size on the volume fraction of the latex is in agreement with theory. It is found, both experimentally and by numerical calculations, that the thickness of the latex sample does not significantly affect the angle and intensity of the diffraction spots, and thus the diffraction spots contain no information about the three-dimensional structure of the latex. From the inner and outer radius of the ring-shaped halo, the mean refractive index of the latex can be determined. This new method is much simpler than the Bragg diffraction method which has been employed by other workers. The volume fraction dependence of the refractive index of the latex was found to be in good agreement with literature results. The dark ring occurs at a radial distance corresponding to an angle which satisfies the Bragg condition. From the radius of the dark ring and the refractive index of the latex, the size of the latex particles can be determined and is found to be in good agreement with the size determined by electron microscopy. Although this darkening effect has not been observed in other materials, it seems to be a general optical phenomenon associated with crystal structure.