Characterization of Optical Diffraction and Crystal Structure in Monodisperse Polystyrene Colloids

Abstract

Bragg diffraction of laser light from crystalline aqueous colloids of polystyrene spheres is examined to determine crystal structure, orientation, and elasticity. A new technique using Kossel rings is described which simultaneously measures structure, lattice spacings, and crystallite orientation. The monodisperse polystyrene sphere latex dispersions crystallize into large single crystals, which, depending on sphere concentration, are either face-centered or body-centered cubic. The interparticle spacings in the crystals are many times larger than the sphere diameter (0.109 μm). The use of tunable lasers to easily determine crystal structure is described, and the technique is further illustrated by the experimental determination of the bulk modulus. The bulk modulus is a macroscopic physical constant which can be used to monitor intersphere potentials and the screening of the particle charges by electrolytes in the solution. Data are presented which suggest that crystallite orientation occurs with the closest packed sphere layers parallel to the sample cell quartz walls.