Microscopic observation of ordered colloids in sedimentation equilibrium and the importance of the Debye-screening length. 9. Compressed crystals of giant colloidal spheres (diam. 2–66 μm)

Abstract

Compressed crystal-like structures of very large polystyrene spheres (specific gravity=1.05, diameter=2–66 μm) are observed directly by a metallurgical microscope in sedimentation equilibrium and in deionized aquueous and 20 vol % methanol aqueous suspensions. Spheres of 2–42 μm in diameter float above the cover glass apart from each other. The center-to-center intersphere distance decreases as the initial concentration of spheres increases, from which Young's elastic moduli (G) for the crystal-like structures are estimated to be between 1 to 10 dyn/cm², irrespective of the spheres size.G's in aqueous suspension are smaller than those in methanol aqueous mixtures. The crystal-like structures are compressed substantially by the gravitational field. These results are consistent with the significant role of the electrical double layers under the influence of purely electrostatic repulsive forces in the effective hard-sphere model.