Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye-screening length. II. Monodispersed polystyrene spheres (specific gravity=1.05) in aqueous and methanol aqueous suspension

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Abstract
A metallurgical microscope is used to observe directly the ordered structures of monodispersed polystyrene spheres (specific gravity=1.05) in water and methanol–water mixtures. Sedimentation occurs for all the spheres examined for which the diameters are between 369 and 1020 nm. The interparticle distances (2D exp) increase with time after the latex sample is set into the cell, and reach equilibrium values after seven to ten days, which shows that complete deionization is essential to get reliable data for 2D exp. 2D exp continues to decrease as the initial concentration of the spheres increases and from these data Young’s modulus for the ordered lattices is estimated to be 1 dyn cm− 2 for spheres of 0.5 μm. The influences on 2D exp by the concentration and charge density of the spheres, and ionic strength show that the electrostatic interparticle repulsion and the elongated Debye‐screening length around the spheres are essential for the ordering phenomenon.