Abstract
A metallurgical microscope is used to directly observe the ordered structures of colloidal silica spheres (340 nm in diameter, specific gravity=1.58) in sedimentation equilibrium and in aqueous suspensions. The two-dimensional distance distribution function is obtained for the crystal-like structures. The center-to-center interparticle distances (2Dexp) in the hexagonal ordering increase with time after the sample is set into the cell, and reach equilibrium values after ∼10 days, which shows that complete deionization is essential to get reliable data for 2Dexp. 2Dexp 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 0.30 Pa. The way 2Dexp is influenced by the concentration and ionic strength shows that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are both essential for the ordering.
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