Microscopic observation of ordered colloids in sedimentation equilibrium and the important role of Debye screening length. 4. Gold particles (specific gravity=19.3) in aqueous suspension.

The ordered structures are observed by a metallurgical microscope for heavy gold particles (specific gravity=19.3) in sedimentation equilibrium. The hexagonal order is observed not only in the deionized state but also in the presence of NaCl, and the centre-to-centre distance between particles, 2 D, decreases with salt concentration. 2 D decreases with increasing initial concentrations of gold partilces, and the lattice structures are compressed under a gravitational field. The Young's modulus of the lattice is estimated as 1.3 dyn · cm. The results confirm that the electrostatic interparticle repulsion and/ or thick electrical double layers around the particles are essential for the ordering of the colloids. [ABSTRACT FROM AUTHOR]