Reappearance of structure in colloidal suspensions

Static structure factors S ( q ) of deionized aqueous suspensions of charged polystyrene particles with similar radii but strongly differing bare charges have been measured for volume fractions 3.5 × 10 −4 ⩽ φ ⩽ 1.55 × 10 −2 , using a cross-correlation light scattering technique which allows for the extraction of single scattered light from highly turbid samples. Measurement of absolute intensities allowed to determine unambiguously normalized values of S ( q ). With decreasing volume fraction, the amplitude of the first peak of S ( q ) reflecting the order of the suspension does not monotonically decrease, but rather shows a distinct minimum at φ ∼ (5 − 8) × 10 −3 . This reappearance of structure is compared with theoretical predictions on the basis of a jellium model for the effective interparticle potential U eff ( r ).