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 Ueff (r).