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Discussion

No consensus has yet emerged regarding the origin of confinement-induced attractions in charge-stabilized colloidal suspensions. Even so, the existing body of experimental evidence constrains possible explanations.

Isolated pairs of like-charged spheres appear to repel each other without exception. And yet evidence for many-body attractions abounds when the number density of the same spheres is increased. On this basis, we provisionally rule out theories which predict pairwise attractions for isolated spheres. This leaves open the question of how neighboring spheres or bounding surfaces induce long-ranged attractions.

Mean-field explanations appear to be excluded by the NSC and Trizac proofs. Fluctuations in the simple-ion distributions are not captured by mean-field treatments and thus offer a fruitful line of inquiry. Existing calculations suggest that such fluctuations do indeed lead to attractions, but that these are doubly-screened and thus short-ranged [52,53]. Surface charge fluctuations due to ion adsorption similarly are found to induce short-ranged attraction [54]. Attractions such as those in Fig. 2.4, on the other hand, are longer-ranged than the singly-screened pair repulsion. If ionic fluctuations are responsible, the mechanism must involve modes of fluctuation not yet considered [43].

The primitive model on which the mean-field theory is based treats the supporting fluid as a continuum. While the solvent's discrete structure induces attractions at molecular length scales [55], it is unlikely to influence colloidal interactions at a range of several micrometers.

Very recently, Squires and Brenner [56] reported a previously overlooked role for the solvent in mediating confined colloidal interactions. The interplay of electrostatic and hydrodynamic interactions between charged spheres near a bounding charged wall can explain at least some anomalous observations, according to their theory. Exploring this suggestion invites an examination of colloidal hydrodynamics in confined geometries.


next up previous
Next: Hydrodynamic Interactions Up: Electrostatic Interactions Previous: Confined Pair Attractions
David G. Grier 2001-01-16