When combined with the results of direct interaction measurements, our observations on metastable colloidal crystals lead us to several insights regarding colloidal interactions. To begin with, the structure and dynamics of the metastable crystals provide clear evidence for strong, long range attractions acting between like-charged colloidal microspheres which are not explained by the standard DLVO theory. The attractive pair potentials measured in the presence of confining glass walls are strong enough to account for the crystals' properties, but do not extend far enough from the walls to stabilize their full three-dimensional structure. Their stability, therefore, suggests that the planes of charged spheres in the crystal act in a manner comparable to charged glass planes in providing the geometrical confinement apparently necessary for attractive interactions. A charged wall with its counterions indeed resembles a concentration of charge-stabilized spheres in a coarse-grained or mean-field description. If this is the case, then confinement-induced attractive interactions might arise in charge-stabilized colloidal suspensions even without glass walls. The nature and mechanism of this and the wall-mediated attraction are open questions and pose a formidable challenge to the theory of colloidal interactions.
Discrepancies between predictions of the DLVO theory and measurements on charge-stabilized colloidal suspensions have fueled an active debate regarding alternatives to the DLVO theory for colloidal interactions including that proposed by Sogami and Ise [30, 31]. The Sogami-Ise theory describes long-range pairwise attractions mediated by counterion distributions at least qualitatively similar to our observations. Very recent direct interaction measurements , however, show that the Sogami-Ise theory does not correctly describe the interactions between unconfined pairs of spheres. Most likely, then, it does not explain the present observations, either.
The existence and behavior of superheated colloidal crystals suggest instead many-body effects may contribute an attractive component to the pairwise interactions between charged microspheres. Given the many useful, interesting, and economically important properties which microscopic interactions impart to colloidal suspensions, this result has significance beyond the immediate application to nonequilibrium phase transitions.