The Coulomb interaction between charged colloidal particles dispersed in a polar solvent is moderated and mediated by a diffuse cloud of atomic-scale ions. These simple ions are much smaller and less highly charged than the macroionic colloid. The resulting disparity in dynamical time scales inspires the notion of an effective macroionic interaction averaged over the simple ions' degrees of freedom. How to formulate this effective interaction has inspired debate for half a century.
Fueling this controversy, experimental observations have raised the surprising possibility that like-charged colloidal spheres sometimes attract each other. If we view the spheres in isolation, their attraction seems counterintuitive. Recalling instead that the overall suspension is electroneutral suggests that unexpected features in the spheres' effective pair potential must reflect unanticipated dynamics in the surrounding medium. The phenomena discussed in the following sections are noteworthy because they appear to contradict long-accepted predictions of mean field theory. Such discrepancies raise concern about our understanding of such related problems as protein folding and colloidal stability.