David Grier's Home Page Grier Group Publications WEAK LONG-RANGED CASIMIR ATTRACTION IN COLLOIDAL CRYSTALS

ABSTRACT. We investigate the influence of geometric confinement on the free energy of an idealized model for charge-stabilized colloidal suspensions. The mean-field Poisson-Boltzmann formulation for this system predicts pure repulsion among macroionic colloidal spheres. Fluctuations in the simple ions ' distribution provide a mechanism for the macroions to attract each other at large separations. Although this Casimir interaction is long-ranged, it is too weak to influence colloidal crystals' properties. Experimental evidence collected over 20 years (1) suggests that similarly-charged colloidal spheres dispersed in water need not simply repel each other. Under some circumstances they instead experience an unexpected long-ranged attraction. For example, like-charge attractions are implicated in the cohesion of metastable superheated colloidal crystals (2); (3) even though isolated pairs of the constituent spheres are observed to repel each other (4); (5). Comparable attractions have been measured for pairs of spheres confined by two (5); (6) charged planar walls. Recent calculations (7); (8) reveal that such confinement-induced attractions cannot be accounted for by local density theory nor by electrohydrodynamic coupling (9); (10). Such anomalous effects in charge-stabilized colloid therefore challenge our genera