The role of effective triplet interactions in charged colloidal suspensions

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Abstract
The traditional theory of the effective interactions between charged colloidal particles (`macro-ions') treats the screening of the counter-ions on a linear response level resulting in the well-known screened Coulomb pair potential proposed by Derjaguin, Landau, Verwey and Overbeek. In general, however, due to non-linear counter-ion screening, the effective interactions involve many-body contributions. Using molecular dynamics simulation of the highly asymmetric primitive model with three macro-ions, we calculate the effective triplet forces and compare them with the pairwise contributions. Near touching of a macro-ion triplet, the triplet interaction represents a significant attractive correction to the repulsive pairwise interaction while it is small for large inter-particle separations. Furthermore, we compare our simulation results for the triplet forces with an analytical expression gained from a density functional perturbation approach. Upon performing a suitable renormalization, we find reasonable agreement for situations of weak and strong coupling as well as for situations with added salt ions.