(585e) Heterogeneity of the Electrostatic Repulsion Between Colloids at the Oil-Water Interface

We present interaction measurements using laser tweezers between polystyrene particles pinned at the oil-water interface. The long-range dipole-dipole force exhibits a distribution of magnitudes that depend on the particle pairs and sample preparation methods. Particles treated with multiple centrifugation and redispersion steps in ultra-pure water exhibit stronger repulsions. A Gamma distribution accurately models the variation for both washed and unwashed particles. Despite this heterogeneity, the multibody interactions measured in small ensembles are pairwise additive. Good agreement is found for the two-dimensional equilibrium suspension structure between experiments and Monte Carlo simulations when a heterogeneous interaction potential is used in the latter. The heterogeneity and long-range interaction accounts for the lower apparent pair interaction derived from the suspension radial distribution function at dilute concentrations and the direct pair interaction measurements made with laser tweezers at nearly infinite dilution.