kT-scale colloidal interactions in high-frequency inhomogeneous AC electric fields. II. Concentrated ensembles.

We report nonintrusive optical microscopy measurements of ensembles of polystyrene colloids in inhomogeneous AC electric fields as a function of field frequency and particle size. By using an inverse Monte Carlo (MC) simulation analysis of time-averaged particle microstructures, we sensitively measure induced dipole-dipole interactions on the kT energy scale. Measurements are reported for frequencies when the particle polarizability is greater and less than the medium, as well as the crossover between these conditions when dipole-dipole interactions vanish. By using measured single dipole-field interactions and associated parameters from Part I as input in the inverse analysis, the dipole-dipole interactions in this work are accurately modeled with no adjustable parameters for conditions away from the crossover frequency (i.e., |f(CM)| > 0). As dipolar interactions vanish at the crossover, a single frequency-dependent parameter is introduced to account for microstructures that appear to result from weak AC electro-osmotic flow induced interactions. By connecting quantitative measures of equilibrium microstructures and kT-scale dipole-field and dipole-dipole interactions, our findings provide a basis to understand colloidal assembly in inhomogeneous AC electric fields.