Binary colloidal suspensions are assembled into planar superlattices using ac electric fields. Either
triangular or square-packed arrays form, depending on the frequency and relative particle concentrations.
The frequency dependence is striking since superlattices develop at low and high frequencies
but not at intermediate frequencies. We explain the low frequency behavior <3 kHz in terms of
induced-dipole repulsion balanced by attraction resulting from electrohydrodynamic (EHD) flow. At
high frequencies (20–200 kHz), EHD flow is negligible but aggregation occurs since dipole-dipole
interactions become attractive.