kT-scale colloidal interactions in high frequency inhomogeneous AC electric fields. I. Single particles.

We report nonintrusive optical microscopy measurements of single micrometer-sized silica and polystyrene colloids in inhomogeneous AC electric fields as a function of field amplitude and frequency. By using a Boltzmann inversion of the time-averaged sampling of single particles within inhomogeneous electric fields, we sensitively measure induced dipole-field interactions on the kT energy scale and fN force 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-field interactions vanish. For all cases, the measured interactions are well-described by theoretical potentials by fitting a nondimensional induced dipole-field magnitude. While silica dipole-field magnitudes are well-described by existing electrokinetic models, the polystyrene results suggest an anomalously high surface conductance. Sensitive measurements of dipole-field interactions in this work provide a basis to understand dipole-dipole interactions in particle ensembles in the same measurement geometry in part II.