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.

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.

Surface-anisotropic polystyrene spheres by electroless deposition.

Surfaces of sulfate-terminated polystyrene microspheres are anisotropically modified with silver nanoparticles covering 20-50% of the sphere surface using electroless deposition. A PDMS templating method is employed. Silver nanoparticles are found to deposit uniformly onto the exposed sphere surfaces. The deposition is diffusion-controlled and the nanoparticles adhere strongly to the polystyrene particles despite extensive exposure to ultrasonication. Silver content is confirmed by EDAX analysis. The final silver coverage is controlled via the PDMS pre-curing conditions.