Interactions and microstructures in electric field mediated colloidal assembly.

We report video microscopy measurements and computer simulations of quasi-two-dimensional configurations of micron sized colloids in 1 MHz ac electric fields between coplanar thin film electrodes. Interactions of induced dipoles (IDs) with each other and inhomogeneous electric fields (IFs) as a function of concentration and field amplitude produced microstructures including confined hard disk fluids, oriented dipolar chains, and oriented hexagonal close packed crystals. Equilibrium measurements and analyses of single colloids within electric fields were used to directly measure ID-IF interactions in the absence of many body effects. Measurements of concentrated systems were characterized in terms of density profiles across the electrode gap and angular pair distribution functions. In concentrated measurements, an inverse Monte Carlo analysis was used to extract the ID-ID interaction. A single adjustable parameter consistently modified the ID-IF potential and the ID-ID potential to account for weakening of ID as the result of the local particle concentration and configuration.