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.

We report confocal laser scanning microscopy (CLSM) measurements, Monte Carlo (MC) simulations, and an analytical model of the assembly of three dimensional (3D) equilibrium colloidal crystals within quadrupole electrodes on microscope cover slip surfaces. Micron sized fluorescent silica colloids in an index matching dimethylformamide (DMF) medium enable three dimensional CSLM imaging to measure particle coordinates. By matching density profiles from CSLM measurements and MC simulations, we obtain electrostatic, dipole–field, dipole–dipole, and gravitational potentials that accurately capture the three dimensional microstructure and morphology. We also report analytical density profiles with fluid–solid coexistence by balancing gravitational and electric field mediated compression against local osmotic pressure, which agree with CSLM and MC results. These results provide fundamental information on the assembly of three dimensional equilibrium colloidal crystals in the presence of multiple external fields and with coexisting inhomogeneous fluid and solid phases.

We investigate the electric field-induced assembly of Janus colloids composed of silica rods patterned with gold patches in both side- and tip-coated motifs. These shape and chemically anisotropic particles assemble into reconfigurable chains, whose structure depends on patch location, AC electric field strength, and frequency.