Electrically induced interactions between colloidal particles in the vicinity of a conducting plane

We address the problem of two-dimensional (2D) colloidal aggregation driven by an ac electrical field, by observing an aqueous dispersion of latex microspheres in contact with a conducting surface. Using micron-sized carboxylated polystyrene particles, we have systematically investigated the aggregation process, as a function of particle size and charge, and of the applied electric field amplitude and frequency. A low-density 2D phase is observed at high frequency (typically above 1 kHz), while at low frequency (below a "contact frequency" nu(c)) the collection of particles collapses into disconnected compact aggregates of crystalline (hexagonal) structure. We argue that this scenario is governed by the competition between an attractive force, of electrohydrodynamic nature, and a repulsive force, basically an electrical dipole-dipole interaction. Both contributions are revealed and analyzed in independent experiments on isolated particle pairs, using optical manipulation and dynamometry.