Dielectrophoresis of suspended polarizable particles is generated by destabilizing a flat interface between two fluids with a spatially uniform ac field. The strong field gradients that are created near the three-phase contact lines of the electrically formed fluid patterns cause the particles to accumulate there and to build a sharp and distinct boundary between regions enriched in and depleted of particles. This straightforward technique does not require the fabrication of complicated electric devices for the dielectrophoretic manipulation and separation of particles.