Controlled formation of colloidal structures by an alternating electric field and its mechanisms.

A detailed phase diagram, revealing a variety of processes including various colloidal structures of monodisperse charged colloidal particles from the colloidal chains, vortex rings, three-dimensional aggregation to a two-dimensional crystal under different frequencies, and strengths of an alternating electric field, is obtained for the first time. The occurrence of different colloidal structures is driven by the electrohydrodynamic interaction and induced dipolar interaction near the polarized layer on the electrode. This simple colloidal system can be employed as a model system to understand the complex phase behavior of the assembly/aggregation of the nanoparticles and biomacromolecules under external perturbation. Detailed phase diagram provides vital guidance for the fabrication of desired colloidal structures with single-particle resolution, which could be employed as a sort of templates for nanolithography or imprinting. Moreover, the sensitivity of the electrohydrodynamic interaction on the particle size and the dependence of the convective flow on the frequency and strength could be utilized in microfluidic devices for manipulating nanoparticles, biomacromolecules, and vesicles.