(123c) Ac Electrohydrodynamic Mobility And Dielectrophoretic Assembly Of "Janus" Particles

The synthesis of ?Janus? particles (whose hemispheres are physically or chemically different) is of growing importance for the development of novel materials, but the behavior of such particles in external fields has not been studied in depth. We will report how Janus particles consisting of a dielectric hemisphere and a conductive one respond to external AC electric fields in water. Numerous field-driven physical effects were observed and will be discussed. The operational diagram of particle response and structure assembled as a function of the frequency and the field intensity will be presented. At low frequencies of the applied AC fields (< 10 kHz), the tangential component of the field on the polarized double layer at the metal-dielectric interface leads to unbalanced ionic flows along the surface. These flows evoke induced-charge electrophoresis (ICEP) and drive the particles to move normal to the electric field with their dielectric hemispheres facing forward. We present the first experimental verification of this effect, which has been recently predicted theoretically. The data are in good correlation with the predictions of the ICEP theory. The characterization of the role of electrolyte concentration, particle size and frequency reveal effects that have yet to be incorporated into a refined theory. The propelling metallodielectric particles could be used as microscopic mixers, "shuttles" and self-propelling on-chip sensors. The ICEP effect is suppressed in AC electric fields of higher frequency (> 15 kHz). In this case the particles self-assemble by dielectrophoresis into new types of metallo-dielectric colloidal crystals, where the metallized halves of neighboring particles align into conductive lanes through the crystals. Such large metallo-dielectic arrays can find applications both in photonics and electronics.