Induced-Charge Electrophoresis of Metallodielectric 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. The application of AC electric fields in aqueous suspensions of anisotropic particles leads to unbalanced liquid flows and nonlinear, induced-charge electrophoretic (ICEP) motion. We report experimental observations of the motion of "Janus" microparticles with one dielectric and one metal-coated hemisphere induced by uniform fields of frequency 100 Hz - 10 kHz in NaCl solutions. The motion is perpendicular to the field axis and persists after particles are attracted to a glass wall. The velocity as a function of field strength, AC frequency and particle size are consistent with ICEP theory in dilute solutions (≤ 100 μM), but we observe anomalous behavior at higher concentrations, including no motion above 10 mM. The metallodielectric Janus particles' motion across uniform AC electric field lines is, to our knowledge, the first report of this fascinating phenomenon that can now guide further development of nonlinear electrokinetic theory. The propelling metallodielectric particles may find applications as microscopic mixers, "shuttles", self-propelling on-chip sensors, microactuators and microsensors in MEMS devices, optoelectronic devices or for separations in microfluidic devices.