Electrorotation of Metallic Microspheres

Electrorotation (ER) experiments of gold-coated micrometer-sized spheres suspended in an electrolyte are presented for three different ionic conductivities over the frequency range of 100 Hz to 40 MHz. The direction of rotation was observed to be counter-field (opposite to the rotating field vector) with a single rotation peak. The maximum in rotation occurs for a frequency on the order of the reciprocal RC time constant for charging the double layer at the gold surface. Dielectrophoresis (DEP) experiments showed that the gold-coated particles undergo negative DEP at low frequencies and positive DEP at high frequencies with the same relaxation frequency as electrorotation. No induced charge electrophoresis was noticeable in ER or DEP experiments.