Electrorotation of a metallic coated Janus particle under AC electric fields

Electrorotations (EROTs) of the Pt-silica Janus particles are measured in different conditions under rotating electric fields. Unlike simple particles, we find that the rotation direction of a Janus particle is mainly opposite to the direction of the electric field (counter-field), which is similar to the metallic particles, while the rotation direction may reverse from counter-field to co-field at the low-frequency region (<1 kHz) and high-frequency region (>1 MHz), depending on the thickness of metallic coating and conductivities of solutions. We also find that EROT of a Janus particle reaches a maximum angular speed at a characteristic frequency, which increases with the thickness of metallic coating and can be one order higher than that of a fully metallic coated particle. These results suggest that the EROT responses of a Janus particle have both dielectric and metallic features and these responses are not simply averaged responses of its both sides. Half side metallic coating reduces the time of polarization due to the lack of fully electric field screening ability comparing with the metallic particles. The special properties of polarization of a Janus particle under electric fields may provide a method to create designable micro-rotors or active particles for applications. Published by AIP Publishing.