Behaviour of iron oxide (Fe3O4) Janus particles in overlapping external AC electric and static magnetic fields

The assembly behaviour of Fe3O4-capped Janus particles with magnetically and electrically anisotropic properties is investigated in overlapping external AC electric and static magnetic fields. Generally, Fe3O4 Janus particles have been shown to form double and staggered chains in external static magnetic and AC electric fields, respectively. Here, the reaction of Fe3O4 Janus particles to sequentially applied AC electric and static magnetic fields in parallel and perpendicular field geometry is studied in order to elucidate to what extend the asymmetric nature of the Janus particle can be addressed independently by each field during assembly. A parallel field geometry leads to contraction of the Janus particle chains, whereas a perpendicular field geometry leads first to the rotation of the Janus particle chains towards the more dominant field followed by rhombohedral stacking of the chains into two-dimensional, two-particle thick sheets as a result of the weaker field. The Janus particle assembly behaviour in each field geometry tested is parameterized by the characteristic angle, theta, and the inter-particle distance, d. The observed response of the Fe3O4 Janus particles is shown to be driven by the interaction of electric and magnetic dipoles generated in the caps by the external AC electric and static magnetic fields, respectively, as well as the polarization of the polystyrene particle. Reversibility of the assembly behaviour correlates strongly with the fact that the electric dipoles disappear immediately after the AC electric field is turned off, whereas the magnetic dipoles remain permanently in the Fe3O4 caps even after the static magnetic field is removed. As a result, the Fe3O4 Janus particle assembly behaviour is responsive to both the electric and the magnetic fields and their behaviour can be manipulated by the sequence and relative orientation of the external fields.