(183d) Assembly of Colloidal Clusters with Homochirality Under Combined Electric and Magnetic Fields

Research involving anisotropic particles has shown great potential in applications ranging from colloidal assembly and microrobot design to medical therapy and drug delivery. One of the more important applications involves the assembly of chiral colloidal clusters, which have potential use as sensors to detect molecules or metamaterials with exotic optical properties. As we have previously reported, applying electric fields can assemble dielectric dumbbell-shaped particles into chiral clusters. However, we typically obtained right- and left-handed clusters with almost equal populations, which is not surprising since they are energetically equivalent. To make homochiral clusters, we coated the dumbbell particles with a thin magnetic layer, which are responsive to both electric and magnetic fields. When we apply an AC electric field perpendicular to the substrate, the particles assemble into chiral clusters with opposite chiralities. However, when we add an additional in-plane circularly rotating magnetic field, the chirality of the particles changed according to the rotating direction of the magnetic field. We find that by controlling the direction of the magnetic field, we can reversibly switch the chirality and rotation of the clusters. In order to gain a more fundamental understanding of our experimental observations, we conduct Brownian Dynamics simulations in which Lennard-Jones, dipolar, electrohydrodynamic, and magnetic forces are present among dumbbells. Chiral clusters are subsequently assembled in response to an orthogonal electric field, while cluster rotation is controlled using an in-plane rotating field. The impact of a variety of tunable parameters including the field strength, frequency, and particle aspect ratio have been investigated and they compare favorably with our experiment observations. Our method provides a convenient route to produce chiral colloidal cluster with single handedness.