(489g) 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. In particular, the assembly of chiral colloidal clusters has potential applications in sensors to detect chiral 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 understandable since they are energetically equivalent. To make homochiral clusters, we applied the layer-by-layer (LBL) method to coat Fe₃O₄ magnetic nanoparticles (MNP) on the dumbbell-shape polystyrene dimer particles, which are responsive to both electric and magnetic fields. The advantage of the LBL method allows us to coat multiple layers of MNP on the dimer particles to tune the magnetic response. When we apply an AC electric field perpendicular to the substrate, the particles assemble into chiral clusters with opposite chiralities. However, when we superimpose an in-plane circularly rotating magnetic field, the chirality of the particles changes 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. The chiral cluster will experience self-rotation under AC electric field due to the electrohydrodynamics (EHD) and induced rotation under the rotating magnetic field. Two rotation effects can be additive or competitive depending on the rotating direction. To gain a more fundamental understanding of our experimental observations, we also conduct Brownian Dynamics simulations in which Lennard-Jones, dipolar, electrohydrodynamic, and magnetic forces are involved. The impact of a variety of tunable parameters, including the field strength, frequency, and particle aspect ratio, have been investigated. They compare favorably with our experimental observations. Our method provides a convenient route for producing chiral colloidal clusters with single-handedness.