Research involving anisotropic particles has shown great potential in applications ranging from colloidal assembly to microrobot. In particular, the assembly of chiral colloidal clusters has potential applications in sensors to detect chiral molecules or metamaterials with exotic optical properties. Here, we report the assembly of colloidal dimers into chiral clusters by applying a low-frequency AC electric field. Interestingly, the assembly relies on the active motion of dimers which arises from the unbalanced electrohydrodynamic flow surrounding the particles. Right- and left-handed clusters with almost equal populations were obtained because they are energetically equivalent. To make homochiral clusters, we superimpose an in-plane circularly rotating magnetic field. The chirality of 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 and induced rotation under the rotating magnetic field. Depending on the magnetic field direction, two rotation effects can be additive or competitive. We also perform Brownian Dynamics simulations in which Lennard-Jones, dipolar, electrohydrodynamic, and magnetic forces are involved to gain a more fundamental understanding of our experimental observations. The impact of various 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.

Active assembly of homochiral colloids under orthogonally applied electric and magnetic fields

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