Electric-field–induced assembly and propulsion of chiral colloidal clusters

Abstract

Chiral molecules with opposite handedness exhibit distinct physical, chemical, or biological properties. They pose challenges as well as opportunities in understanding the phase behavior of soft matter, designing enantioselective catalysts, and manufacturing single-handed pharmaceuticals. Microscopic particles, arranged in a chiral configuration, could also exhibit unusual optical, electric, or magnetic responses. Here we report a simple method to assemble achiral building blocks, i.e., the asymmetric colloidal dimers, into a family of chiral clusters. Under alternating current electric fields, two to four lying dimers associate closely with a central standing dimer and form both right-and left-handed clusters on a conducting substrate. The cluster configuration is primarily determined by the induced dipolar interactions between constituent dimers. Our theoretical model reveals that in-plane dipolar repulsion between petals in the cluster favors the achiral configuration, whereas outof-plane attraction between the central dimer and surrounding petals favors a chiral arrangement. It is the competition between these two interactions that dictates the final configuration. The theoretical chirality phase diagram is found to be in excellent agreement with experimental observations. We further demonstrate that the broken symmetry in chiral clusters induces an unbalanced electrohydrodynamic flow surrounding them. As a result, they rotate in opposite directions according to their handedness. Both the assembly and propulsion mechanisms revealed here can be potentially applied to other types of asymmetric particles. Such kinds of chiral colloids will be useful for fabricating metamaterials, making model systems for both chiral molecules and active matter, or building propellers for microscale transport.hirality is a fundamental concept presented ubiquitously in the molecular world. For example, small molecules such as the amino acids, phospholipids, and sugars with specific handedness build many biomacromolecules whose chirality is essential to living organisms. Although the right-and left-handed molecules are identical in chemical composition, the catalytic activity (1), pharmacological impact (2, 3), biological recognition (4), and optical response (5) can be strikingly different. Extending the chiral structure to microscopic objects such as colloids has become increasingly desirable for several reasons. First, the chiral arrangement of colloidal particles can exhibit unusually strong optical, electric, and magnetic responses (6-10) that are not manifested either at the single-particle level or in achiral forms. Therefore, chiral clusters can be potentially used to build metamaterials (11-14) with exotic properties or sensors for detection of molecules. Second, chiral particles can be conveniently characterized by real-time optical microscopy. As the macroscopic analog of chiral molecules, they can be used to study fundamental questions related to the crystallization (15) or enantiomeric separation (16, 17) of a racemic...

Emergence of collective dynamical chirality for achiral active particles

“…For instance, establishment of leftright asymmetry in embryonic development, one of the most intriguing biological phenomena, involves coordinated activity of many cells [15,16] where the ability of cells to distinguish between left and right is evident in systems of chiral patterns formed by collective motion of identical cells confined in circular island or ring/stripeshaped micropatterns [17][18][19][20]. On the other hand, CDC may also inspire new routines for fabrication of complex chiral architectures by dynamically self-assembling simple and achiral building blocks [21], e.g., chiral clusters of asymmetric colloidal dimers have been successfully assembled by using alternating current electric fields [22]. Revealing how CDC arises from groups of active units is then very important for the understanding of the formation mechanism.…”

A spatially adaptive high-order meshless method for fluid–structure interactions

“…Lastly, we solved a more complicated but interesting problem, regarding the collective dynamics of multiple active asymmetric colloidal dimers. This study was motivated by an experimental work [3], where each dimer was composed of two overlapped spheres of different radii. As the dynamics of interest for these dimers occurred only along the substrate plane in the experiment, it may be posed as a 2D problem.…”

“…The fluid was assumed to be water with ρ = 1.0 × 10 3 kg/m 3 and ν = 1.0 × 10 −6 m 2 /s. Due to the small length scale and slow dynamics [3], this may be treated as a Stokes problem.…”

All-optical reconfigurable chiral meta-molecules

“…The diversity of materials, sizes, and functionality of colloids provides abundant building blocks for the assembly of complex chiral colloidal molecules [18][19][20]. For example, using magnetic colloids or asymmetric colloidal dimers as building blocks, chiral colloidal molecules have been successfully synthesized via an external magnetic field [21] or electric field [22]. Atomic force microscopy has been used to manipulate and assemble metallic colloids into chiral structure [23], while it is not an efficient approach with extremely low throughput.…”