Chiral molecules with opposite handedness exhibit distinct physical, chemical, and biological properties. They pose significant challenges as well as opportunities in understanding the phase behavior of soft condensed matter, designing enantioselective catalysts, and manufacturing single-handed pharmaceuticals. Microscopic particles, arranged in a chiral configuration, could also exhibit unusual optical, electric, and magnetic responses that are essential to build metamaterials with exotic properties. Here we report a surprisingly simple method to assemble achiral building blocks, the asymmetric colloidal dimers, into a family of chiral clusters. Under a perpendicularly applied AC electric field, two to four lying dimers associate closely with a central standing dimer and form both right- and left-handed clusters on a conducting substrate. Although the assembly is initiated by an electrohydrodynamic flow, the final configuration of the cluster is primarily determined by the dipolar interactions between constituent dimers. Our theoretical model accounting for interactions between induced dipoles reveals that in-plane dipolar repulsions between petals favor the achiral configuration, while out-of-plane attractions between the central dimer and surrounding petals favor a chiral arrangement. It is the competition between these two interactions that dictates the final configuration. The theoretical chirality phase diagram vs dimer bond length and lobe diameter ratio is found to be in excellent agreement with experimental observations.  We further demonstrate that the broken symmetry induces an unbalanced electrohydrodynamic flow surrounding the chiral clusters. As a result, they behave like micro-propellers and rotate in opposite directions according to their handedness. The assembly mechanism reported here could be applied to other types of asymmetric particles under electric fields. Such chiral colloids could be used for fabricating metamaterials, model systems for chiral molecules and active matter, and propellers for microscale transport.

Electric-field assembly and propulsion of chiral colloidal clusters

249th National Meeting (2015)

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