Electric-Field Induced Formation and Propulsion of Chiral Colloidal Clusters

Chirality is a fundamental concept present ubiquitously in the molecular world. Extending the chiral structure to microscopic objects such as colloidal particles has become increasingly desirable because the chiral arrangement of colloidal particles can exhibit collectively strong optical, electric, and magnetic responses than chiral molecules. In addition, chiral micro-particles, as the macroscopic analogue of chiral molecules, could be used to study crystallization or enantiomeric separation of a racemic mixture. Here we report using AC electric field directed-assembly method to assemble achiral building blocks, i.e., the asymmetric colloidal dimer, into chiral colloids.  At low frequencies, one standing dimer and several lying dimers attract each other and assemble into a variety of chiral clusters with different numbers of petals. Both right- and left-handed chiral clusters with equal populations have been observed. Our theoretical model reveals that the chiral configuration is energetically favorable due to dipolar attraction between the central dimer and surrounding petals. Interestingly, the broken symmetry induces an unbalanced electrohydrodynamic flow surrounding the chiral colloids and causes them to rotate in opposite directions according to their handedness.