Tunable Two-Dimensional Crystals Comprising Aspherical Colloids

Aspherical colloidal particles were assembled in a reversible and tunable manner by means of dielectrophoresis (DEP). Spherical particles with single spherical cavities (referred as “dimples”) adopted lattices with non-closed packed cmm plane group symmetry and a packing fraction of 0.68 at low electric field strengths. With increasing electric field strength, the packing structure reverted to the entropically favored close-packed structure with p6m symmetry and a packing fraction of 0.90 typically observed for spherical colloids. Open packing structures were also achieved in systems comprising cubic hematite colloids with cylindrical arms protruding from each face, referred to as “hexapods”. Upon application of an electric field, perpendicular to the field direction, the arms of the hexapods prevented close packing. The presence of a magnetic field was found to anneal the hexapod crystals, decreasing the number of defects. The magnetic field could also be used to manipulate the orientation and packing structure of the crystals. These findings suggest that the interaction between external fields and aspherical colloidal geometries can be used to induced open-packed and tunable structures in two-dimensional crystals.