(366d) Directed Assembly of Colloidal Dimers Under Electric Field

Anisotropic colloids are colloids that have been tailored for their presence of anisotropy in chemical compositions, surface functionalities, geometric sizes, or shapes. Therefore, they can recognize each other and assemble themselves via directional and specific interactions, potentially making large numbers of close-packed and non-close-packed crystalline structures that the isotropic spheres cannot form. In this talk, we will present our results in the directed assembly of the simplest form of anisotropic particles, colloidal dimers, under electric field. A thin film of dimer suspension is sandwiched between two conducting substrates, and an electric field perpendicular to the substrate is applied. We have studied the responses of both individual dimers and their collective motions under DC and AC electric field.   

We have found rich behaviors in terms of both equilibrium phases and transient states. Under a DC field, individual dimers always orient perpendicularly to the electric field (parallel to the substrate) and they can aggregate into amorphous or semi-crystalline solids when the field strength is high enough. Increasing the frequency of the electric field, however, enables individual dimer particles to rotate and re-orient themselves in the direction parallel to the electric field (perpendicular to the substrate, i.e., “standing” dimer). Large areas of “standing” dimer crystals can be created within a window of intermediate frequencies. At even higher frequencies, the lateral dipole-dipole repulsions between dimers become dominant so that vertical chains of dimers span between top and bottom substrates. In addition, other complex patterns and phase behaviors will be presented. The experimental observations can be explained based on the electrokinetic theory of non-spherical colloids in bulk and electro-hydrodynamic flow near the substrate. The observed two- dimensional crystals of aligned dimers indicates a promising route to making photonic crystals using colloidal dimers as the building blocks.