(128f) Electric Field Directed Assembly of Anisotropic Colloids

Colloids with anisotropic interactions are good mimics of molecules. They could potentially assemble into more complex and unique structures which are impossible for isotropic spherical particles. Here we study the impact of geometric anisotropy on the assembly of asymmetric colloidal dimers under an external AC electric field. Experimental results clearly indicate that the assembly behaviors of asymmetric dimers strongly depend on their orientations. For example, the standing (lying) and standing (lying) dimers are well separated, while lying and standing dimers attract each other and assemble into a variety of chiral clusters, such as chiral trimers, tetramers, and pentamers. When all dimers stand on the substrate, dimers with opposite standing orientations attract each other and form one- and two- dimensional Ising lattice structures, including small clusters, linear chains, and squares. On the other hand, dimers with same standing orientations always repel each other. Our theoretical model based on electrostatic interactions agrees well with experimental results and provide further insights on electric-field assisted assembly of anisotropic particles. 

The assembly of isotropic spherical colloids under external electric field is also investigated. Although the spherical particles are isotropic, “anisotropic” interactions between spherical particles can still be generated by applying an external AC electric field. This experiment is done within a previously unexplored experimental regime: low salt concentrations and low frequencies. The spherical particles assemble into well defined zig-zag oligomers from trimers, tetramers, to nonamers. Moreover, these oligomers can further connect and form two-dimensional non-close-packed networks.  The assembly behaviors of spherical particles can be explained by the delicate balance among 1) offset dipolar attraction between top and bottom spheres, 2) dielectrophoretic attraction between top sphere and substrate, and 3) double layer repulsion between bottom spheres. These non-close-packed structures could be used as building blocks for making photonic crystals and plasmonic structures.