(392d) Assembly of “Anisotropic” Colloidal Dimers and Spheres Under Applied Electric Fields

Colloids possessing anisotropic interactions could potentially assemble into a much wider range of crystalline arrays and meso-structures than spherical particles with isotropic interactions. Here, we investigate the impact of geometric anisotropy on the assembly of colloidal dimers on conducting substrates under external electric fields. By systematically tuning the size ratio between two lobes on dimers, we have found that interactions between dimers strongly depend on their relative orientations. For example, the interaction between lying and standing dimers on the substrate is attractive. When all dimers stand on the substrate, the interaction between neighboring dimers with alternating orientations is also attractive. Otherwise, it is repulsive. Such kind of orientation-dependent interactions generate a good variety of new structures, such as chiral clusters and dimer crystals with alternating orientations. We will discuss the physical origin of those orientation-dependent interactions and the impacts of experimental conditions such as the ionic concentrations and surface charges. Our numerical model based on electrostatics agrees well with experimental observations and provide further insights on electric-field assisted assembly of anisotropic particles. 

In addition, the assembly of isotropic spherical colloids under applied electric fields is investigated. By applying an external AC electric field, we show that the apparently isotropic particles experience “anisotropic” interactions. New types of sphere-packing have been observed within a previously unexplored experimental regime: low salt concentrations (<10^-3M) and low frequency regime (100 Hz to 10 kHz). At low particles concentrations, a family of well-defined oligomers, ranging from 3 to 10 was observed. At high particles concentrations, the colloidal clusters will further assemble and connect themselves into a good variety of two-dimensional non-close-packed networks. We attribute these new types of non-planar structures to the competition among double layer repulsion, dielectrophoretic attraction, and dipolar interactions. The double layer and in-plane dipolar repulsion could make bottom particles in the clusters separate from each other. While the out-of-plane dipolar attraction and particle-substrate dielectrophoretic attraction could be responsible for the formation of the clusters, i.e., the top central sphere is associated with the bottom spheres. The effects of salt concentration and frequency on the geometry of those colloidal molecules will be discussed. These non-close-packed structures could be used as building blocks for making photonic crystals and plasmonic structures.