Electric-field assembly of colloids with anisotropic interactions

Abstract

Assembly of colloidal particles has been a hot research topic for past two decades. Scientifically, these studies have significantly enriched our fundamental understanding in the physics of soft materials, including crystal nucleation and growth, phase transition, and glass formation. Practically, the assembled structures can uniquely interact with a broad range of electromagnetic waves and they are envisioned as important building blocks for metamaterials or photonic crystals with exotic properties. Previous studies, however, lack the diversity in the assembled structures, precise tunability of the colloidal interactions, and systematic investigation of different types of anisotropy. In this thesis, we apply external electric fields to manipulate and assemble anisotropic particles, particles that possess asymmetric properties in geometry, surface functionality, or chemical composition. We have obtained new and rich structures assembled from both spherical particles with anisotropic interactions and dimers with anisotropic material properties. Spherical colloids can acquire anisotropic dipolar interactions under AC electric fields and assemble into a variety of well-defined oligomers. Colloidal dimers with equal lobe sizes also show rich phase behavior and different assembly regimes. In particular, the formation of two dimensional close-packed crystals of perpendicularly aligned dimers shows promise in fabricating 3D photonic crystals based on dimer-like colloids. When dimers are asymmetric, the pair interaction is orientation-dependent. At low frequencies, two to four lying dimers associate closely with a central standing dimer and form chiral clusters. At high frequencies, we observe a series of novel structures that closely resemble one- and two-dimensional antiferromagnetic lattices. In addition to various types of equilibrium structures, we also discover a new particle propulsion mechanism that arises from the unbalanced electrohydrodynamic flow surrounding an asymmetric dielectric dimer when we tune the lobe size, chemical composition, and zeta potential on two lobes differently. Both the propulsion direction and speed can be conveniently modulated by field strength and frequency. The propulsion mechanism revealed here is not limited to colloidal dimers and should be universal for other types of asymmetric particles. Such knowledge is important for both building intelligent colloidal robots and studying the out-of-equilibrium behavior of active matter.