Understanding and Controlling Anisotropic Particle Assembly using Electric Fields

Understanding the ensemble microstructure, interactions, and dynamics of anisotropic particle structure addresses the broader goal of assembling complex and hierarchical micro- and nano-structures. Hierarchical and reconfigurable materials from define emerging synthetic meta-materials with non-trivial optical and interfacial properties including photonic crystals, sensors, and biomimicry applications. To create these structures, bottom-up approaches like electric field-directed self-assembly allow for rapid fabrication and controlled reconfiguration of diverse microstructures using scalable arrays. While particle-scale manipulation has been demonstrated on the micro- and nano-scale, there is a lack of fundamental understanding of the interactions driving assembly and reconfiguration in anisotropic particle systems. First, we uncover analytical potentials for dipole-field and dipole-dipole interactions of anisotropic particles in high frequency AC electric fields by comparing experimental and simulated epoxy colloidal particles. Optimal fits from inverse analysis show two potentials that capture the system well using ellipsoidal or spherical dipole model. Next, we quantify phases of superellipse particles in two-dimensions in simulation. By investing large systems of disks, squares, rectangles, ellipses, and rhombuses, we establish dominant trends in phases based on curvature and symmetry while cataloging benchmark shapes. Additionally, short-range order parameters are compared to traditional correlation functions when quantifying structural features. Then, we create these stable phases experimentally in electric fields using monolayer assemblies of disks, ellipses, squares, and rectangles. We quantify the formation of liquid, liquid crystal and crystal phases using image analysis based on particle position and orientation. The net superposition of dipole-field, dipole-dipole and hard-body interactions allows for faster and more diverse microstructures than with gravitational assembly alone. Focusing on rectangular particles, we kinetically transition between phases while uncovering other metastable states depending on the applied electric field including string fluid, binematic, smectic, and four-fold crystal states. We report accessible and inaccessible transition pathways depending on the staring and target state while demonstrating real-time reconfiguration. Overall, this work demonstrates how a fundamental understanding of interactions and microstructure can lead to proof-of-principle reconfiguration of anisotropic structures in real-time and real-space as a foundation for ordered metamaterials