(220b) How to Make Particles Interact in Multiple Directions and Form Interconnected Networks: Colloidal Assembly Based on Field-Induced Multipoles

Authors: 
Velev, O. D., North Carolina State University
Bharti, B., North Carolina State University
The assembly of new materials based on particle networks and ordered phases requires the application of â??interaction toolboxâ? that allows independent control of the particle interactions in multiple directions by external field(s). It also necessitates a closely coordinated experimental and theoretical effort, as the enormous parameter space of particle types, interactions, densities, equilibrium/non-equilibrium assembled states and corresponding dynamics are nearly impossible to explore by experimental means only. We will present the experimental and theoretical basis of two strategies for such multidirectional assembly. The first strategy is based on using complex asymmetric particles. Metallo-dielectric particles, such as Janus and patchy spheres and cubes, acquire complex polarization patterns in AC electric and magnetic fields, which can lead to multidirectional interactions. They can be controlled through the field frequency and strength. We will demonstrate how quadrupolar, hexapolar and multipolar polarization interactions induced by external electric and magnetic fields can be used to assemble new types of staggered chains, networks of unusual symmetry and crystals. We will show how the particle assembly motifs and long-scale structures can be analyzed via numerical modeling. The second strategy for multidirectional assembly is to use simple particles that polarize and interact in complex external fields. We show how this can be achieved by simultaneous application of electric and magnetic fields to dispersions of superparamagentic microspheres. We investigate the dynamics of the assembly process, and identify and classify the intermediate states formed. The concurrent field application lead to the formation of bidirectional chains, colloidal networks, and discrete crystals. The morphologies of these structures are in good correlation with theoretical predictions based on Brownian Dynamics simulations. We distinguish two separate kinetic routes for bidirectional assembly, colloidal â??polymerizationâ? and â??re-polymerizationâ?, leading to the formation of structures with specific symmetry, connectivity, directional response or unusual transport properties.