Interactions and self-assembly of anisotropic colloidal particles in electric fields

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0930549FurstRecent results in our laboratory demonstrate the surprisingly rich role particle shape has on the disorder to order transition of anisotropic particles in electric fields. These suggest new routes to forming complex, higher order structures from dispersions via self assembly. While spherical particles rapidly and reversibly form ordered hexagonal close packed arrays in AC electric fields, colloidal ellipsoids and dicolloids, particles resembling two fused spheres, can form unique aggregate geometries (chains at an angle with the field, particle chains with alternating orientations) and open, ordered arrays with a centered rectangular symmetry. However, particle shape may also play a critical role in the self assembly kinetics by frustrating the path of the disorder to order transition. For instance, the the lack of registry between chains of dicolloid particles initially formed in the field direction frustrates assembly into centered rectangular arrays. However, this also suggests unique possibilities for creating complex colloidal structures using mixtures of spherical and anisotropic colloids that resemble molecular compounds. In this work, we will study the field directed self assembly of homo-dicolloid particles with symmetric lobes. Even this relatively simple anisotropic shape leads to complex interparticle interactions, packing, and self assembly kinetics. We will exploit the large parameter space to create new, complex colloidal structures. This includes varying the degree of separation between the particle lobes, from slightly aspherical to kissing spheres, and altering the bulk dielectric, surface chemistry or surface conductivity of the particles, even making Janus dicolloids, using adsorbed polymers, surfactants or particle monomer chemistries. We will study the order disorder transition, including the characterization of self assembled structures and kinetics, as well as the the field induced interactions between anisotropic particles. The latter will elucidate the mechanisms of the particle polarizability and the roles of the double layer, bulk conductivity, particle dielectric properties and particle surface conductivity. Combined with the physical insight provided by our previous work on direct colloidal interaction measurements between spherical particles, this will enable us to understand and control the fieldinduced colloidal interactions on the molecular level to tailor particle self assembly. Furthermore, by mixing particles with different polarizabilities, which controls particle orientation in the field, self assembled structures with even greater complexity may be attainable. Other novel aspects of field directed assembly will be developed, including pulsed fields to anneal structures and assisted assembly using holographic optical tweezers.Intellectual Merit: Solution phase self assembly promises to be the technologically and economically optimal approach in the realization of industrial scale nano materials and devices. In essence, harnessing self assembly for man made applications mimics nature's route to the formation of functional nanostructures. The goal of this work is to develop and fundamentally validate novel approaches to self assembled structures using colloidal building blocks and external fields. We will discover new routes to forming complex self assembled structures and gain a fundamental understanding of the underlying mechanisms of particle interactions and self assembly in electric fields. The latter will lead to broad scalability of our findings across a vast parameter space of physico chemical conditions, including particle size, shape, composition (dielectric properties), surface chemistry and solution conditions.Broader impacts. In addition to the broad technical impacts, the proposed work will develop the human resources needed to sustain and grow national excellence in the science and engineering of colloidal and nanoparticle suspensions. The education and outreach impact will be amplified by sponsoring a secondary school Science, Technology, Engineering, and Mathematics (STEM) teacher as a summer research fellow in our laboratory, in coordination with the Delaware?s NSF sponsored Nature InSpired Engineering Research Experiences for Teachers (NISE-RET) program.