title: Assembly of optical-scale dumbbells into dense photonic crystals.
abstract: We describe the self-assembly of nonspherical particles into crystals with novel structure and optical properties combining a partial photonic band gap with birefringence that can be modulated by an external field or quenched by solvent evaporation. Specifically, we study symmetric optical-scale polymer dumbbells with an aspect ratio of 1.58. Hard particles with this geometry have been predicted to crystallize in equilibrium at high concentrations. However, unlike spherical particles, which readily crystallize in the bulk, previous experiments have shown that these dumbbells crystallize only under strong confinement. Here, we demonstrate the use of an external electric field to align and assemble the dumbbells to make a birefringent suspension with structural color. When the electric field is turned off, the dumbbells rapidly lose their orientational order and the color and birefringence quickly go away. In this way, dumbbells combine the structural color of photonic crystals with the field addressability of liquid crystals. In addition, we find that if the solvent is removed in the presence of an electric field, the particles self-assemble into a novel, dense crystalline packing hundreds of particles thick. Analysis of the crystal structure indicates that the dumbbells have a packing fraction of 0.7862, higher than the densest known packings of spheres and ellipsoids. We perform numerical experiments to more generally demonstrate the importance of controlling the orientation of anisotropic particles during a concentration quench to achieve long-range order.
authors: Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, USA.
authors: Jason D
authors: JD
authors: Forster
authors: Jin-Gyu
authors: JG
authors: Park
authors: Manish
authors: M
authors: Mittal
authors: Heeso
authors: H
authors: Noh
authors: Carl F
authors: CF
authors: Schreck
authors: Corey S
authors: CS
authors: O'Hern
authors: Hui
authors: H
authors: Cao
authors: Eric M
authors: EM
authors: Furst
authors: Eric R
authors: ER
authors: Dufresne
mesh_terms: D016880:Anisotropy; D003116:Color; D004560:Electricity; D050866:Liquid Crystals / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D055620:Optical Phenomena; D017785:Photons