title: Exploiting imperfections in the bulk to direct assembly of surface colloids.
abstract: We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals (LCs) to assemble colloidal particles trapped at the LC interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic LC 4-cyano-4'-pentylbiphenyl create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here, we report on complex open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic LC in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Because topographically complex substrates are easily fabricated and LC defects are readily reconfigured, this work lays the foundation for a versatile, robust mechanism to direct assembly dynamically over large areas by controlling surface anchoring and associated bulk defect structure.
authors.affiliation: Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19104.
authors.forename: Marcello
authors.initials: M
authors.lastname: Cavallaro
authors.forename: Mohamed A
authors.initials: MA
authors.lastname: Gharbi
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Beller
authors.forename: Simon
authors.initials: S
authors.lastname: Čopar
authors.forename: Zheng
authors.initials: Z
authors.lastname: Shi
authors.forename: Tobias
authors.initials: T
authors.lastname: Baumgart
authors.forename: Shu
authors.initials: S
authors.lastname: Yang
authors.forename: Randall D
authors.initials: RD
authors.lastname: Kamien
authors.forename: Kathleen J
authors.initials: KJ
authors.lastname: Stebe
mesh_terms: D000388:Air; D001713:Biphenyl Compounds; D003102:Colloids / Q000737:chemistry*; D055119:Elastic Modulus; D050866:Liquid Crystals / Q000737:chemistry*; D046911:Macromolecular Substances / Q000138:chemical synthesis* / Q000737:chemistry; D008956:Models, Chemical; D008968:Molecular Conformation; D009570:Nitriles
chemical_list: D001713:Biphenyl Compounds; D003102:Colloids; D046911:Macromolecular Substances; D009570:Nitriles; C433919:4-cyano-4'-pentylbiphenyl
keywords: 2D superstructures; directed assembly; elastic interaction; nematic interface; topology