  We exploit the long-ranged elastic fields inherent to confined nematic liquid
crystals to assemble colloidal particles trapped at the liquid crystal
interface into reconfigurable structures with complex symmetries and packings.
Spherical colloids with homeotropic anchoring trapped at the interface between
air and the nematic liquid crystal 5CB 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 liquid crystal 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. Since
topographically complex substrates are easily fabricated and liquid crystal
defects are readily reconfigured, this work lays the foundation for a new,
robust mechanism to dynamically direct assembly over large areas by controlling
surface anchoring and associated bulk defect structure.
