Exploiting imperfections: Directed assembly of surface colloids via bulk topological defects

TL;DR

This paper demonstrates how bulk topological defects in confined nematic liquid crystals can be exploited to assemble colloidal particles into reconfigurable, complex structures with potential applications in dynamic material design.

Contribution

It introduces a novel method of directing colloidal assembly using bulk defect structures in liquid crystals, enabling reconfigurable and complex arrangements.

Findings

Defect rings in nematic liquid crystals can organize colloids into ring-like structures.

High surface density colloids form rings near defects and hexagonal lattices elsewhere.

Reconfigurable defect structures enable dynamic control of colloidal assemblies.

Abstract

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.