Colloidal structures in confined nematics offer novel routes for designing complex optical materials with micrometre and submicrometre functionality. In this paper, we review some of our recently assembled colloidal structures that form in confined nematic cells. We present effective elastic binding via nematic distortion as a mechanism for the assembly of two-dimensional colloidal crystals of elastic dipoles and elastic quadrupoles. We introduce entangled colloids as novel types of structures, where particles are topologically bound by delocalised defect loops, producing robust and possibly chiral structures. The concept of hierarchical assembly is demonstrated in colloids with particles of various scales. In cholesteric blue phases, the assembly of three-dimensional colloidal crystals is shown based on naturally occurring three-dimensional arrays of trapping sites produced by blue phases.Keywords: colloids; nematic liquid crystal; self-assembly; entanglement; blue phases
IntroductionLiquid crystal colloids are novel materials (1-3) with promising applications in optics, including photonic crystals (4), tunable lasers (5) and metamaterials (6). They can be used to design optical patterns at scales ranging from nanometres to micrometres, using selfassembly (7), application of external fields (8, 9) and trapping by domain boundaries (10).The current main research focus in liquid crystal colloids is on nematic-based materials, which can provide for large-scale assembly of chains (2), clusters (11), two-dimensional (2D) lattices at interfaces (12, 13) and 2D colloidal crystals (7). Additionally, such colloidal assemblies have been functionalised by using facetted particles (14, 15), ellipsoids (16), rods (17) and Janus particles (18). Recently, chiral cholesteric and blue phase colloids are also attracting growing interest. Self-organisation of planar particles in cholesteric (19), size-dependent defects and non-Stokesian microrheology (20), stabilisation of blue phases by nanoparticles (21, 22) and stabilisation of blue phases by guest components (23) have been demonstrated. This growing interest in blue phases was started by applications and recent advances in materials synthesis. Blue phase materials with a wide temperature range have been synthesised (24), where flexoelectricity was interpreted as an important contributor to the phase stabilisation (25). Blue phase materials can yield high switching times, which is attractive for display applications (26).