Chiral nematic liquid crystals are known to form blue phases—liquid states of matter that exhibit ordered cubic arrangements of topological defects. Blue-phase specimens, however, are generally polycrystalline, consisting of randomly oriented domains that limit their performance in applications. A strategy that relies on nano-patterned substrates is presented here for preparation of stable, macroscopic single-crystal blue-phase materials. Different template designs are conceived to exert control over different planes of the blue-phase lattice orientation with respect to the underlying substrate. Experiments are then used to demonstrate that it is indeed possible to create stable single-crystal blue-phase domains with the desired orientation over large regions. These results provide a potential avenue to fully exploit the electro-optical properties of blue phases, which have been hindered by the existence of grain boundaries.

Blue phase (BP) liquid crystals are chiral fluids wherein millions of molecules self-assemble into cubic lattices that are on the order of hundred nanometers. As the unit cell sizes of BPs are comparable to the wavelength of light, they exhibit selective Bragg reflections in the visible. The exploitation of the photonic properties of BPs for technological applications is made possible through photopolymerization, a process that renders mechanical robustness and thermal stability. We review here the preparation and characterization of stimuli-responsive, polymeric photonic crystals based on BPs. We highlight recent studies that demonstrate the promise that polymerized BP photonic crystals hold for colorimetric sensing and dynamic light control. We review using Landau–de Gennes simulations for predicting the self-assembly of BPs and the potential for using theory to guide experimental design. Finally, opportunities for using BPs to synthesize new soft materials, such as highly structured polymer meshes, are discussed.

Significance: Blue phases represent distinct liquid states of matter having a high viscosity, finite shear modulus, and Bragg reflections in the visible spectrum. These properties arise from a highly ordered defect structure, unique amongst complex liquids, which is stable only over a narrow range of temperature. The number and characteristics of the corresponding unit cells could in principle be altered by confinement. In this work we show that the stability of blue phases can be increased by preparing them into small droplets. We demonstrate that defect structure, color, and morphology can be manipulated by controlling droplet size, temperature, and anchoring, thereby offering intriguing opportunities for optical devices based on chiral liquid crystals.

Significance: The processes that mediate crystal nucleation and growth, and the transformation between crystal structures having different lattice symmetries, are of fundamental importance to a wide range of scientific disciplines. Here, we use single crystals of liquid-crystal blue phases with a controlled orientation to study the liquid analog of a crystal–crystal transformation. In contrast to traditional atomic crystals, the transitions that arise in blue phases take place over submicron length scales. They do, however, occur in a diffusionless manner, with characteristics that are reminiscent of traditional martensitic transformations in atomic crystals.