Curvature-Induced Instability in Crystallization of Chiral Liquid Crystals

Crystallization in curved entities such as viral capsids, Radiolaria, and ice-freezing is ubiquitous in nature, wherein the spatial coordination of constituent materials adjusts to adequately accommodate within curvature, resulting in exotic collective properties. Herein, we investigated the nucleation and growth of blue phase (BPs) soft crystals within curved topological confinement. BPs represent 3D periodic cubic crystals while they have fluidity. They have the potential to assemble into two crystalline symmetries: BPI with a body-centered cubic structure and BPII with a simple cubic structure. These cubic crystal lattices are a few hundred nanometers and show selective Bragg's reflection with rapid sub-millisecond response times, which makes them attractive for sensing and photonic applications. They, however, exist in a very narrow temperature range and generate polycrystalline structures when confined in flat films, preventing their widespread use in most practical applications. Moreover, the growing interest in the integration of these ordered materials into miniaturized, flexible devices brings about the need to understand the effect of geometrical confinement and curved boundaries on the structure of BPs and their stability and photonic responses.<br/>To understand how the interplay of curvature, confinement, and surface anchoring impacts the structural stability and optical response of BPs, we have confined BPs within core-shell and microdroplets with precise size and shell thicknesses. Our observations in shells indicate that increased curvature and strong spatial confinement destabilize BPI promoting its transition into configuration and optical characteristics resembling BPII. Although BPs in shells appeared polycrystalline, the inherent 3D symmetrical curvature in microdroplets facilitated the formation of monodomain crystals, eliminating the need for any specific surface treatments. Moreover, using photo-polymerization technique to stabilize defects in droplets led to an expanded thermal stability range for BPs. Yet, the phase transition temperature in these microdroplets was found to be strongly size-dependent. We have delved into the underlying principles of these molecular arrangements through both experimental and theoretical simulations. These findings can pave the way for designing optically active microstructures, addressing BP-related challenges, offering potential for cutting-edge photonic and sensing applications especially in flexible and wearable devices.