Innovative Applications Enabled by the Versatile Structural Color of Cholesteric Liquid Crystals

Cholesteric liquid crystals (CLCs) are famous for their ability to self-assemble into Bragg reflectors of visible light, yielding intense structural color with a single circular polarization, despite flowing like a liquid. This review focuses on a selection of entirely new opportunities to apply CLCs to solve problems with high societal and industrial relevance, as demonstrated in proof-of-concept experiments with a transition to commercial application underway, in contexts quite far from the more traditional applied role of CLCs as thermometers. We now see a renaissance of applied CLC research resulting in exciting new functional materials taking advantage of CLC photonics, often displaying unique types of responsiveness. This development has been enabled, first, by recent advances in formulating CLC mixtures with reactive mesogens such that they can be processed as a liquid but used as a hard glass or rubber after polymerization and cross-linking, keeping the photonic performance generated by CLC self-assembly intact. Second, the rapid development of advanced liquid processing methods like microfluidic production of multiple emulsions, 3D printing and composite fiber spinning have allowed the CLCs to be processed into unconventional form factors prior to cross-linking. The review focuses, first, on CLC-templated hard spheres exhibiting omnidirectional circularly polarized Bragg reflection, so-called Cholesteric Spherical Reflectors, or CSRs. They can be used to make artificial "fingerprints" for physical objects that act as Physical Unclonable Functions, of great interest in secure authentication, or to print QR-codes or similar machine-readable patterns in a way that they remain invisible to humans while appearing to the intended machines with exceptional contrast. Since each CSR is effectively a pixel of structural color, we can also use them as a versatile solution for coloring without absorption or scattering, also enabling nonspectral colors like shades of gray that are normally not obtainable with structural color. A related application discussed is the camouflage of solar panels using polymerized CLC films to replace their visually obtrusive black appearance with color generated by CLCs, with almost no loss of energy conversion efficiency thanks to its origin in Bragg reflection. We then move to soft rubbery CLC elastomer (CLCE) films and fibers which change their color in response to strain. We highlight a new application opportunity in structural health monitoring, demonstrated by coating CLCE films onto surfaces where we wish to detect crack formation, e.g., in reinforced concrete constructions: the localized strain in the CLCE where a crack appears leads to a strong color change that allows immediate detection of the crack, whereas the crack in the uncoated surface remains invisible until it has grown to much greater width. The colorimetric strain monitoring is also possible with CLCE fibers, where the 1D form factor lends itself to applications in, e.g., fashion, medicine and sports. We end by discussing the key remaining challenges, in particular related to scale-up of production.