Robust cholesteric liquid crystal elastomer fibres for mechanochromic textiles

in Nature Materials (2022), 21(12), 14411447

Mechanically responsive textiles have transformative potential in many areas from fashion to healthcare. Cholesteric liquid crystal elastomers have strong mechanochromic responses that offer attractive ... [more ▼]

Mechanically responsive textiles have transformative potential in many areas from fashion to healthcare. Cholesteric liquid crystal elastomers have strong mechanochromic responses that offer attractive opportunities for such applications. Nonetheless, making liquid crystalline elastomer fibres suitable for textiles is challenging since the Plateau–Rayleigh instability tends to break up precursor solutions into droplets. Here, we report a simple approach that balances the viscoelastic properties of the precursor solution to avoid this outcome and achieve long and mechanically robust cholesteric liquid crystal elastomer filaments. These filaments have fast, progressive and reversible mechanochromic responses, from red to blue (wavelength shift of 155 nm), when stretched up to 200%. Moreover, the fibres can be sewed into garments and withstand repeated stretching and regular machine washing. This approach and resulting fibres may be useful for applications in wearable technology and other areas benefiting from autonomous strain sensing or detection of critically strong deformations. [less ▲]

Encoding Hidden Information onto Surfaces Using Polymerized Cholesteric Spherical Reflectors

in Advanced Functional Materials (2021)

The omnidirectional Bragg reflection of cholesteric liquid crystals molded into spheres turns them into narrow-band retroreflectors with distinct circular polarization. It is shown that these cholesteric ... [more ▼]

The omnidirectional Bragg reflection of cholesteric liquid crystals molded into spheres turns them into narrow-band retroreflectors with distinct circular polarization. It is shown that these cholesteric spherical reflectors (CSRs) can encode information onto surfaces for far-field optical read-out without false positives, as the selective retroreflectivity allows the background to be easily subtracted. In order to hide the encoding from detection by the human eye, the retroreflection band is tuned to the near-UV or IR spectra, allowing ubiquitous deployment of CSRs in human-populated environments. This opens diverse application opportunities, for instance, in supporting safe robotic navigation and in augmented reality. A key breakthrough is our ability to permanently embed CSRs in a binder such that undesired scattering and reflections are minimized. This is achieved by realizing CSRs as shells that are polymerized from the liquid crystalline state. The resulting shrinkage around an incompressible fluid deforms the thinnest region of each shell such that it ruptures at a well-defined point. This leaves a single small hole in every CSR that gives access to the interior, allowing complete embedding in the binder with optimized refractive index, minimizing visibility. [less ▲]

Cholesteric Liquid Crystal Shells as Enabling Material for Information-Rich Design and Architecture.

in Advanced Materials (2018)

The responsive and dynamic character of liquid crystals (LCs), arising from their ability to self-organize into long-range ordered structures while maintaining fluidity, has given them a role as key ... [more ▼]

The responsive and dynamic character of liquid crystals (LCs), arising from their ability to self-organize into long-range ordered structures while maintaining fluidity, has given them a role as key enabling materials in the information technology that surrounds us today. Ongoing research hints at future LC-based technologies of entirely different types, for instance by taking advantage of the peculiar behavior of cholesteric liquid crystals (CLCs) subject to curvature. Spherical shells of CLC reflect light omnidirectionally with specific polarization and wavelength, tunable from the UV to the infrared (IR) range, with complex patterns arising when many of them are brought together. Here, these properties are analyzed and explained, and future application opportunities from an inter- disciplinary standpoint are discussed. By incorporating arrangements of CLC shells in smart facades or vehicle coatings, or in objects of high value subject to counterfeiting, game-changing future uses might arise in fields spanning infor- mation security, design, and architecture. The focus here is on the challenges of a digitized and information-rich future society where humans increasingly rely on technology and share their space with autonomous vehicles, drones, and robots. [less ▲]

Liquid crystals in micron-scale droplets, shells and fibers

in Journal of Physics : Condensed Matter (2017), 29

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this ... [more ▼]

The extraordinary responsiveness and large diversity of self-assembled structures of liquid crystals are well documented and they have been extensively used in devices like displays. For long, this application route strongly influenced academic research, which frequently focused on the performance of liquid crystals in display-like geometries, typically between flat, rigid substrates of glass or similar solids. Today a new trend is clearly visible, where liquid crystals confined within curved, often soft and flexible, interfaces are in focus. Innovation in microfluidic technology has opened for high-throughput production of liquid crystal droplets or shells with exquisite monodispersity, and modern characterization methods allow detailed analysis of complex director arrangements. The introduction of electrospinning in liquid crystal research has enabled encapsulation in optically transparent polymeric cylinders with very small radius, allowing studies of confinement effects that were not easily accessible before. It also opened the prospect of functionalizing textile fibers with liquid crystals in the core, triggering activities that target wearable devices with true textile form factor for seamless integration in clothing. Together, these developments have brought issues center stage that might previously have been considered esoteric, like the interaction of topological defects on spherical surfaces, saddle-splay curvature-induced spontaneous chiral symmetry breaking, or the non-trivial shape changes of curved liquid crystal elastomers with non-uniform director fields that undergo a phase transition to an isotropic state. The new research thrusts are motivated equally by the intriguing soft matter physics showcased by liquid crystals in these unconventional geometries, and by the many novel application opportunities that arise when we can reproducibly manufacture these systems on a commercial scale. This review attempts to summarize the current understanding of liquid crystals in spherical and cylindrical geometry, the state of the art of producing such samples, as well as the perspectives for innovative applications that have been put forward. [less ▲]

Transmission polarized optical microscopy of short-pitch cholesteric liquid crystal shells

in Proceedings of SPIE - The International Society for Optical Engineering (2016, March 07), 9769

We recently demonstrated that colloidal crystal arrangements of monodisperse droplets or shells of planar-aligned cholesteric liquid crystal exhibit intricate patterns of circularly polarized reflection ... [more ▼]

We recently demonstrated that colloidal crystal arrangements of monodisperse droplets or shells of planar-aligned cholesteric liquid crystal exhibit intricate patterns of circularly polarized reflection spots of different colors. The spots appear as a result of photonic cross communication between droplets, hence the patterns reflect the macroscopic arrangement of droplets or shells. Apart from being an interesting optical phenomenon, it offers attractive application opportunities in photonics and beyond, due to the unique characteristics of the patterns. It turns out that the optical quality of shells is much enhanced compared to that of droplets, hence we focus our attention primarily on shells, of varying thickness. Here we analyze and explain the intriguing textures arising when studying planar-aligned short-pitch cholesteric shells in transmission polarizing optical microscopy. In this case, the texture reflects the properties of each individual shell, without any sign of cross communication, yet also this pattern holds some fascinating mysteries. These can only be elucidated by considering all the peculiar optical properties of cholesterics together, as well as the unusual situation given by the spherical shell geometry. [less ▲]