Cholesteric Research Articles

Effect of smectic polymers on cholesteric liquid crystals

Effect of smectic polymers on cholesteric liquid crystals

Stable Non-equilibrium Structures in Chiral Nematics under Microfluidic Flow.

Cholesteric liquid crystals (CLCs) are compelling responsive materials with applications in next-generation sensing, imaging, and display technologies. While electric fields and surface treatments have been used to manipulate the molecular organization and, subsequently, the optical properties of CLCs, their response to controlled fluid flow has remained largely unexplored. Here, we investigate the influence of microfluidic flow on the structure of thermotropic CLCs that can exhibit structural coloration. We demonstrate that the shear forces that arise from microfluidic flow align the helical axis of CLCs; alignment is a prerequisite for harnessing the promising photonic properties of CLCs. Moreover, we show that microfluidic flow can generate non-equilibrium structures exhibiting photonic band gaps that are inaccessible in the stationary cholesteric phase. Our findings have implications for the use of CLCs in applications involving flow processing such as additive manufacturing.

The influence of thermally induced cholesteric liquid crystal structural colour for information encryption

ABSTRACT Cholesteric liquid crystal (CLC) is particularly remarkable due to its ability to exhibit structural colour. Structural colour materials offer solutions for information encryption that feature good stability, ease of perceivability, and arbitrary patterns. This work investigates the temperature-dependent variation of CLC structural colour and provides a quantitative analysis of the pitch and helix twist power (HTP) at different temperatures, further exploring its potential applications in information encryption. By recording quick response (QR) code patterns in the LC Polymer of the LC cell, single-pattern and dual-pattern LC cells are prepared, and their applications in information encryption are demonstrated. The QR pattern will be hidden or revealed as the temperature changes by CLC in the LC cell. Additionally, dual-pattern encryption requires polarisers to address the crosstalk issue under white light, making the LC cell a safer solution for access control systems. These results lay a theoretical foundation in CLC stimulus-responsive structural colours and offer valuable insights in information security and anti-counterfeiting applications.

Microstructures, crystallography and growth patterns of serpulid tubeworms (Class Polychaeta)

Serpulid polychaetes are marine worms that secrete calcium carbonate tubes in which they live. Despite extensive previous research on their microstructures, there are no crystallographic data and their biomineralization process remains unclear. Here, the microstructures of the tubes of seven serpulid species were studied, including their chemical composition, mineralogy and crystallography, using X-ray diffraction, Raman and Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam, electron backscatter diffraction, and thermogravimetric analysis. Generally, serpulid tubes have a high amount of organic matter (~ 7.5 wt%), consisting of chitin and proteins, and the calcite is always present as medium to high magnesium calcite. Three main microstructures were identified: granular-prismatic and lamello-fibrillar calcite, and fibrous aragonite. They all displayed an axial texture, which is stronger in the lamello-fibrillar calcite, with the c-axis aligned with the elongation axis of the crystals. These findings demonstrate that only some instances of the granular-prismatic and the lamello-fibrillar calcite are biogenic (primary) microstructures. Conversely, other instances of the granular-prismatic calcite and the fibrous aragonite are a consequence of a recrystallization process (i.e. secondary). Replacement may occur on either primary or secondary calcitic microstructures (replaced by aragonite). Secondary microstructures retain remnants of the previously replaced microstructures, such as vestigial crystals or major growth increments. The high-Mg nature of the calcite favors recrystallization. The plywood arrangement of the lamello-fibrillar calcite is hypothesized to result from the ordering of a chitin fibrillar precursor into a cholesteric liquid crystal phase, with the calcite subsequently growing by oriented nucleation onto the organic fibrils.

Photonics of Two-dimensional Structures Formed by Cholesteric Liquid Crystals

Photonics of Two-dimensional Structures Formed by Cholesteric Liquid Crystals

A Multiresponsive Ferrocene‐Based Chiral Overcrowded Alkene Twisting Liquid Crystals

AbstractThe reversible modulation of chirality has gained significant attention not only for fundamental stereochemical studies but also for numerous applications ranging from liquid crystals (LCs) to molecular motors and machines. This requires the construction of switchable molecules with (multiple) chiral elements in a highly enantioselective manner, which is often a significant synthetic challenge. Here, we show that the dimerization of an easily accessible enantiopure planar chiral ferrocene‐indanone building block affords a multi‐stimuli‐responsive dimer (FcD) with pre‐determined double bond geometry, helical chirality, and relative orientation of the two ferrocene motifs in high yield. This intrinsically planar chiral switch can not only undergo thermal or photochemical E/Z isomerization but can also be reversibly and quantitatively oxidized to both a monocationic and a dicationic state which is associated with significant changes in its (chir)optical properties. Specifically, FcD acts as a chiral dopant for cholesteric LCs with a helical twisting power (HTP) of 13 μm−1 which, upon oxidation, drops to near zero, resulting in an unprecedently large redox‐tuning of the LC reflection color by up to 84 nm. Due to the straightforward stereoselective synthesis, FcD, and related chiral switches, are envisioned to be powerful building blocks for multi‐stimuli‐responsive molecular machines and in LC‐based materials.

Three-Level Chirality Transfer and Amplification in Liquid Crystal Supramolecular Assembly for Achieving Full-Color and White Circularly Polarized Luminescence.

Chiral liquid crystal supramolecular assembly provides an ideal strategy for constructing excellent circularly polarized luminescence (CPL) materials. However, the chirality transfer in chiral liquid crystals normally occurs at two levels from the configurational chirality to the supramolecular phase chirality. The more precise and more levels of chirality transmission are fascinating but remain challenging. The present work reports the first success of three-level chirality transfer and amplification from configurationally point chirality of small molecules to conformationally helical chirality of helical polymers and finally to supramolecular phase chirality of cholesteric liquid crystals composed of chiral nonfluorescent polymers (P46) and nematic liquid crystals. Noticeably, the helical twisting power of P46 is five-fold larger than its monomer. Full-color and white CPL with maximum luminescence dissymmetry factor up to 1.54 and photoluminescence quantum yield up to 63.8% are realized utilizing helical supramolecular assembly combined with selective reflection mechanism. Also significantly, the electrically stimuli-responsive CPL switching device as well as anti-counterfeiting security, information encryption, and chiral logic gate applications are developed. This study deepens the understanding of chirality transfer and amplification across different hierarchical levels.

Simultaneous monitoring of humidity and temperature by polarization volume gratings utilizing responsive cholesteric liquid crystals

Optical sensing devices are utilized for noncontact operation in harsh environments due to their advantage of local separation between the measurement and detector systems. Cholesteric liquid crystals (CLCs) with vivid structural colors are integrated into optical sensors, attracting significant interest for naked-eye detection. To enable vision-based, real-time remote monitoring, we propose a cost-effective method to convert the stimuli-responsive reflection change of CLCs into recordable diffraction patterns using polarization volume gratings (PVGs). This enables real-time remote readout of environmental parameters. PVGs are constructed through holographic polarization interference and photoalignment technologies. The diffraction efficiency of PVGs varies with relative humidity (RH). To improve sensing accuracy and extend the monitoring range of RH, we employ two methods: an orthogonal grating structure and dual-wavelength detection optical path. In addition, we demonstrate the simultaneous measurement of humidity and temperature by cascading independently responding PVGs. We believe that this work will provide a broader perspective and expand the application of light dimensions in LC-based optical sensing.

All-aqueous liquid crystal emulsions for 4D laser printing of dual-stimuli response printlets with on-demand remote control of drug release

The combination of responsive materials with 3D printing can lead to functional materials and devices for use in health, biofabrication, and biosensing. Here, we utilized an all-aqueous two-phase system made from chitosan and carrageenan to create all-water emulsion bodies, a groundbreaking group of encapsulated emulsions for possible drug delivery. Specifically, we described the development of an original all-aqueous cholesteric bicontinuous liquid crystal (LC) emulsion, which was accomplished by homogenization of a rod-like cellulose nanocrystal (CNC) with two thermodynamically immiscible, phase-separating carrageenan and chitosan solutions. Stable water-in-water emulsions were effectively prepared by mixing the respective CNC/biopolymer dispersions, showing micrometric CNC/chitosan dispersed droplets and a CNC/carrageenan continuous phase. The LC-based emulsion showed a confined 3D percolating bicontinuous network with cholesteric self-assembly of CNC within the carrageenan phase, while the nanoparticles in the chitosan phase remain isotropic. After laser 3D printing of freeze-dried LC-based emulsion, 3D porous printlets loaded with curcumim were produced with a dual stimuli response toward temperature and redox. The curcumin-loaded 3D printlets exhibited superior thermosensitive properties and maintained a sustained drug release profile, exhibiting much greater on-demand release mode at human body temperature. Notably, the 3D printlets demonstrated a gated response in an aqueous environment, showcasing potential advancements in the realms of supramolecular sensors and drug delivery systems.

Biomimetic Structural Hydrogels Reinforced by Gradient Twisted Plywood Architectures.

Naturally structural hydrogels such as crustacean exoskeletons possess a remarkable combination of seemingly contradictory properties: high strength, modulus, and toughness coupled with exceptional fatigue resistance, owing to their hierarchical structures across multiple length scales. However, replicating these unique mechanical properties in synthetic hydrogels remains a significant challenge. This work presents a synergistic approach for constructing hierarchical structural hydrogels by employing cholesteric liquid crystal self-assembly followed by nanocrystalline engineering. The resulting hydrogels exhibit a long-range ordered gradient twisted plywood structure with high crystallinity to mimic the design of crustacean exoskeletons. Consequently, the structural hydrogels achieve an unprecedented combination of ultrahigh strength (46±3MPa), modulus (496±25MPa), and toughness (170±14MJ m-3), together with recorded high fatigue threshold (32.5kJ m-2) and superior impact resistance (48±2kJ m-1). Additionally, through controlling geometry and compositional gradients of the hierarchical structures, a programmable shape morphing process allows for the fabrication of complex 3D hydrogels. This study not only offers valuable insights into advanced design strategies applicable to a broad range of promising hierarchical materials, but also pave the ways for load-bearing applications in tissue engineering, wearable devices, and soft robotics.

Enhanced electrically-induced bandwidth broadening via stratification of polymer stabilized cholesteric liquid crystals

Enhanced electrically-induced bandwidth broadening via stratification of polymer stabilized cholesteric liquid crystals

Sign Inversion of Circularly Polarized Luminescence in Cholesteric Liquid Crystals Induced by Mercury Ions through Binaphthyl Dopants′ Conjugation Control

AbstractStimuli‐responsive circularly polarized luminescence (CPL) materials based on cholesteric liquid crystal (CLC) platforms show great promise for applications in information encryption and anticounterfeiting. In this study, we constructed a mercury ion‐responsive CPL system in CLCs by controlling the conjugation degree of axially chiral binaphthyl derivatives. Two chiral binaphthyl derivatives (R/S‐1 and R/S‐2) were initially used as chiral dopants to demonstrate that CPL inversion (glum values from 0.5/‐0.44 to −0.53/0.48) in CLCs could be achieved by modulating the conjugation degree of the chiral binaphthyls. Based on this concept, the thioacetal binaphthyl R‐2S was developed and used as a mercury‐responsive chiral dopant in CLCs. Under Hg ion treatment, the CPL sign inverted (glum value changed from 0.22 to −0.29) due to the transformation of the thioacetal into an aldehyde group. Additionally, the mercury ion‐responsive CPL material was applied in information encryption.

Temperature‐Tunable Cholesteric Liquid Crystal Optical Combiners for Extended Reality Applications

Recent advancements in extended reality (XR) showcase their potential ability to enhance the user experience of traditional displays such as liquid crystal displays and organic light‐emitting diode screens. To achieve this goal, the upcoming generation of head‐mounted displays (HMDs) necessitates a seamless transition between different XR modes, including augmented reality (AR) and virtual reality (VR) modes. Herein, an innovative cholesteric liquid crystal‐ (CLC‐) based optical combiner for HMDs is introduced. This approach enables the display device to switch between AR, VR, and transparent modes via temperature modulation. Since CLC materials demonstrate varying optical properties at different temperatures, a real‐time temperature monitoring system that is paired with an external controller is integrated. This allows for dynamic temperature adjustments of the optical combiner and enables smooth transitions between display modes. Furthermore, the Berreman 4 × 4 matrix method is used to simulate the optical properties of the optical combiner, finding strong agreement with the experimental results. The performance of the tunable optical combiners using 450, 532, and 635 nm laser sources is explored. The findings demonstrate that CLC‐based optical combiners can effectively switch between the three distinct modes at corresponding temperatures, paving the way for versatile applications in future HMDs.

Direct Ink Writing of Cephalopod Skin‐Like Core‐Shell Fibers From Cholesteric Liquid Crystal Elastomers and Dyed Solutions

AbstractDirect ink writing (DIW) of core‐shell structures allows for patterning hollow or composite structures for shape morphing and color displays. Cholesteric liquid crystal elastomers (CLCEs) with liquid crystal mesogens assembled in a helix superstructure are attractive for generating tunable iridescent structural colors. Here, by fine‐tuning the rheology of the core and shell materials, respectively, this study creates droplets or a continuous filament in the core from the precursors of polydimethylsiloxane (PDMS) or poly(vinyl alcohol), whereas CLCE forms the outer shell. By introducing a dye in the droplets, the skin structures of cephalopods, consisting of chromatophores and iridocytes, are mimicked for enhanced color saturation, lightness, and camouflage. After removal of the core material, a CLCE hollow fiber is obtained, which can switch colors upon mechanical stretching and pneumatic actuation, much like papilla along with iridocytes. Further, liquid crystal mesogens assembled in the bulk of the fiber are in polydomain. Thus, the skin appears opalescent at room temperature, much like how leucophores enhance reflectins. Upon heating above the nematic to isotropic transition temperature, the skin becomes transparent. Lastly, a cephalopod model is constructed, where different parts of the model can change colors independently based on different mechanisms.

Overlooked Ionic Contribution of a Chiral Dopant in Cholesteric Liquid Crystals.

This study focuses on the ionic contribution by a chiral dopant added into a nematic host for preparing cholesteric liquid crystals (CLCs). Chiral structures were designated by individually incorporating two enantiomers, R5011 and S5011, into the nematic E44 to construct right- and left-handed CLCs, respectively. Characterized by the space-charge polarization, the dielectric spectra of the CLCs were investigated in the low-frequency regime, where f ≤ 1 kHz. The role of the individual chiral dopant, R5011 or S5011, at concentrations of 0-4.0 wt.% in altering the ionic properties of the CLC material was analyzed by deducing the electrical conductivity, ion density, and ion diffusivity. Regardless of the cell structure to be antiparallel or twisted by 90°, a significant ionic response was observed in the right-handed CLCs in comparison with the left-handed counterparts, suggesting that excess ions originating from our R5011 were introduced into the mesogenic mixtures. This work alarms the potential contribution of notorious impurity ions by a chiral dopant, which is often ignored in fabricating CLCs for electro-optical applications.

Circularly Polarized Organic Ultralong Room‐Temperature Phosphorescence: Generation, Enhancement, and Application

AbstractCircularly polarized luminescent (CPL) materials have garnered tremendous attention owing to their expanded optical properties beyond emission wavelength and intensity. Among these, the emerging circularly polarized organic ultralong room‐temperature phosphorescence (CP‐OURTP) materialsdemonstrating elegant and distinct features are of significant importance for their extended emission lifetime, which represent a novel frontier in research with promising scientific and technological applications across diverse fields. This review systematically outlines the traditional strategies to achieve CP‐OURTP including organic crystals, copolymerization, host–guest doping, a combination of the copolymerization and host–guest doping, spinning and twisting technology, and supramolecular polymer assembly. Importantly, the recent significant progress of CP‐OURTP in the chiral soft materials, such as liquid crystals (LCs) involving lyotropic LCs (cellulose nanocrystals, CNCs) and chiral thermotropic LCs (cholesteric LCs and chiral LC elastomers), is showcased. Finally, the practical applications of CP‐OURTP materials are summarized, and the review concludes with the perspectives on the current challenges and future opportunities for CP‐OURTP materials. This review aims to inspire the further innovations in the fabrication of advanced CP‐OURTP materials and enrich their promising applications.

Local Orientation Transitions to a Lying Helix State in Negative Dielectric Anisotropy Cholesteric Liquid Crystal

Orientation transitions in a cholesteric liquid crystal (CLC) layer with negative dielectric anisotropy, under the influence of a non-uniform spatially periodic electric field created using a planar system of interdigitated electrodes, were studied experimentally and numerically. In the interelectrode space, transitions are observed from a planar Grandjean texture, with the helix axis perpendicular to the layer plane, to states with a lying helix, when the helix axis is parallel to the layer plane and perpendicular to the electrode stripes. It was found that the relaxation time of the induced state in the Grandjean zones, corresponding to two or more half-turns of the helix, significantly exceeded the relaxation time for the first Grandjean zone with one half-turn. An analysis of experimentally observed and numerically simulated textures shows that slow relaxation to the initial state in the second Grandjean zone, as well as in higher-order zones, is associated with the formation of local topologically equivalent states. In these states, the helix has a reduced integer number of helix half-turns throughout the layer thickness or unwound into the planar alignment state.

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Enhanced Laser Emission by Incorporating Helically Twisted Silver Nanoflakes within Polymer-Stabilized Cholesteric Liquid Crystals

Propagation of periodic director and flow patterns in a cholesteric liquid crystal under electroconvection

The electroconvection of liquid crystals is a typical example of a dissipative structure generated by complicated interactions between three factors: convective flow, structural deformation, and the migration of charge carriers. In this study, we found that the periodic structural deformation of a cholesteric liquid crystal propagates in space, like a wave, under an alternating-current electric field. The existence of convection and charge carriers was confirmed by flow-field measurements and dielectric relaxation spectroscopy. Given that the wave phenomenon results from electroconvection, we suggest a possible model for describing the mechanism of wave generation. The validity of the model was examined using the Onsager variational principle. Consequently, it was suggested that wave generation can be described by four effects: the electrostatic potential, mixing entropy, anisotropic friction due to charge migration, and viscous dissipation of the liquid crystal.

Multi-stage and multi-colour liquid crystal reflections using a chiral triptycene photoswitchable dopant.

The photomodulation of the helical pitch of cholesteric liquid crystals results in dynamic and coloured canvases that can potentially be used in applications ranging from energy-efficient displays to colour filters, anti-counterfeiting tags and liquid crystal (LC) lasers. Here we report on the analysis of a series of photoswitchable chiral dopants that combine the large geometrical change and bistability of hydrazone switches with the efficient helical pitch induction of the chiral motif, triptycene. We elucidate the effects that conformational flexibility, dispersion forces and π-π interactions have on the chirality transfer ability of the dopant. We then use the irradiation time with visible light (442 nm) combined with a simple digital light processing microscope projection set-up to draw numerous stable multi-coloured images on an LC canvas, showcasing the fine control this dopant yields over the LC assembly.