Chemically Responsive Gels Prepared from Microspheres Dispersed in Liquid Crystals

Liquid‐crystalline materials are a promising class of stimuli‐responsive materials that have been demonstrated to undergo surface‐induced orientational ordering transitions that can be highly sensitive and specific to chemical species. However, past studies demonstrating surface‐induced transitions in liquid crystals (LCs) have employed thin films of low‐molecular‐weight LCs that are difficult to stabilize (due to dewetting of the LC on a surface). Here, it is reported that it is possible to prepare liquid‐crystalline gels using a mixture of polystyrene microspheres and nematic LCs that undergo changes in orientational order, and thus optical appearance, in response to exposure to specific chemical compounds. These colloid‐in‐liquid‐crystal (CLC) gels are mechanically stable and can be molded on chemically functionalized surfaces into thin films containing micrometer‐sized LC‐rich domains that span the two interfaces of the gels. In contrast to other reports of LC gels, where the presence of a polymeric or self‐assembled small‐molecule gelator network within a nematic LC frustrates ordering transitions from propagating through the gels over distances, it is demonstrated that thin films of CLC gels, when supported on chemically functionalized surfaces, do undergo easily visualized ordering transitions upon exposure to organophosphonate compounds. Because these optically responsive CLC gels are mechanically robust and can be molded, this class of composite LC material may be broadly useful for the design of chemically responsive LC devices.     

Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

The ability to control light direction with tailored precision via facile means is long‐desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next‐generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli‐controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.     

Responsive Liquid‐Crystal‐Clad Fibers for Advanced Textiles and Wearable Sensors

A simple process to clad conventional monofilament fibers with low‐molecular‐weight liquid crystals (LCs) stabilized by an outer polymer sheath is demonstrated. The fibers retain the responsive properties of the LCs but in a highly flexible/drapable format. The monofilament core makes these fibers much more rugged with a magnified response to external stimuli when compared to previously reported LC‐core fibers produced by electrospinning or airbrushing. The microscopic structure and the optical properties of round and flattened fibers are reported. The sensitivity of the response of individual fibers can be tuned over a broad range by varying the composition of the LCs. Complex fabrics can be easily woven from fibers that respond to different external stimuli, such as temperature variation, chemical concentrations, and pressure. The fabrics can be fashioned into garments that can sense and report the state of health of the wearer or the status of their environment.     

Colloid‐in‐Liquid Crystal Gels that Respond to Biomolecular Interactions

This paper advances the design of stimuli‐responsive materials based on colloidal particles dispersed in liquid crystals (LCs). Specifically, thin films of colloid‐in‐liquid crystal (CLC) gels undergo easily visualized ordering transitions in response to reversible and irreversible (enzymatic) biomolecular interactions occurring at the aqueous interfaces of the gels. In particular, LC ordering transitions can propagate across the entire thickness of the gels. However, confinement of the LC to small domains with lateral sizes of ～10 μm does change the nature of the anchoring transitions, as compared to films of pure LC, due to the effects of confinement on the elastic energy stored in the LC. The effects of confinement are also observed to cause the response of individual domains of the LC within the CLC gel to vary significantly from one to another, indicating that manipulation of LC domain size and shape can provide the basis of a general and facile method to tune the response of these LC‐based physical gels to interfacial phenomena. Overall, the results presented in this paper establish that CLC gels offer a promising approach to the preparation of self‐supporting, LC‐based stimuli‐responsive materials.     

Planar Anchoring of C70 Liquid Crystals Using a Covalent Organic Framework Template

The surface‐induced anchoring effect is a well‐developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer‐scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF‐1) can offer an anchoring effect to template C₇₀ molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C₆₀ system, a stepwise breakdown in ordering of C₇₀ LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface‐confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.     

Selectivity of Threefold Symmetry in Epitaxial Alignment of Liquid Crystal Molecules on Macroscale Single‐Crystal Graphene

Epitaxial alignment of organic liquid crystal (LC) molecules on single‐crystal graphene (SCG), an effective epitaxial molecular assembly template, can be used in alignment‐layer‐free liquid crystal displays. However, selectivity among the threefold symmetric easy axes of LCs on graphene is not well understood, which limits its application. Here, sixfold symmetric radial LC domains are demonstrated by dropping an LC droplet on clean SCG, which reveals that the graphene surface does not have an intrinsic preferential direction. Instead, the first contact geometry of the LC molecules determines the direction. Despite its strong anchoring energy on graphene, the LC alignment direction is readily erasable and rewritable, contrary to previous understanding. In addition, the quality of the threefold symmetric alignment is sensitive to alien residue and graphene imperfections, which can be used to detect infinitesimal impurities or structural defects on the graphene. Based on this unique epitaxial behavior of LCs on SCG, an alignment‐layer‐free electro‐optical LC device and LC alignment duplication, which can result in practical graphene‐based flexible LC devices, are realized.     

Stimuli-Responsive Dynamic Metaholographic Displays with Designer Liquid Crystal Modulators

Flat optics, realized by the artificially created 2D material platform called optical metasurfaces, is currently undergoing a science-to-technology transition. However, “real-time” active operations of such flat optical devices remain yet unresolved. Here, liquid crystals (LCs)-integrated metaholograms for ultracompact dynamic holographic displays are proposed. The anisotropic nature of the LCs allows facile and repeatable manipulation of the polarization of light. Specifically designed (“designer”) LCs and efficient helicity-encoded metaholograms are combined to realize stimuli-responsive dynamic displays. The designer LC modulators are used as switches that enable a variety of external stimuli (e.g., electric field, heat, surface pressure) to operate holographic images in real-time. Such a dynamic metaholographic platform will provide a path to external stimuli-driven “smart” sensing and display applications such as hologram labels for temperature/pressure/touch monitoring and interactive holographic displays with haptic motion recognition.     

Influence of pretilt angle on disclination lines of liquid crystal lens

This article investigates the effect of pretilt angle on disclination lines of liquid crystal (LC) lenses. When the pretilt angle of LCs is higher than 7°, the disclination lines are reduced and are moved to the boundary of the LC lens. The disclination lines at the boundary do not influence the focused beam profile of the LC lens. As the pretilt angle of LCs further increases, the disclination lines at the boundary become almost invisible. However, the interference rings become asymmetrical. The response time of an LC lens with a pretilt angle higher than 7° is ～60% of the conventionally homogeneous LC lens. This value is a result of the decrease in the rotation angle of the LCs and the reduced disclination lines.     

液晶分子在受限条件下的液晶相行为（Ⅱ）某些化学受限环境对液晶相行为的影响

综述了液晶分子在与聚合物嵌段、碳纳米管（CNT）和多面低聚倍半硅氧烷（POSS）等通过共价键形成的化学受限环境下的液晶相行为。聚合物嵌段微区的受限作用使侧链液晶嵌段共聚物的液晶有序度减小,相变温度降低。由于碳纳米管的受限作用,液晶聚合物接枝CNTs的有序结构被破坏,液晶性丧失。POSS以共价键的方式引入到液晶分子中,明显提高了液晶相的稳定性。但是当POSS含量高于某临界值后使液晶分子表现出单向性液晶相行为,甚至使其丧失液晶性。     

Polymer‐Stabilized Micropixelated Liquid Crystals with Tunable Optical Properties Fabricated by Double Templating

Self‐organized nano‐ and microstructures of soft materials are attracting considerable attention because most of them are stimuli‐responsive due to their soft nature. In this regard, topological defects in liquid crystals (LCs) are promising not only for self‐assembling colloids and molecules but also for electro‐optical applications such as optical vortex generation. However, there are currently few bottom‐up methods for patterning a large number of defects periodically over a large area. It would be highly desirable to develop more effective techniques for high‐throughput and low‐cost fabrication. Here, a micropixelated LC structure consisting of a square array of topological defects is stabilized by photopolymerization. A polymer network is formed on the structure of a self‐organized template of a nematic liquid crystal (NLC), and this in turn imprints other nonpolymerizable NLC molecules, which maintains their responses to electric field and temperature. Photocuring of specific local regions is used to create a designable template for the reproducible self‐organization of defects. Moreover, a highly diluted polymer network (≈0.1 wt% monomer) exhibits instant on–off switching of the patterns. Beyond the mere stabilization of patterns, these results demonstrate that the incorporation of self‐organized NLC patterns offers some unique and unconventional applications for anisotropic polymer networks.     

Manipulation of photonic nanojet using liquid crystals for elliptical and circular core-shell variations

We present theoretical studies on a tunable photonic nanojet (PNJ) created by adapting a shell and liquid crystal (LC) core architecture. The shell is made of indium tin oxide and the core is infiltrated with nematic LCs. The application of an external static electric field to the LCs modifies their refractive index, and this allows tuning the PNJ effect in the proposed system. In addition to nonresonant excitation, resonant PNJ excitation is also obtained from a hybrid structure. Both nonresonant and resonant internal field excitations of circular and elliptical PNJ configurations are examined by using a high-resolution finite-difference time-domain method. The calculated results indicate that the proposed PNJ configurations with tunable refractive indices lead to significant changes in some parameters such as decay length, focal distance, full width at half maximum and electric field intensity. Such PNJ designs can be employed in high-resolution optical sensors, optical trapping, and high-density data storage.     

Tunable Diffraction Gratings from Biosourced Lyotropic Liquid Crystals

Diffraction gratings are important for modern optical components, such as optical multiplexers and signal processors. Although liquid crystal (LC) gratings based on thermotropic LCs have been extensively explored, they often require expensive molecules and complicated manufacturing processes. Lyotropic LCs, which can be broadly obtained from both synthetic and natural sources, have not yet been applied in optical gratings. Herein, a facile grating fabrication method using a biosourced lyotropic LC formed by cellulose nanocrystals (CNCs), a material extracted from plants, is reported. Hydrogel sheets with vertically aligned uniform periodic structures are obtained by fixing the highly oriented chiral nematic LC of CNCs in polymer networks under the cooperative effects of gravity on phase separation and a magnetic field on LC orientation. The hydrogel generates up to sixth-order diffraction spots and shows linear polarization selectivity, with tunable grating periodicity controlled through LC concentration regulation. This synthesis strategy can be broadly applied to various grating materials and opens up a new area of optical materials from lyotropic LCs.     

Photochemical modulation of alignment of liquid crystals and photonic applications

The recent highlights in the photoinduced modulation of the liquid-crystalline (LC) alignment and their applications to photonics are reviewed. The molecular alignment of LCs can be controlled by either the photochemical process or the photophysical process and we can change the birefringence as well as refractive index of the material by the modulation of the LC alignment. The photoresponsive LCs show very quick response to stimulus light: the response time is ∼200 ns. Furthermore, the extremely large refractive index modulation (Δn=∼10⁻¹) can be also induced. The photoresponsive LCs, therefore, are potential materials for holographic recording and other photonic device.     

Inside Front Cover: Homeotropic Alignment of Columnar Liquid Crystals in Open Films by Means of Surface Nanopatterning (Adv. Mater. 6/2007)

Columnar liquid crystals (LCs) are found to spontaneously form homeotropically‐aligned films when deposited on surfaces fabricated via friction transfer of polytetrafluoroethylene. The inside cover schematically shows the film structure of a phthalocyanine derivative together with the corresponding X‐ray diffraction pattern (inset). The results, reported on p. 815 by Dmitri Ivanov and co‐workers, indicate that not only are the columns homeotropically oriented but also that they form a single monodomain of macroscopic size. These findings can have an important impact for fabrication of LC‐based organic solar cells.     

Quantitative methods based on twisted nematic liquid crystals for mapping surfaces patterned with bio/chemical functionality relevant to bioanalytical assays

We report methods for the acquisition and analysis of optical images formed by thin films of twisted nematic liquid crystals (LCs) placed into contact with surfaces patterned with bio/chemical functionality relevant to surface-based assays. The methods are simple to implement and are shown to provide easily interpreted maps of chemical transformations on surfaces that are widely exploited in the preparation of analytic devices. The methods involve acquisition of multiple images of the LC as a function of the orientation of a polarizer; data analysis condenses the information present in the stack of images into a spatial map of the twist angle of the LC on the analytic surface. The potential utility of the methods is illustrated by mapping (i) the displacement of a monolayer formed from one alkanethiol on a gold film by a second thiol in solution, (ii) coadsorption of mixtures of amine-terminated and ethylene glycol-terminated alkanethiols on gold films, which leads to a type of mixed monolayer that is widely exploited for immobilization of proteins on analytic surfaces, and (iii) patterns of antibodies printed onto surfaces. These results show that maps of the twist angle of the LC constructed from families of optical images can be used to reveal surface features that are not apparent in a single image of the LC film. Furthermore, the twist angles of the LC can be used to quantify the energy of interaction of the LC with the surface with a spatial resolution of <10 microm. When combined, the results described in this paper suggest nondestructive methods to monitor and validate chemical transformations on surfaces of the type that are routinely employed in the preparation of surface-based analytic technologies.     

Polymer Carpets

The fabrication of defined polymer objects of reduced dimensions such as polymer‐coated nanoparticles (zero‐dimensional (0D)), cylindrical brushes (1D), and polymer membranes (2D), is currently the subject of intense research. In particular, ultrathin polymer membranes with high aspect ratios are being discussed as novel materials for miniaturized sensors because they would provide extraordinary sensitivity and dynamic range when sufficient mechanical stability can be combined with flexibility and chemical functionality. Unlike current approaches that rely on crosslinking of polymer layers for stabilization, this report presents the preparation of a new class of polymer material, so‐called “polymer carpets,” a freestanding polymer brush grown by surface‐initiated polymerization on a crosslinked 1‐nm‐thick monolayer. The solid‐supported, as well as freestanding, polymer carpets are found to be mechanically robust and to react instantaneously and reversibly to external stimuli by buckling. The carpet mechanics and the dramatic changes of the film properties (optical, wetting) upon chemical stimuli are investigated in detail as they allow the development of completely new integrated micro‐/nanotechnology devices.     

25th Anniversary Article: Dynamic Interfaces for Responsive Encapsulation Systems

Encapsulation systems are urgently needed both as micrometer and sub‐micrometer capsules for active chemicals' delivery, to encapsulate biological objects and capsules immobilized on surfaces for a wide variety of advanced applications. Methods for encapsulation, prolonged storage and controllable release are discussed in this review. Formation of stimuli responsive systems via layer‐by‐layer (LbL) assembly, as well as via mobile chemical bonding (hydrogen bonds, chemisorptions) and formation of special dynamic stoppers are presented. The most essential advances of the systems presented are multifunctionality and responsiveness to a multitude of stimuli – the possibility of formation of multi‐modal systems. Specific examples of advanced applications – drug delivery, diagnostics, tissue engineering, lab‐on‐chip and organ‐on‐chip, bio‐sensors, membranes, templates for synthesis, optical systems, and antifouling, self‐healing materials and coatings – are provided. Finally, we try to outline emerging developments.     

基于葡萄糖的星型胆甾相液晶化合物的合成及相行为

以葡萄糖为手性中心,6-（4-（苯甲酸基）苯）己二酸单酯（M1）、6-（4-（4-烷氧基苯甲酸基）苯）己二酸单酯（M2～M3）为侧臂,采用N,N′-二环己基碳化二亚胺（DCC）/4-二甲氨基吡啶（DMAP）成酯法,合成了星型化合物（GM1～GM3）。通过红外光谱（FT-IR）、核磁共振（1H-NMR）、偏光显微镜（PO...     

Capacitive Transduction for Liquid Crystal Based Sensors, Part II: Partially Disordered System

This paper presents the capacitive transduction technique involved with liquid crystal (LC) based sensors in partially disordered systems. These sensors have the potential applications in chemical and biological systems. The theory for tracking the average molecular deformation (state of alignment) and degree of ordering of anisotropic and partially disordered LC film via capacitive sensing is investigated. This system is modeled using the Q-tensor approach in modeling uniaxial LC material. The proposed sensor design is an interdigitated electrodes structure. Transverse and fringing capacitances as function of the molecular deformation are calculated. It is verified that three capacitance measurements are required to track the average molecular orientation and the degree of disorder in the LC film. The sensitivity for the sensor at different alignments and ordering degree is also studied. Toward practical sensor, neuro-fuzzy system is modeled to simulate the capacitive transduction and to monitor the LC profile. Sensors are fabricated and tested. Both the experimental and calculated capacitances are presented and compared.      

Terahertz properties of liquid crystals with negative dielectric anisotropy

We present what is believed to be the first terahertz time-domain study of a set of liquid crystals (LCs) with negative dielectric anisotropy. From the measured data, refractive indices, and absorption coefficients for ordinary and extraordinary polarization are extracted. We find that the investigated materials exhibit a much smaller absorption than LCs with positive dielectric anisotropy. Thus, these materials are more useful for switchable terahertz devices. Moreover, the LC 1808 shows what is to our knowledge the largest terahertz birefringence reported so far.