Liquid Crystal Core Research Articles

A Comprehensive Study on Electrospun Cholesteric Liquid Crystal Core Fibers for Detecting Volatile Organic Compounds

Encapsulation of the cholesteric liquid crystal (CLC) into the core of poly(vinylpyrrolidone) (PVP) microfibers is accomplished via coaxial electrospinning technology and is applied to detect volatile organic compounds (VOCs). The impacts of various process parameters on the morphology of CLC fibers are deeply explored. The periodic arrangement of CLC molecules within the fibers enables the reflection of specific wavelengths. Consequently, changes in the reflected color of the CLC fiber membrane allow for the measurement and analysis of targeted gas molecules. The dynamic response of CLC fibers upon exposure to organic vapor is observed by a polarizing optical microscope, and its gray scale is used to quantitatively analyze the changes of reflected light signals of CLC fibers. Furthermore, the study investigates the reaction time for the failure of CLC molecular planar alignment in the CLC fiber membrane when subjected to perturbations induced by organic gases. The CLC fiber sensor exhibits heightened sensitivity to gases characterized by greater polarity. In light of potential future advancements, it is suggested that the spun fibers imbued with responsive properties through the incorporation of a liquid crystal core exhibit promise as a next-generation sensor technology in textile form, well-suited for applications in wearable technology.

The good, the bad and the ugly faces of cyanobiphenyl mesogens in selected tracks of fundamental and applied liquid crystal research

ABSTRACT Liquid crystal-forming cyanobiphenyls are truly extraordinary molecules that have had an enormous impact on liquid crystal research and applications since they were first synthesised. This impact is, on the one hand, due to the exceptionally convenient physical properties of the main characters, 5CB and 8CB, allowing easy experiments at room temperature, as well as their commercial availability at reasonable cost. On the other hand, the cyanobiphenyl chemical structure leads to some quite peculiar characteristics in terms of organisation at the molecular scale, which are sometimes well recognised and even utilised, but often the awareness of these peculiarities is not strong. This perspective article reviews the use of cyanobiphenyls in making liquid crystal shells and liquid crystal core fibres, in sensing, as a medium for simultaneously aligning and dispersing carbon nanotubes, and as highly useful solvents for reactive mesogens that can be polymerised into anisotropic networks. This choice is very much motivated by how cyanobiphenyls have impacted our group’s research throughout the years, which is the basis for the examples I provide. Nevertheless, I believe they serve well to illustrate the immense usefulness of cyanobiphenyls in innovating research and applications related to liquid crystals.

Functional Liquid Crystal Core/Hydrogel Shell Microcapsules for Monitoring Live Cells in a 3D Microenvironment.

Three-dimensional (3D) cell culture, even as a simple microspheroid model, can be used to recapitulate the native biological microenvironment of cells. Examining the biochemical characteristics of cells in multicellular hydrogel microspheroids using microsensors is usually limited to monitoring the medium around the microspheroids. Here, functional liquid crystal (LC) core/hydrogel shell microcapsules loaded with cells were prepared using droplet microfluidic technology for monitoring live cells in a 3D microenvironment. These microcapsules have a distinctive core/shell structure; cells can be cultured in the hydrogel shell of this 3D model. The functional LC core responds to the acidic microenvironment of cells, showing an axial-to-bipolar transfiguration. 3D cell culture and visual monitoring of the cell microenvironment can be simultaneously achieved in a single microcapsule. Therefore, this novel method may enable a standard approach for monitoring multiple ions or molecules in a 3D model of the cell microenvironment.

Low threshold electro-optic switch and TE/TM polarization selector using liquid crystal channel waveguide

We demonstrate a Liquid Crystal (LC) core channel waveguide-based low threshold electro-optic switch by employing in-plane switching configuration and show its application for polarization selection device. The waveguide is fabricated on an indium tin oxide (ITO) coated glass substrate using the laser direct writing lithography technique. We have experimentally shown that the Freedericksz transition voltage of LC molecules is ∼ 1 V, and it relatively requires a low threshold voltage of ∼ 2 V to switch on the waveguide. The measured extinction ratio of the device is greater than 18 dB, along with a low insertion loss of ∼ 3.27 dB as compared to the conventional LC waveguides. The fabricated device is further demonstrated as a voltage-controlled polarization selector that passes either TM or TE-polarized light with a polarization extinction ratio of 28 dB and 25 dB, respectively. The proposed device exhibits the features such as low voltage control, low optical and insertion loss, high extinction ratio, and an externally controlled polarization selector, which are the key requirements in integrated photonics.

Electrically controlled continuous laser beam steering in a liquid crystal based electro-optic waveguide

Non-mechanical and continuous beam steering devices are highly desirable in recent laser scanning applications and telecommunication advancements. We report a liquid crystal (LC) core waveguide-based electrically controlled, non-mechanical, and continuous laser beam steering device. The waveguide uses nematic 5CB (4-cyano-4′-pentylbiphenyl) LC as a propagation medium surrounded by PVA (polyvinyl alcohol) polymer cladding, which is realized on ITO coated glass substrates. A pattern of five triangular electrodes is transferred on the substrate using photolithography to create prisms of tunable refractive index in the LC core of the waveguide. The beam steering for transverse electric (TE) and transverse magnetic (TM) polarization is theoretically studied and experimentally characterized using a He-Ne laser of λ = 633 nm. A steering angle of −5.4° and +5.7° at 7 Vpp is obtained for TE and TM polarization, respectively, in a waveguide with the prism’s apex angle of 13°. Similarly, a higher steering angle up to 7.3° for TM polarization is demonstrated in the waveguide with an apex angle of 22°. The device is developed using ease of fabrication process and cost-effective materials that operate at relatively low voltages.

Highly sensitive temperature sensor based on a liquid crystal selectively infiltrated two-core photonic crystal fiber

This article proposes a temperature sensor based on liquid crystal selectively filled two-core photonic crystal fiber featuring high sensitivity and compact structure. In order to achieve the purpose of compact structure and temperature sensing, we consider to combine the temperature-sensitive material liquid crystal (LC) E7 and photonic crystal fiber to study the performance of temperature sensing. Since the applied electric field and temperature have great influence on the arrangement of liquid crystal molecules, LC as a temperature sensitive material can improve the sensing performance. An air hole filled with LC isolates the two horizontal cores, where the two horizontal cores act as two separate waveguides and the LC core is used to regulate the mode coupling of the two horizontal cores. In the process of transferring light energy from one core to another, the position of the transmission spectrum of the sensor is changed by optimizing the diameter of LC core and horizontal two-core, and then a sensor with high sensitivity and stable performance is designed. The coupling length and transmission spectrum of the sensor are analyzed by using full vector finite element method. By analyzing the wavelength dips of the transmission spectrum, the maximum sensitivity of the proposed temperature sensor can be reached −6.69 nm/℃ at 30 ℃ for Y-polarized (Y-Pol) direction. The main advantages of the sensor proposed in this paper are simple structure and easy to manufacture, high sensitivity and high fitting coefficient.

Mode Size Converter Based on Periodically Segmented Liquid Crystal Core Waveguide

We report the numerical and experimental study of a liquid crystal (LC) core periodically segmented waveguide (PSW) based mode size converter, fabricated on an ITO coated glass substrate. The effective index and mode size of the proposed device can be dynamically tuned by changing the applied voltage. The 5CB (4-cyano-4′-pentylbiphenyl) nematic LC has been used as the guiding layer, surrounded by the cladding of negative photoresist AZ15nXT. An equivalent waveguide model has been used to determine the waveguide characteristics, and we have shown that the effective index and mode size of the guided modes change with the duty cycle and the applied voltage. For a fixed duty cycle, we have experimentally demonstrated that the size of the fundamental mode can be tuned by the applied voltage. The advantage of using LC-PSW for mode size transformation is that the effective index and mode size in the fabricated waveguide can be varied by an externally applied electric field. It has also been experimentally demonstrated that the coupling efficiency between the LC-PSW and a single mode fiber can be maximized by appropriate choice of the applied voltage. The proposed mode size converter can be used in LC photonic lightwave circuits to couple the light between optical devices with maximum efficiency.

Dynamic Interference Colors in Electrospun Microfibrous Mats

AbstractThe design of soft materials that change their optical properties in response to a variety of external stimuli is one step toward mimicking the dynamic color changing systems that are observed in nature. A simple, multiresponsive system is presented where the colors of core/sheath microfibers and microfibrous mats can be tuned by confinement (i.e., fiber diameter), temperature, and UV–vis irradiation. Microfibers are fabricated using coaxial electrospinning with a nematic liquid crystal core and a polymer sheath. This results in soft, flexible mats of material that display uniform color when viewed between crossed‐polarizers both under a microscope and by the naked eye. The observed interference color is dictated by the inner fiber diameter and the birefringence of the liquid crystal core. While the fiber diameter is set during mat fabrication, continuous in situ color change by tuning the birefringence of the liquid crystal is demonstrated using temperature or photochemical control of azobenzene‐based surfactants embedded in the polymer sheath. The expected colors are calculated and easy‐to‐use color charts are generated to accurately predict fiber behavior.

Tunable liquid crystal core refractive index sensor based on surface plasmon resonance in gold nanofilm coated photonic crystal fiber.

We propose, to the best of our knowledge, a novel kind of tunable liquid crystal core refractive index (RI) sensor based on photonic crystal fiber (PCF) covered with a nanoring gold film. The finite element method is used to discuss and analyze the sensing performance of the RI sensor. Gold is used as the excitation material for surface plasmon resonance, and a gold nanoring is embedded around the first cladding of the PCF. The liquid analytes penetrate the outermost layer of the cladding, and the central hole is filled with liquid crystal E7. Complete coupling and incomplete coupling are excited as the analyte RI increases, and the resonance strength of complete coupling is stronger than that of incomplete coupling. It can be proved by calculation that at different wavelengths, resonant coupling of fifth-order and sixth-order surface plasmon polaritons is obtained. The RI of liquid analytes ranges from 1.405 to 1.445. The diameters of the liquid crystal cores are 0.2, 0.4, 0.6, and 0.8 µm; their average sensitivities are 10700, 10566, 10167, and 9166 nm/RIU; and the linear fitting constants are 0.98025, 0.97387, 0.96597, and 0.95507, respectively. In short, the RI sensor has the advantages of tunability, wide range, and high sensitivity, and is expected to be applied in various fields.

Preliminary study on the self-assembly of a pair of azobenzene liquid crystals derived from alanine

A series of azobenzene liquid crystals containing one amino acid residue were synthesized. They exhibit smectic mesophases. The compounds derived from alanine can self-assemble into twisted nanoribbons. The molecules organized in a monolayer structure within the nanoribbon. Circular dichroism characterization indicated that the neighboring liquid crystal cores packed in a chiral structure. FT-IR spectra indicated that the nanoribbons were caused by H-bonding and association of hydrocarbon chains.

Fiber Optic Electric Field Intensity Sensor Based on Liquid Crystal-Filled Photonic Crystal Fiber Incorporated Ring Laser

Optical fiber sensors are of splendid strength for electrical field intensity sensor due to characteristics including the immunity to electromagnetic interference, lightweight, high sensitivity, and large bandwidth. In this paper, we proposed an electric field intensity sensor based on Mach-Zehnder interferometer (MZI) based liquid crystal (LC) filled photonic crystal fiber (PCF) embedded in optical fiber ring laser (FRL). The air hole of PCF combines the LC and fiber core together. When LC is introduced into air holes, it can maintain the waveguide based on external parameters. The photonic bandgap effect significantly improved the sensitivity between light and external electrical field intensity. Thanks to the FRL demodulation, a high signal to noise ratio (SNR) spectrum about 35 dB is obtained. Besides, in comparison with traditional LC-PCF structures, the sensitivity of ours is as high as 1.1 nm/Vrms which is about twice than traditional sensors. At the same time, the stability of proposed sensor was verified which fluctuation was 0.15 nm around 2.5 hours. Therefore, our structure is expected to practical applications in remote electric field monitor and such electric modulate electro-optical devices.

Combining responsiveness and durability in liquid crystal-functionalised electrospun fibres with crosslinked sheath

ABSTRACT Responsive functional composite fibre mats that are mechanically stable and impervious to water exposure are produced by coaxial electrospinning of thermotropic liquid crystal (LC) core inside a water-based solution of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) forming the sheath. Because thermotropic LCs usually cannot be spun inside water-based solutions due to excessive interfacial tension , a n enabling step is the addition of ethanol or dioxane to the LC as a co-solvent compatible with both core and sheath fluids. This reduces sufficiently that coaxial jet spinning is possible. After spinning, thermal cross-linking of the PVA+PAA sheath yields LC-functionalised fibres that can be manipulated by hand and remain intact even upon full immersion in water. The LC core retains its behaviour, nematics showing well-aligned birefringence and transitioning to isotropic upon heating above the clearing point, and cholesterics showing selective reflection which is even enhanced upon water immersion due to the removal of sheath scattering. Our results pave the way to producing LC-functionalised responsive fibre mats using durable polymer sheaths, thereby enabling numerous innovative applications in wearable technology, and they also open new opportunities to study LCs in confinement, without visible impact of the container walls.

Directed ortho and Remote Metalation-Suzuki-Miyaura Cross Coupling Route to Azafluorenol Core Liquid Crystals.

Two new smectic C* mesogens containing a hexyloxy side chain and an azafluorenone (3a) or azafluorenol (3b) core were synthesized using a combined directed ortho metalation-directed remote metalation-Suzuki-Miyaura cross-coupling strategy. 3b was formed in 10 steps and 25% overall yield based on starting benzamide 1a. 3a forms a nematic phase, while 3b forms a smectic A phase. The large temperature range of the smectic phase for the azafluorenol 3b is indicative of mesophase stabilization by intermolecular hydrogen bonding between the hydroxyl group and pyridine nitrogen of neighboring 3b molecules.

Quantitative volatile organic compound sensing with liquid crystal core fibers

SummaryPolymer fibers with liquid crystals (LCs) in the core have potential as autonomous sensors of airborne volatile organic compounds (VOCs), with a high surface-to-volume ratio enabling fast and sensitive response and an attractive non-woven textile form factor. We demonstrate their ability to continuously and quantitatively measure the concentration of toluene, cyclohexane, and isopropanol as representative VOCs, via the impact of each VOC on the LC birefringence. The response is fully reversible and repeatable over several cycles, the response time can be as low as seconds, and high sensitivity is achieved when the operating temperature is near the LC-isotropic transition temperature. We propose that a broad operating temperature range can be realized by combining fibers with different LC mixtures, yielding autonomous VOC sensors suitable for integration in apparel or in furniture that can compete with existing consumer-grade electronic VOC sensors in terms of sensitivity and response speed.

A Broadband polarization filter based on liquid crystal core and gold-coated microstructure fiber

A Broadband polarization filter based on liquid crystal core and gold-coated microstructure fiber

Liquid crystal core polymer fiber mat electronic gas sensors

ABSTRACT Electrospun liquid crystal fibres are promising media for gas sensing. However, accurately quantifying their response has been a challenge and until now limited to optical changes which utilise lasers and bulky detectors hindering the use for wear-able sensors. Herein, we make use of the resistive changes of the liquid crystal core polymer fibre mats (LCC-PFM) when exposed to volatile organic compounds (VOC). We found that these LCC-PFMs show reversible sensing of acetone at room temperature with response (recovery) times of ~14 (21) seconds and sensitivity as low as 10 ppm. This is comparable to the currently used metal-oxide-based sensors which typically operate at temperatures above 200 °C. These fibre mats are flexible, lightweight and in principle, can be easily miniaturised for use as resistive sensors to as small as 10 μm linear dimensions.

Rib Waveguide Based Liquid Crystal EO Switch

We report electrically re-configurable liquid crystal core rib waveguide based electro-optic switch. It is proposed to use single side anchored in-plane switching (IPS) configuration for optical switching in view of its low threshold voltage and low switching voltage in comparison to that in a double-side anchored LC waveguide switch. Further, the proposed LC waveguide based switch has better response time than that in a double side anchored configuration at operating voltage close to threshold voltage of a double side anchored. The device has been realized on a BK7 glass substrate using E7 LC. We also show wavelength re-configurability of the switch over a wide wavelength range (1 - 3 μm) by externally applied voltage. Proposed work would be useful for optical signal processing on integrated optic platform.

Synthesis and properties of 5,6-dihydro-4H-cyclopenta[b]thiophene-based nematic liquid crystals: A new access to mesogens with high birefringence and large dielectric anisotropy

A novel liquid crystal core unit 5,6-dihydro-4H-cyclopenta[b]thiophene was designed and synthesized in four steps and three mesogens containing terminal cyano group were then obtained. The effects of this skeleton on the mesophase and physical properties of liquid crystals were then investigated by compared with two series of reference compounds based on 1,4-disubstituted phenylene and disubstituted phenylcyclohexane and found this unit can benefit the formation of the nematic phase. Besides, both the birefringence (Δn) and dielectric anisotropy (Δε) values were increased after the introduction of this core unit. Density function theory calculations suggested that the combination of thiophene and cyclopentane had increased the height/width ratio and linearity of the mesogens which can stabilise the liquid crystal phase. Five different liquid crystals based on this new core were synthesized at last to further illustrate the usefulness of this core in constructing high birefringence and large dielectric anisotropy liquid crystals with wide nematic phase interval.

Growth and Termination of Cylindrical Micelles via Liquid-Crystallization-Driven Self-Assembly

Growth and termination of cylindrical micelles with cholesteric liquid crystal (LC) cores (seeds) were achieved experimentally, following the simulation studies of liquid-crystallization-driven sel...

Highly negative dispersion dual-core liquid crystal photonic crystal fiber

Abstract A novel design for dispersion compensation based on dual core liquid crystal photonic crystal fiber (DC-LCPCF) is reported and analyzed. The new structure is based on the optical coupling between the dual cores of the suggested PCF. The full vectorial finite element method (FVFEM) is used to study the dispersion characteristics of the DC-LCPCF. A very large negative chromatic dispersion of 27101.8 ps/(nm-km) is achieved at λ = 1.542 μm with circular air holes only. In addition, the proposed PCF has good tunability with the temperature. Further, the coupling can be switched between transverse electric (TE) and transverse magnetic (TM) modes through external electric field due to the infiltration of the liquid crystal (LC) material. Furthermore, the DC-LCPCF with elliptical air holes has an ultra-high negative dispersion of 289,118 ps/(nm-km) at λ = 1.543 μm.