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Dual-Mode Switchable Liquid-Crystal Window
Abstract
Radiant energy-flow control and privacy control are two important features for smart windows (or glass). Current smart window technologies can, however, only control one of them: radiant energy flow or privacy. Therefore, a dual-mode smart window is highly desirable. Here, we report a dual-mode switchable liquid-crystal window that can control both radiant energy flow and privacy. The switchable liquid-crystal window makes use of dielectric and flexoelectric effects. In the absence of an applied voltage, the window is clear and transparent, and radiant energy can flow through it and the scenery behind the window can be seen. When a low-frequency (50 Hz) voltage is applied, the window is switched to an optical scattering and absorbing state by a flexoelectric effect, and thus, privacy is protected. When a high-frequency (1 kHz) voltage is applied, the window is switched to an optical absorbing but nonscattering state through a dielectric effect, and thus, radiant energy flow is controlled.
... Liquid crystal (LC) materials have been widely used in the field of electro-optical devices due to their unique optical properties and stimuli responsiveness. [1][2][3][4][5][6][7] However, pure LC materials have a low mechanical strength and durability, and high complexity to process and control, which limits their applications in many fields such as flexible electronics and wearable sensors. Polymer-liquid crystals (PLCs) are a kind of materials with stable structures formed by introducing the polymer matrix into pure LCs, which may effectively solve the above problems. ...
Polymer-liquid crystals (PLC) is a common material for smart windows. However, PLC smart windows usually require high driving voltage to maintain transparency. In this work, we synthesized a novel PLC smart film by doping multi-wall carbon nanotubes (MWCNT) into a reverse-mode polymer network liquid crystal (R-PNLC). It was found that doping MWCNT could effectively reduce the threshold voltage ( V th ) of R-PNLC from 19.0V to 8.4V. Due to co-orientation between MWCNT and LC molecules, the doped R-PNLC was able to maintain a high transmittance of visible light (~80%) without an applied electric field. We found that doping MWCNT could change the frequency modulation property of R-PNLC. The doped R-PNLC exhibits a wider frequency modulation range up to 40,000Hz, while the frequency modulation of the undoped R-PNLC reached to a saturation at 23,000 Hz. We also tested the electromagnetic interference (EMI) shielding efficiency of R-PNLC and found that the EMI shielding efficiency could be improved by doping only 0.01wt% MWCNT into the system. The total shielding effectiveness (SE T ) value of 0.01wt% MWCNT doped R-PNLC was up to 14.91 dB during frequency band of 5.38 GHz-8.17 GHz. This study demonstrates that the films are potentially useful for low-energy-consumption smart windows with enhanced electromagnetic shielding capability.
... In one voltage condition, the windows are transparent with high transmittance. In another voltage condition, they are optically scattering (Long and Ye, 2014;Fuh et al., 2017;Jiang et al., 2019). ...
... Smart optical devices using LC materials can realize multiband protection, light intensity regulation, and fast responses to multiple stimuli, among other functions. Consequently, LC materials 68 have been widely used in light protection 69 . The next section discusses the application of LCs in the field of light protection from the perspective of principles of protection. ...
With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection. With the development of optical technology, LCs with intelligent light protection function become more and more important. This paper discusses the research progress and challenges of LCs for light protection.
... In one voltage condition, the windows are transparent with high transmittance. In another voltage condition, they are optically scattering (Long and Ye, 2014;Fuh et al., 2017;Jiang et al., 2019). ...
Global warming is becoming a more and more severe crisis for humans. One way to resolve the concern is to reduce energy consumption. Smart switchable windows for office and residential buildings and vehicles can help reduce energy consumption. An ideal smart window should be able to control radiant energy flow and privacy. We investigated the capability of switchable windows based on liquid crystal/polymer composites, such as polymer dispersed liquid crystal (PDLC), polymer stabilized liquid crystal (PSLC), and polymer stabilized cholesteric texture (PSCT), to control the privacy and radiant energy flow. Through a systematic study, we identified methods to improve their capabilities. We demonstrated that PDLC and PSCT windows of sufficient thick film thickness can control both privacy and energy flow.
... The dielectric anisotropy of MAT978 is À4.0 and the dielectric aniso- tropy of ZLI4330 is À1.9. The dielectric anisotropies of CB7CB and CB9CB are about + 2 [63]. The reason to use the two dimers is that if only one dimer is used, it is difficult to obtain large flexoelectric coefficients because its limited solubility in the regular nematic LCs. ...
Reconfigurable optical diffraction grating is of great importance for monochromator, spectrometer, optical fiber telecommunication, laser protection, antenna and more. We report a liquid crystal diffraction grating based on flexoelectric effect. When no voltage is applied, the liquid crystal is unidirectionally aligned and does not diffract light. When a DC voltage is applied, the liquid crystal is switched to a 1-dimensional periodic structure due to flexoelectric interaction and becomes diffracting. The periodicity and refractive index undulation of the periodic structure are determined by the elastic energy, due to the spatial variation of the liquid crystal director, and flexoelectric interaction energy. The diffraction angle and efficiency can be controlled by the applied voltage. The liquid crystal diffraction grating has the advantages of low driving voltage, high diffraction efficiency and independency of the polarization of incident light.
Windows are used everywhere, from buildings to vehicles. Smart switchable windows are highly sought‐after in order to reduce energy consumption. An ideal smart window should have two functions. First, it should be able to control privacy. Second, it should be able to control radiant energy flow. Here, a smart window is reported that possesses the two functions. The smart window is based on a dichroic dye‐doped cholesteric liquid crystal. It exhibits two stable states in the absence of applied voltage. One of them is the planar state that is optically absorbing without haze and can control radiant energy flow. The other state is the focal conic state that is optically scattering and can control privacy. Furthermore, when voltage is applied, the liquid crystal is switched to a heliconical state, where the transmittance can be tuned continuously. Thanks to its superior performance, this smart window has a big potential for applications in architectural and vehicle windows.
In this paper, we reported a highly thermal stable YP‐11 polymer stabilized cholesteric liquid crystal (YP‐11 PSCT) for smart windows that can be electrically switched from transparent to opaque. The polymerization between photo‐sensitive monomers (RM 257) has been characterized using a polarized optical microscope (POM), which has been found to become much firmer and denser by raising the doping concentration. The 3D polymer matrices remarkably affect the electro‐optical performances of YP‐11 PSCT, and the cholesteric liquid crystals (Ch LCs) within polymer matrices were found merely able to be reversibly switched until the doping concentration of RM 257 was raised up to 3%. Specifically, the threshold voltage, raising time and decaying time of a 3% YP‐11 PSCT cell were determined as 22.908 V, 85.442 ms and 18.617 ms, respectively. The critical thermal‐stable temperature of the 3% YP‐11 PSCT was further evaluated using a hot stage and was confirmed as high as up to 100 °C. These findings in this paper will significantly contribute to the development of smart windows.
Photochromic or thermochromic liquid crystal (LC) smart windows have attracted wide attention due to their spontaneous transmittance modulation under different environments. There remains a challenge for the LC smart windows that can be modulated with light and temperature simultaneously owing to the difficulty in selecting photothermal molecules. Herein, we selected a photothermal molecule, isobutyl-substituted diimmonium borate (IDI), which shows excellent characteristics of a photothermal material used in smart windows, such as transparency in the visible light range with a slight brown color, good compatibility with the LC system, and excellent photothermal effect compared with common photothermal materials. Thus, a photothermal dual-driven smart window is developed by doping IDI into chiral LC mixtures, which can efficiently modulate the transmittance at different temperatures (or light intensities) by varying the phase state from the homeotropically oriented smectic phase (transparent) to the focal conic cholesteric phase (opaque). The transmittance is high (70%) when the ambient temperature is low and the light intensity is weak, allowing more sunlight to enter the room. The transmittance is low (20%) when the ambient temperature is high and the light intensity is strong, which prevents sunlight from entering the room. The proposed smart window will have a promising application in terms of energy saving and personalized privacy protection.
Glass with multiple light control functions has received considerable attention in recent years. This study demonstrates a multifunctional smart glass composed of hybrid‐aligned ion‐doped nematic liquid crystals (LCs). In the absence of an applied electric field, the hybrid aligned LC is transparent and can be used for light field shaping. The transmittance of the glass can be limited to less than 2% with a 1D oblique viewing angle of >50°. In addition, by varying the frequency and strength of the electric field applied to the device, tinted and scattering states can be achieved with polarization rotation and the electrohydrodynamic effect, respectively. The nonscattering tinted state can control transmittance in the range from 38% to 0.2%. The scattering state provides a 99% high‐haze performance. These results demonstrate that the proposed smart glass can be used for meeting the novel requirements of the next generation of green and smart buildings and vehicles. A multifunctional smart glass capable with light field shaping, tinted, and scattering control is achieved herein. With geometric orientation of liquid crystal in space, the smart glass can exhibit the light field with 1D transmitting. While manipulating with electric field, the designed smart glass can be transited into the tinted or scattering state according to the applied frequency and strength.
We report a light waveguide liquid crystal display (LCD) based on the flexoelectric effect. The display consists of two parallel flat substrates with a layer of flexoelectric liquid crystal sandwiched between them. A light-emitting diode (LED) is installed on the edge of the display and the produced light is coupled into the display. When no voltage is applied, the liquid crystal is uniformly aligned and is transparent. The incident light propagates through the display by total internal reflection at the interface between the substrate and air, and no light comes out of the viewing side of the display. The display appears transparent. When a voltage is applied, the liquid crystal is switched to a micrometer-sized polydomain state due to flexoelectric interaction and becomes scattering. The incident light is deflected from the waveguide mode and comes out of the viewing side of the display. We achieved thin-film-transistor active matrix compatible driving voltage by doping liquid crystal dimers with large flexoelectric coefficients. The light waveguide LCD does not use polarizers as in conventional LCDs. It has an ultrahigh transmittance near 90% in the voltage-off state. It is very suitable for transparent display, which can be used for head-up display and augmented reality display.
We present a flexoelectro-optic liquid crystal (LC) analog phase modulator with phase range at a 1 kHz switching frequency. The chiral nematic LC mixture consists of the bimesogen CBC7CB with chiral dopant R5011, aligned in the uniform lying helix mode. The mixture exhibits rotation of the optic axis for a drive voltage of ( ). The rotation of the optic axis is converted into a phase modulation with the aid of a reflective device configuration incorporating a LC cell, a polarizer, two quarter-wave plates, and a mirror. The residual amplitude modulation is found to be . This flexoelectro-optic phase modulator combination has the potential to enable analog spatial light modulators with very fast frame rates suitable for a range of applications.
Although LCDs are the leading technology for flat panel displays, their energy efficiency is low. One way to improve the energy efficiency is to decrease the driving frequency when static images are displayed. As the driving frequency is decreased, the transmittance of the display may vary with time, known as flickering. We carried out both experimental and simulation studies to investigate the origins that cause the flickering problem. Our results show that flexoelectric effect caused by non‐uniform liquid crystal director configurations and ions in the liquid crystal are the main factors responsible for the flickering. We quantitatively analyzed the flickering caused by the two factors.
We demonstrated that hybrid anchoring of dye-doped cholesteric liquid crystals (ChLCs) could be used for the simultaneous control of haze and transmittance. Hybrid anchoring of ChLCs can be obtained by the vertical anchoring at one substrate surface and planar anchoring at the other substrate surface. In a ChLC cell with hybrid anchoring, the LCs near the planar alignment layer are in the planar state, while those near the vertical alignment layer are in the focal-conic state. In the initial opaque state, the incident light can be absorbed by the LC mixture in the planar state and scattered by the LCs in the focal-conic state. The ChLC cell with hybrid anchoring exhibited lower transmittance and haze value than those of a focal-conic state in a ChLC cell with vertical anchoring. The cell can be switched to the transparent state without use of a complicated drive scheme at a driving voltage significantly lower than that for a double-layered cell.
We demonstrate a normally transparent smart window based on a cholesteric liquid crystal with negative dielectric anisotropy. The window is electrically switchable between clear and diffuse states. The clear state exhibits a well-aligned planar cholesteric texture and is stable in the absence of an electric field, while the diffuse state is switched on when applying a field higher than the undulation instability threshold. The degree of translucency can be controlled by varying the field strength. When the applied field is removed, the smart window relaxes back to the clear state spontaneously. It is also found that a much faster diffuse–clear process can be stimulated by reducing the field below the instability threshold. The smart window presents itself a promising projector screen for augmented reality applications as it features single stable state, sub-second switching speed, and polymerization-free fabrication.
We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rod-like cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist bend nematic, NTB, and the oblique helicoidal cholesteric, ChOH, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pre-transitional regions near the N-NTB and N-to-isotropic (I) transitions. The splay constant K11 is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K33 and twist K22 constants are measured by DLS. K33 being the smallest of the three constants, shows a strong non-monotonous temperature dependence with a negative slope in both N-I and N-NTB pretransitional regions. The measured ratio K11/K22 is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist and bend fluctuations in the N phase are comparable to those of nematics formed by rod-like molecules. All three show strong temperature dependence, increasing sharply near the N-NTB transition.
We demonstrate a giant flexoelectro-optic behavior of liquid crystal dimer CB7CB. Flexoelectric properties of CB7CB experimentally characterized by measured angle of an in-plane rotation of helical axis (HA) in polymer stabilized uniform lying helix cholesteric liquid crystal. The 45° rotation of HA providing full intensity modulation of transmitted through a pair of crossed polarizers light, is achieved with 4.5 V/μm with a sub-millisecond electro-optic switching time. Reported properties enable application of CB7CB in applications of the flexoelectric effect in fast switching photonic and electro-optic devices.
Suspended particle device (SPD) switchable glazing can change optical transmission from "opaque" state to "transparent" state in the presence of an alternating current (AC) power supply. It can be applied to control internal temperatures in buildings. Thermal characterisation of both SPD and same area of a double-glazing sample was accomplished using an outdoor test cell in Dublin, Ireland. The overall heat transfer coefficients (U value) were calculated for both systems from the experimental data. The average U values for SPD and double glazing samples were found to be 5.9W/m2K and 2.98W/m2K, respectively. Addition of double-glazing to this SPD switchable single glazing offered a U value of 1.99W/m2K.
We propose a light shutter device using dichroic-dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is set to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). A dye-doped long-pitch ChLC cell is switchable between the dark planar state and the transparent homeotropic state. It has the advantages of high transmittance, low operation voltage, and an easy fabrication process relative to previous LC light shutter devices. The proposed light shutter device is expected to achieve high visibility for transparent organic light-emitting diode displays and emerging smart windows, which can be used in airplanes, cars, and other similar applications.
We developed a bistable polymer stabilized cholesteric texture (PSCT) light shutter which can be switched between a transparent state and an opaque state by voltage pulses. The PSCT light shutter is switched to a transparent state by a low frequency voltage pulse and remains transparent after the pulse. It is switched to an opaque state by a high frequency voltage pulse and remains opaque after the pulse. It can be used for architectural and greenhouse windows and is very energy-efficient. (C) 2010 The Japan Society of Applied Physics DOI: 10.1143/APEX.3.021702
Due to the existence of a macroscopic helical structure, oriented cholesteric liquid crystals (CLCs) can selectively reflect light. The wavelength bandwidth for these Bragg reflections is limited: typically a few tens of nanometers due to limited birefringence values available in organic mesomorphic compounds. Here, we show that larger bandwidths may be reached by associating two layers of polymer-stabilized CLCs (PSCLCs). In the case of near-IR spectrum, it is shown that the optical properties of the bilayer system are not the sum of the individual properties. Time stability is investigated because the very existence of an interdiffusion between soft matter layers. Optical characteristics of the PSCLC can be tuned by an electric field and transitions between broadband reflecting, scattering, and transparent states are possible. For applications, switchable broadband reflections in liquid concentration media are relevant for white-or-black polarizer-free flat displays and “smart” windows in buildings. © 2001 American Institute of Physics.
CHOLESTERIC liquid crystals, in which the orientation of the molecules varies in a helical fashion, are used for optical filtering of circularly polarized light, for example in liquidcrystal displays. They reflect circularly polarized incident light of the same handed ness as the cholesteric helix, and in a wavelength band that depends on the helical pitch (repeat distance). This pitch can be selected by careful design of the liquidcrystal molecules or by mixing cholesteric materials with nematic (linearly oriented) liquid crystals, which tend to increase the pitch. Stable optical filters are produced by crosslinking the cholesteric molecules by photopolymerization1; these filters typically have a reflection wavelength bandwidth of ~50 nm. Here we show that, by introducing a gradient in the pitch of the cholesteric helix, we can obtain reflection of one of the two circularly polarized components over the entire visible spectrum. Polarizers with such broadband reflectivity would greatly improve the light yield and energy efficiency of liquidcrystal display devices.
The cholesteric-liquid-crystalline structure, which concerns the organization of chromatin, collagen, chitin, or cellulose, is omnipresent in living matter. In technology, it is found in temperature and pressure sensors, supertwisted nematic liquid crystal displays, optical filters, reflective devices, or cosmetics. A cholesteric liquid crystal reflects light because of its helical structure. The reflection is selective - the bandwidth is limited to a few tens of nanometers and the reflectance is equal to at most 50% for unpolarized incident light, which is a consequence of the polarization-selectivity rule. These limits must be exceeded for innovative applications like polarizer-free reflective displays, broadband polarizers, optical data storage media, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. Novel cholesteric-liquid-crystalline architectures with the related fabrication procedures must therefore be developed. This article reviews solutions found in living matter and laboratories to broaden the bandwidth around a central reflection wavelength, do without the polarization-selectivity rule and go beyond the reflectance limit.
A linear electro-optic effect in a cholesteric liquid crystal is described and attributed to the flexoelectric effect. An electric field applied perpendicular to the helix axis rotates the director about an axis parallel to the field. This produces a periodic splay-bend pattern in the helix, which couples flexoelectrically to the field.
Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.
Power consumption of a liquid crystal display, when it is used to display static images, can be reduced by using low frequency driving. However, when the driving frequency is decreased, the brightness of the display may change with time, a phenomenon known as image flickering. One factor responsible for the flickering issue is flexoelectric effect which is sensitive to the polarity of the applied voltage. We show that the flickering in fringe field switching (FFS) LCD can be significantly reduced by doping a liquid crystal dimer and using polymer stabilization. We demonstrated that 2 Hz driving frequency can be used to display static images.
Some classes of nematic liquid crystals can be driven through turbulent regimes when forced by an external electric field. In contrast to isotropic fluids, a turbulent nematic exhibits a transition to a stochastic regime that is characterised by a network of topological defects. We study the deformations arising after the electric field has been switched-off. In contrast to the turbulent regime, the relaxation of this topological-defect regime involves the annihilation of an interlacement of defect lines. We show that these defect lines separate regions of the nematic having topologically non-equivalent textures.
We construct a flexoelectric liquid-crystal display based on the in-plane-switching mode. The display uses a hybrid alignment cell where there are intrinsically splay and bend deformations of the liquid crystal, which promote the flexoelectric effect. In the absence of applied electric fields, the liquid crystal is aligned in a vertical plane. When a horizontal electric field is applied, the liquid crystal is azimuthally switched out of the vertical plane through the flexoelectric interaction, which is much stronger than the dielectric interaction in regular in-plane-switching displays. Also, the azimuthal rotation of the liquid crystal is sensitive to the polarity of the applied electric field. A positive field produces a counterclockwise rotation, while a negative field produces a clockwise rotation. We experimentally measure the flexoelectric coefficients and study the static and dynamic electro-optical properties of the display. We observe that the turn-on and turn-off times are linearly proportional to the inverse of the applied electric field. We also carry out theoretical and simulation studies and obtained results which agree well with the experimental results. Because the flexoelectric interaction is sensitive to the polarity of the applied voltage, both turn-on and turn-off times can be significantly reduced by applying voltages.
We report wormlike flexoelectric structures evolving deep in the Freedericksz state of a nematic layer of the liquid crystal cyanobiphenyl-(CH2)7-cyanobiphenyl. They form in the predominantly splay-bend thin boundary layers and are built up of solitary flexoelectric domains of the Bobylev-Pikin type. Their formation is possibly triggered by the gradient flexoelectric surface instability that remains optically discernible up to unusually high frequencies. The threshold voltage at which the worms form scales as square root of the frequency; in their extended state, worms often appear as labyrinthine structures on a section of loops that separate regions of opposite director deviation. Such asymmetric loops are also derived through pincement-like dissociation of ring-shaped walls. Formation of isolated domains of bulk electroconvection precedes the onset of surface instabilities. In essence, far above the Freedericksz threshold, the twisted nematic layer behaves as a combination of two orthogonally oriented planar half-layers destabilized by localized flexoelectric distortion.
In the present work, the fabrication processes and display characteristics of reverse mode polymer stabilized cholesteric texture (PSCT) of the light shutters are reviewed. Unlike the operation of normal mode PSCTs, the reverse-mode PDLC films remain transparent at external field OFF and are opaque in the presence of an external electric field ON, respectively. Display devices based on reverse-mode PSCT films are suitable useful for large area windows displays, light shutters, reflective and transmissive displays; because such technology offers an excellent clear voltage-off state, large viewing angle, fast switching speed and very small hysteresis. The current work provides a comprehensive review of the advantageous features of the reverse mode PSCTs, their operational principles and their fabrication techniques. It further reviews some of the past and current ongoing research employed for the development into this display field. In addition, it also provides a detail discussion on the various factors that influence the electro optical performance of reverse mode PSCT light shutters.
Electric field-induced patterns in liquid crystals have been observed and studied for about 50 years. During this time, a great variety of structures, detected under different conditions, have been described; theoretical descriptions were also developed parallel with the experiments and a huge number of papers have been published. The non-vanishing interest in the topic is due to several factors. First, most experimentalists working with new (or even well-known) liquid crystals apply sooner or later an electric field for different purposes and, as a response, often (maybe undesirably or unexpectedly) have to face with emergence of patterns. Second, understanding the complexity of the formation mechanism of regular patterns in a viscous, anisotropic fluid is an extremely challenging theoretical task. Third, specialists in display fabrication or in other applications are also interested in the results; either to make use of them or in order to avoid field-induced patterns. In this review, we attempt to provide a systematic overview of the large amount of published results, focusing on recent achievements, about the three main types of electric field-induced patterns: transient patterns during the Freedericksz transition, flexoelectric domains and electroconvection. As a result of different instability mechanisms, a variety of pattern morphologies may arise. We address the physical background of the mechanisms, specify the conditions under which they may become effective, discuss the characteristics of the patterns, and summarize the possibilities of morphological transitions induced by frequency, voltage or temperature variations. Special emphasis is given to certain topics, which recently have gained enhanced interest from experimental as well as theoretical point of view, like driving with ultra-low frequencies or non-sinusoidal (superposed) waveforms, and the dynamics of defects and embedded colloidal particles. Assisting newcomers to the field, we also mention some, yet unresolved, problems, which may need further experimental and/or theoretical studies.
A light shutter, which consists of a dye-doped cholesteric liquid crystal (ChLC) layer and a polymer-dispersed liquid crystal (PDLC) film, for simultaneous control of haze and transmittance is demonstrated. In the opaque state, it can not only provide a black color by using the dye-doped ChLCs but also hide the objects behind the display panel by using the PDLC film. The proposed light shutter shows a high haze value of 90.7% with a low specular transmittance of 1.20%. By switching the proposed light shutter placed at the back of a see-through display, we can choose between the see-through mode and the high-visibility mode in a see-through display.
Liquid crystals have been used to make electrically switchable light shutters (windows), but most of them are monostable: opaque in the absence of applied voltage and transparent when a voltage is applied. Here we report a bistable switchable light shutter based on cholesteric liquid crystal doped with tetrabutylammonium bromide. The salt makes it possible for the liquid crystal to have different electro-optical responses to applied voltages with different frequencies. The shutter can be either transparent or opaque in the absence of applied voltage. It can be switched from the transparent state to the opaque state by applying a low frequency (60 Hz) voltage pulse and switched back to the transparent state by applying a high frequency (2 kHz) voltage pulse. Because of the bistability, it can be used for energy-saving switchable privacy control and architectural windows.
Liquid Crystal Devices are crucial and ubiquitous components of an ever-increasing number of technologies. They are used in everything from cellular phones, eBook readers, GPS devices, computer monitors and automotive displays to projectors and TVs, to name but a few. This second edition continues to serve as an introductory guide to the fundamental properties of liquid crystals and their technical application, while explicating the recent advancements within LCD technology. This edition includes important new chapters on blue-phase display technology, advancements in LCD research significantly contributed to by the authors themselves. This title is of particular interest to engineers and researchers involved in display technology and graduate students involved in display technology research.
We constructed a cholesteric liquid crystal/polymer network system and studied the aligning effects of the polymer network on the liquid crystal. We stabilized different liquid crystal states by varying the polymer concentration. Using a proper polymer network, we developed a bistable polymer stabilized cholesteric texture (PSCT) light shutter at zero field. The PSCT light shutter is switched to a transparent state by a voltage and remains transparent after removal of the voltage. When the shutter is heated to elevated temperatures, it is switched into a scattering state and remains scattering when cooled to low temperatures. (C) 2009 The Japan Society of Applied Physics
A smart window is fabricated from a composite consisting of elastomeric poly(dimethylsiloxane) embedded with a thin layer of quasi-amorphous silica nanoparticles. The smart window can be switched from the initial highly transparent state to opaqueness and displays angle-independent structural color via mechanical stretching. The switchable optical property can be fully recovered after 1000 stretching/releasing cycles.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
In this paper, a new integrated multifunctional flexible device called the Energy Storage Smart Window (ESS window) was designed and fabricated. The proposed ESS window comprises an integrated supercapacitor and electrochromism function in one flexible device using ordered polyaniline nanowire arrays as electrodes. The ESS window showed high areal capacitance and high stability as a supercapacitor, and in situ optical measurements proved its electrochromic function. Furthermore, the ESS window can be integrated with a conventional solar cell to form a smart device system, which can simultaneously harvest, store, and use renewable energy efficiently.
A theory of the relationship between curvature strains and electric polarization in liquid crystals is developed in analogy to piezoelectric theory in ordinary crystals. The theory may explain some recently observed phenomena in nematic liquid crystals.
The control of sunlight can be achieved either by electrochromic or polymer-dispersed liquid crystal (PDLC) smart windows. We have recently shown that it is possible to homeotropically align fluid mixtures of low molecular mass liquid crystal with a negative dielectric anisotropy, and a liquid crystalline monomer, in order to obtain electrically switchable chromogenic devices. They are new materials useful for external glazing. In fact, they are not affected by the classical drawbacks of PDLCs. In this paper we present a new self-switchable glazing technology based on the light-controlled transmittance in a PDLC device. The self-adjusting chromogenic material, which we obtain, is able to self-increase its scattering as a function of the impinging light intensity. The relationship between the electro-optical response and the physical–chemical properties of the components has been also investigated.
— Polymer-stabilized cholesteric-texture light shutters are made of cholesteric liquid crystal and polymer-network dispersions. In the polymer-stabilized cholesteric-texture reverse-mode light shutter, the dispersed polymer network stabilizes the transparent planar texture of the liquid crystal at zero field. When an electric field is applied to the dispersion, the liquid crystal is switched to the focal conic texture, which is optically scattering. We systematically studied the effects of the pitch of the liquid crystal on the electro-optical properties of the light shutters. The dispersion can be used to make projection and reflective displays.
We developed a dual frequency cholesteric light shutter which utilizes a cholesteric liquid crystal; the dielectric anisotropy of the material changes from positive to negative when the frequency of applied voltage is increased. The shutters are switched to the transparent homeotropic texture by low frequency voltages, and switched to the reflecting planar texture by high frequency voltages. We designed a waveform which reduced the transition time from the homeotropic texture to the planar texture to less than 100 ms. © 1997 American Institute of Physics.
Cholesteric liquid crystal displays employ cells consisting of parallel glass plates which have indium-tin-oxide coating as the electrode and polyimide as the alignment layer. We have performed a systematic study of the reflections from the interfaces and the liquid crystal. We measured the reflection spectra using various incident light, various polarizations, and detection polarizations. Using the Berreman 4×4 method, we simulated the reflection spectra without fitting parameters; the results agreed well with the experimental results. © 1998 American Institute of Physics.
▪ Abstract Cholesteric liquid crystals possess a helical structure and exhibit two stable states at zero field: the planar texture and the focal conic texture. In the planar texture, they reflect circularly polarized light, whereas in the focal conic texture, they scatter light in forward directions. They can be switched from the planar texture to the focal conic texture by a low-voltage pulse and switched from the focal conic texture to the planar texture by a high-voltage pulse. The wavelength of the reflected light is easily adjusted by varying the pitch of cholesteric liquid crystals. They can be used to make reflective displays that do not need a back light and have a good readability under room-light conditions. We first review the optical properties of bistable cholesteric reflective displays and discuss the techniques used to achieve high contrast and large viewing angle. We then discuss the transitions among the cholesteric textures and the drive schemes used to address bistable cholesteric refle...
The response times and operating voltages of light shutters formed from polymer dispersed liquid crystals (PDLCs) have been studied experimentally and the results compared with calculations based on non-sperhically shaped nematic droplet models. The experiments were performed on light shutters with elongated and uniformly aligned droplets where the relaxation time and voltage response were measured. It is shown that the droplet shape can be a dominant factor, particularly for the relaxation time, and the data are compared with equations derived in terms of the aspect ratio of the droplet l = a/b, where a and b are the lengths of the semi-major and semi-minor axes, respectively, of the elongated droplet. It is further demonstrated that the electric field inside the droplet can be considerably smaller than the applied field, due to the conductivity and dielectric properties of the polymer and liquid crystal materials. These data are used to obtain values for the ratio of the conductivities of the polymer binder and liquid crystal droplet, as well as the anisotropy of the conductivity in the liquid crystal.
We have measured the optical transmittance of UV-cured epoxy PDLC films at normal incidence as a function of applied voltage for a number of different wavelengths. By controlling the cure rate of each film, we were able to study a range of droplet sizes from approximately 0.5 microns to 2.5 microns. By assuming a simple exponential decay law for the transmitted intensity, we have extracted the wavelength dependence of the effective droplet scattering cross-section. This cross-section depends strongly on both the applied voltage and the droplet size. We compare the results with a model based on the anomalous diffraction description of the scattering and find good quantitative agreement.
We have investigated the light switching properties of polymer dispersed liquid crystal (PDLC) films, which can be characterized by the spectral change in transmittance and reflectance due to switching by application of a suitable electrical voltage. The PDLC samples were prepared using the standard liquid crystal mixture E7 in a UV-curable matrix material. Using these samples, a transmission reduction of up to 30% was achieved in the solar spectral range. A strong dependence of the light switching behaviour on the mass ratio of liquid crystal and matrix material as well as on the curing rate of the matrix material was found, whereas the film thickness has only a small influence. Light scattering measurements allowed us to determine the average size of the scattering centres. In the samples investigated the liquid crystal droplets have radii below 2 mu m. Additional measurements of the electric properties of the PDLC films showed that the capacity and the resistivity of the PDLC films depend on both the applied AC voltage and its frequency. The power consumption of the above PDLC films is in the range of 5 W m-2, if powered with a line frequency of 50 Hz; at lower frequencies (e.g. 20 Hz) the power consumption can be reduced to about 2.5 W m-2 without lowering the change in transmittance significantly.
A large contrast ratio (> 60%) and rapid switching (0.3–1 s) electrochromic (EC) polymer device that consists of a laminated two-layer structure between two electrodes was prepared. The new design consists of an indium tin oxide (ITO) glass electrode, a cathodic EC polymer film, a solid electrolyte, and a counterelectrode that replaces the anodic EC polymer and ITO electrode. Four EC polymers including two new EC polymers, Poly[3-methyl-3-propyl-3,4-dihydro-2H-thieno(3,4-b)(1,4)dioxepine] (PProDOT-MePro) and Poly[3,3-diethyl-3,4-dihydro-2H,7H-(1,4)dioxepino(2,3-c)pyrrole] (PProDOP-Et 2), were synthesized as cathodic EC polymers. A carbon-based counterelectrode was prepared for comparison with an Au-based counterelectrode. Several kinds of polymer gel electrolytes were prepared for comparison. The devices (windows) were increased in area from 0.028 × 0.04 in. 2 , 1 × 1 in. 2 to 3 × 3 in. 2 Three main components, the EC polymer film, the gel electrolyte, and the counterelectrode, were studied and their optical properties, conductivities, and repeatabilities were compared. The effects of window size on the contrast ratio, switching speed, power usage, and repeatability were studied.
This paper covers three aspects of electrochromic smart windows. Their energy efficiency is discussed, and it is argued that
a control strategy considering whether a room is in use or not can lead to large savings of the energy needed for space cooling.
With regard to durability, it is shown that chemical compatibility between the electrolyte and electrochromic films of tungsten
oxide and nickel oxide can be achieved without loss of optical transparency. Finally, we consider device manufacturability
and present data on precharging of electrochromic nickel oxide films by ozone treatment.
We studied the electro‐optical response of a bistable cholesteric texture (BCT) display to ac voltage pulses. The material can be driven into states where planar and focal conic textures coexist at zero field and gray scale memory is achieved. According to the properties of the BCT display we designed two drive schemes; one for binary operation and the other for gray scale operation. We made a 320×320 pixel reflective display with a resolution of 80 lines/in. on a passive matrix. Measurement in an integration chamber showed that the display has higher contrast and better viewing angle than a reflective super twisted nematic display.
A new dispersion involving a polymer in low concentration with a cholesteric liquid crystal is reported. Two types of light modulators from these materials are described as a normal mode shutter which is opaque (light scattering) in the field‐OFF state and transparent in the field‐ON state and a reverse‐mode shutter with the opposite field conditions. The transparent state of both cells is haze‐free for all viewing angles making the material attractive for window applications and direct view displays.
The light scattering and electro‐optic response of new material with display potential are investigated. The materials consist of microdroplets of nematic liquid crystals which are spontaneously formed in a solid polymer at the time of its polymerization. Droplet size, spacing, and distribution are readily controlled in these materials to allow optimization of displays based upon electrically controlled light scattering from the liquid crystal droplets. Preliminary experimental and theoretical studies of the light scattering properties show these materials to offer new features suitable for many display applications.
A survey on prototype and currently commercial dynamic tintable smart windows has been carried out. The technologies of electrochromic, gasochromic, liquid crystal and electrophoretic or suspended-particle devices were examined and compared for dynamic daylight and solar energy control in buildings. Presently, state-of-the art commercial electrochromic windows seem most promising to reduce cooling loads, heating loads and lighting energy in buildings, where they have been found most reliable and able to modulate the transmittance up to 68% of the total solar spectrum. Their efficiency has already been proven in hot Californian climates, but more research is necessary to validate the products for colder climates, and to improve furthermore the commercial products in order to control the indoor climate in a more energy efficient way by reducing both heating and cooling loads.
Electrochromic devices have the ability to produce reversible and persistent changes of their optical properties. The phenomenon is associated with joint ion and electron transport into/out of an electrochromic thin film, in most cases being a transition metal oxide. This paper outlines the various applications of such devices in smart windows suitable for energy-conscious architecture, in variable-reflectance mirrors, and in display devices. Critical materials issues and design concepts are discussed. The paper also covers two specific research topics: computed electronic structure of crystalline WO3 incorporating ionic species, showing how reflectance modulation emerges from a first-principles calculation; and Li+ dynamics in heavily disordered Ti oxide, illustrating how diffusion constants derived from impedance spectroscopy can be reconciled with the Anderson—Stuart model.
Une résolution complète du problème d'instabilité flexoélectrique est donnée dans le cas d'un échantillon d'épaisseur finie et sans approximations sur les constantes élastiques. Total solution of flexoelectric instability problem is given for finite thickness sample and real elastic moduli.
We present a continuum theory model of switching in a bistable nematic liquid crystal device. The bistability of the device investigated relies on the fact that one of the cell surfaces exhibits two stable anchoring states, that is, two surface director orientations are locally stable. Since the other surface exhibits monostable, homeotropic anchoring there are two possible ground state director orientations within the cell, depending on the director orientation at the bistable surface. We first investigate the stability of these base states and find a critical surface anchoring strength below which only one of the states is stable. We also investigate the process of switching between the two stable states through the application of an electric field and the presence of a flexoelectric polarization. At high field strengths the dielectric interaction with the applied field will dominate the flexoelectric effect and may hinder switching. We find, therefore, that a window of possible field strengths exists within which switching occurs.
An eco-efficiency analysis was conducted using indicators suitably defined to evaluate the performance of an electrochromic window acting as an energy saving component in buildings. Combining the indicators for various parameters (control scenario, expected lifetime, climatic type, purchase cost) significant conclusions are drawn for the development and the potential applications of the device compared to other commercial fenestration products. The reduction of the purchase cost (to 200 euros/m2) and the increase of the lifetime (above 15 years) are the two main targets for achieving both cost and environmental efficiency. An electrochromic device, implemented in cooling dominated areas and operated with an optimum control strategy for the maximum expected lifetime (25 years), can reduce the building energy requirements by 52%. Furthermore, the total energy savings provided will be 33 times more than the energy required for its production while the emission of 615 kg CO2 equivalent per electrochromic glazing unit can be avoided.