Dynamic Scattering Mode Research Articles

Convective Structure of Nematic Liquid Crystal in Parallelepiped Transverse Cell

The convective structures of electroconvection (EC) have been observed in parallelepiped transverse cells. The systems were William domain (WD), Sawtooth pattern (STP), and Dynamic Scattering Mode (DSM) in the convective area. However, the patterns did not appear in the non-convective area. In addition, the domain width of the WD pattern was examined to find their relation to the electric field, E*x. The domain width decreased when the higher voltage and the non-convective areas were expended.

Stable hazy states by electrohydrodynamic convection of ultraviolet-treated chiral nematic liquid crystal

In this paper, we show that electrohydrodynamic convection of ultraviolet (UV)-treated chiral nematic liquid crystals (CNLCs) generates stable foggy states. UV-irradiation produces relatively heavier and larger molecular ions at CNLC mixture, which activates electroconvection, and leads to easily dynamic scattering mode (DSM) with strong turbulence. Chirality of LC combined with the DSM builds two stable states (planar state and focal conic state) more strongly, unlike the conventional method generating the two states in the CNLCs. The molecular ion effect enables state conversion to a much lower switching voltage than that of the conventional cholesteric devices. The focal conic state connected to DSM2 exhibits a stable deep hazy state, the mixed state combined with DSM1 displays a stable weak hazy state, and the planar state activated in a particular frequency region creates a stable and very clear transparent state due to the chiral pitch of the infrared region. Therefore, a smart window with the tri-stable-hazy state specialized in low power consumption can be realized. The tri-stable-hazy state device also generates various hazy states according to the applied voltage; thus, it can provide a proper indoor temperature for users. Furthermore, a stable planar state with a chiral pitch designed in the infrared region may save energy through selective reflection of infrared rays from the sunlight in the summer. The proposed technology is suitable for commercial use in the next-generation smart window.

Electroconvection in Zwitterion‐Doped Nematic Liquid Crystals and Application as Smart Windows

AbstractElectroconvection (EC) in liquid crystals (LCs) is extensively studied for the formation of regular nonequilibrium structures, and the applications that require strong light scattering and high haze. Doping the LCs with ionic species has proven to be an effective method to obtain uniform and high‐quality textures. Herein, zwitterion is explored as a new class of ionic dopant that has little problem of ion accumulation as compared to typical organic electrolytes. A rich variety of EC patterns are observed and characterized for both positive and negative LCs doped with a zwitterion Reichardt's dye. The initial homogeneous or homeotropic alignment, as well as the voltage and frequency of the applied electric field, is shown to influence the formation of EC patterns, among which the dynamic scattering mode (DSM) is specially examined for the application as privacy‐protecting smart windows. Using an LC mixture doped with zwitterions, the smart window demonstrates outstanding performance, as well as much improved durability for a wide range of operating conditions. Such a method offers a simple strategy for the fabrication of polymer‐free smart windows.

Low and High-Frequency Electro-Hydrodynamic Patterns in Nematic Liquid Crystal Aligned Using Dual Layer Alignment Produced by Nanofiber

The optical properties and electro-hydrodynamics (EHD) patterns of 4-methoxybenzylidene-4-butylaniline (MBBA) which is parallel oriented by nanofibers has been observed. The alignment of the liquid crystals was made using polyvinyl alcohol (PVA) nanofiber by used modified electrospinning. Electrospinning method was modified using two commercial copper (Cu) printed circuit board (PCB) as a gap collector. These planar samples area is 20 mm x 25 mm. The optical characteristic of these samples was studied by using an optical polarizing microscope. The optical intensity changes by a rotation of crossed polarizers are observed. When an electric field was applied to these samples, the electrohydrodynamic (EHD) pattern was observed. At the low till high frequency, by increasing the voltage, the patterns were formed such as Williams domain (WD), fluctuating Williams domain (FWD), grid patterns (GP) and dynamic scattering Mode (DSM).

Selective variable optical attenuator for visible and mid-Infrared wavelengths.

This work demonstrates a variable optical attenuator (VOA) using dynamic scattering mode (DSM) in ion-doped liquid crystals with negative dielectric anisotropy. The mechanism of attenuation comes from optical scattering, which is generated by the electrically induced instability of undulation of LC textures. Electric fields are applied to switch the initial transparent state of the designed VOA to scattering states, varying the transmittance. The electric field also changes the size of the scattering domain from the LC texture and causes the designed device to exhibit an ultra-broadband selective operation in a visible to mid-IR spectral range. Furthermore, the VOA can selectively block one visible or mid-IR wavelength of light while letting other light pass. Such a VOA has many superior optical switching properties, such as high on/off contrast, insensitivity to polarization, and spectral selectivity; therefore, it has the potential to be used in practical optical systems.

Low-frequency electrohydrodynamic convection patterns in nematic liquid crystal aligned using parallel-oriented nanofiber

Parallel-aligned poly(vinyl alcohol) (PVA) nanofiber with a diameter of 240 ± 60 nm and an alignment parameter (S) of 0.95 ± 0.16 was obtained by a gap collector electrospinning that used copper (Cu) as a collector. The sandwiched cells (the horizontal-view and longitudinal-view) nematic liquid crystal was prepared by treating glass surfaces with the aligned PVA nanofiber to provide uniform anchoring of the director. When an electric field was applied to these samples, the electrohydrodynamic convection (EHC) pattern was observed. In the longitudinal-view cells, above a threshold voltage at low frequency, a typically low-frequency EHC rolls i.e., a Williams domain (WD) pattern was observed. By increasing the voltage, a fluctuating Williams domain (FWD) and grid patterns (GPs) could also be observed. In the transverse-view cells, at low-frequency regimes, WD, sawtooth patterned (STP), and dynamic scattering mode (DSM) patterns were observed. By replacing the conventional rubbing method with the use of parallel-aligned nanofibers, the well-known EHC phenomenon also could be observed.

The history of liquid-crystal displays

The modem history of liquid crystals has been dominated by the development of electronic displays. These developments began in 1964, when Heilmeier of RCA Laboratories discovered the guest-host mode and the dynamic-scattering mode. He thought a wall-sized flat-panel color TV was just around the corner From that point on, twisted-nematic (TN) mode, super TN mode, amorphous-Si field-effect transistor and room-temperature liquid crystals were developed. In the beginning, liquid-crystal displays (LCDs) were limited to niche applications such as small-size displays for digital watches, pocket calculators, and small handheld devices. That all changed with the development of the notebook computer industry. In 1988, Washizuka et al. of Sharp Corporation demonstrated an active-matrix full-color full-motion 14-in display using a thin-film-transistor array. The electronics industries now recognized that Heilmeier's 25-year dream of a wall-hanging television had become reality. LCDs could be used to replace existing cathode ray tubes. Through the cooperation and competition of many electronics giants, the LCD industry was firmly established.

Dynamic Image Analysis for Dynamic Scattering Modes in a Nematic Liquid Crystal

We have investigated the pattern fluctuation in a highly developed electro-hydrodynamic convection (EHC) by dynamic analyses of spatial Fourier coefficients of images. We measured the power spectrum and the two-time correlation function of spatial Fourier coefficients of images in the two turbulent states of EHC: the dynamic scattering mode (DSM)1 and DSM2 states. It was found that in the DSM1 state both the power spectrum and the correlation time show spatial anisotropy and have a characteristic peak at a wave number corresponding to the thickness of the cell in the rubbing direction. The peaks of the power spectrum and the correlation time decrease as the applied electric field is raised, and vanish entirely when the system undergoes the transition from DSM1 to DSM2. In the DSM2 state, there is no anisotropy in both the power spectrum and the correlation time.

Dynamic Image Analysis for Electro-Hydrodynamic Convection in Nematic Liquid Crystal

We have investigated the fluctuation in the electro-hydrodynamic convection(EHC) by developing a method for dynamic image analyses. The fluctuation of images is characterized by the two-time correlation function of the spatial Fourier coefficients of images. We measured the time correlation function of patterns in the two turbulent states of EHC:(i) the dynamic scattering mode(DSM) 1 and (ii)DSM2 states. It was found that in the DSM2 state, the correlation time, defined by the time at which the correlation function becomes a half of the initial value, decreases monotonically as the increase of the wave number. In the DSM1 state, on the other hand, the correlation time along the direction perpendicular to the rubbing direction has a peak at the wave number corresponding to the thickness of the cell. The peak value decreases as the applied electric field is raised, and vanishes entirely when the system undergoes the transition from DSM1 to DSM2.

Description of Dynamic Scattering Mode in Liquid Crystals by Analogy with Blackbody Radiation

Abstract A technique for experimental investigation of scattered intensities of LC elements showing dynamic light scattering mode has been developed on the basis of methods of the similarity theory conventionally used for description of blackbody spectral chacteristics.

Thermotropic homopolyesters. III. Preparation and properties of polymers based on 4′‐hydroxyphenyl‐4‐hydroxycinnamate

AbstractLow‐molecular‐weight 4′‐acetoxyphenyl‐4‐acetoxyoinnamate, as well as several polyesters synthesized from this monomer and aliphatic dibasic acids, exhibit thermotropic nematic phases. DSC heating curves for all of the polymers exhibit multiple transitions. The amount of crystallinity of these polymers at room temperature is small and the degree of order along the chain axis in the crystalline phase is poor. For the lower homologues the nematic phase exists over a broad temperature range of approximately 100°C. The polyester from chiral (+)‐3‐methyl adipate forms a thermotropic cholesteric phase. Both the diacetoxy monomer and azelate polymers of low molecular weight adopt the homeotropic texture on glass slides, but with increasing molecular weight the planar texture becomes preferred. Investigation of the effects of electric fields in the conduction regime upon the nematic phase of the diacetoxy monomer revealed that Williams domains are formed only with difficulty. In most cases, a stationary pattern appeared instead. At higher voltage the dynamic scattering mode (DSM) was obtained, and above this a field‐induced transition to the isotropic phase. The azelate polyesters exhibited Williams domains and the DSM in the conduction regime. The formation time for Williams domains was fairly short for polymers having ηinh < 0.44 dL/g, but increased to 80 min when ηinh = 0.68 dL/g. The DSM was only observed for polymers having ηinh < 0.61 dL/g. For these polymers the critical frequency separating the conduction and dielectric regimes exhibits a stronger temperature dependence than that of low‐molecular‐weight nematogens. A new instability pattern is reported for the azelate polyesters in the dielectric regime.

Transmission of Liquid Crystals in the near Infrared

Abstract The dynamic scattering mode (DSM) in nematic liquid crystals of negative dielectric anisotropy has been the subject of extensive fundamental and applied research, but almost all of these experiments have been performed in the visible spectral domain. In this wavelength range, when an increasing voltage (dc or low frequency ac) exceeding a threshold below 10 V is applied to a nematic thin layer held between two transparent electrodes, the incident light is scattered due to electrohydrodynamic instabilities, and the cell transmission decreases rapidly towards a low value (a few percent); this latter then remains constant, even if the applied voltage is high (80 V, for example).

A comparative market survey of liquid crystal displays: the United States, Japan and Europe

The earliest LCD principle to find commercial application was the dynamic scattering mode (DSM) based on ideas proposed by an American group of scientists in 1968. Three years later a Swiss group succeeded in making a vital contribution to the prosperity of the LCD industry by suggesting field effect-twisted nematic (TN)LCCDs, which eventually created a large-volume LCD market. In the meantime, Japan introduced both DSM and TN LCD technologies and now supplies a major proportion of the world's LCD market. But for chemical compounds, it is the Europeans who have long dominated, almost exclusively, the international market of basic LC materials. With stable markets, most LCD manufacturers are now putting a lot of emphasis on their research into dot matrix technology and colour-— mainlyguest-host —LCDs.

Effects of dielectric anisotropy on DSM-type display devices

The transmission of dynamic-scattering-mode (DSM) liquid-crystal cells was studied as a function of dielectric anisotropy and surface alignment. Dielectric anisotropy of about -0.2 was found to be optimum for the devices from the viewpoints of threshold voltage, contrast ratio, and recovery time. Vertical-aligned cells were found to have higher contrast ratios and longer recovery times than parallel-aligned cells.

Dynamic Analysis of the Growing Process to DSM in Nematic MBBA

The growing process to dynamic scattering mode (DSM) or a turbulent flow caused by the electrohydrodynamic instability in nematic MBBA is studied by dynamical analysis of the transmitted intensity of light. It is found that an autocorrelation function of the fluctuation in the intensity and its Fourier spectral function show an anomalous behavior at a time of the transition to DSM from metastable state.

Infrared Spectra of a Nematic Liquid Crystal in Dynamic Scattering Mode

Infrared Spectra of a Nematic Liquid Crystal in Dynamic Scattering Mode

Further studies of the dynamic scattering mode in nematic liquid crystals

Evaluation of fabrication procedures and operating life have indicated that moisture can play a major role in determining the performance of nematic liquid crystal displays based on the dynamic scattering mode (DSM) in anisylidene-p-aminophenyl-acetate. There is strong evidence that the presence of moisture promotes electron injection by lowering the potential barrier between the metal electrode and the liquid crystal. This could arise from a lowering of the effective work function of the metal by adsorption and/or increasing the electron affinity of the liquid crystal by molecular attachment. In any case the injected electron is captured by a neutral molecule and transported as a negative ion. The electron is removed by an oxidation process at the anode leaving the original molecular species free to repeat the process. It was also found that a short pulse (shorter than a space charge build-up time) could be used to electrically quench dynamic scattering and appreciably reduce the relaxation time. The use of this new scheme significantly improves the brightness and contrast of matrix addressed, real time displays based on dynamic scattering. A novel laminate structure for realizing such displays is also presented.

Scattering of Coherent Light from Nematic Liquid Crystals in the Dynamic Scattering Mode

Measurements of the scattering of laser light by anisylidene-para-amino-phenylacetate (APAPA) in the ``dynamic scattering'' mode associated with the passage of electric current have been made. The results have been interpreted by means of a model in which the light scattering is due to amplitude and phase modulation resulting from orientation fluctuations induced by the current flow. Coherence distances for the liquid crystal film are derived which are expected to give an inverse measure of the turbulence in it. The coherence distances are found to depend very strongly on the polarization of both incident and scattered light and to decrease markedly with either increased applied voltage or increased thickness; these results are explained in terms of the theoretical model.

Reflective liquid-crystal television display

A new reflective television display, operating off the air, is described. The display uses the dynamic scattering mode (DSM) in nematic liquid crystals and can be viewed in high-brightness ambients. The liquid crystal cell, 6 to 12 µm thick, was addressed in real time by means of an electron beam using a wire-mosaic faceplate on a demountable cathode ray tube. Two different liquid crystal materials--anasylidene paraamino phenyl acetate (APAPA) and an RCA proprietary room temperature liquid crystal--were tested with similar results except for the difference in operating temperature (82 to 110° C and room temperature, respectively). The resolution of the 3.1-cm-square display was somewhat less than that required for commercial television (∼150 lines), limited by the mosaic structure but not the liquid crystal. Adequate resolution should be achievable in slightly larger displays. The contrast was not optimized (7.5 to 1) but several techniques are available to improve this value to 15 or 20 to 1 as has been demonstrated in static displays. The subjective evaluation of this display points out the attractiveness of reflective television panels; under suitable external illumination the panel has a wide viewing angle (±45° approximately). The current and voltage requirements to achieve the DSM in liquid crystals are such as to raise hopes that an integrated addressing approach suitable for flat television panels can be found.

Reflective liquid crystal television display

A new reflective television display, operating off the air, is described. The display uses the dynamic scattering mode (DSM) in nematic liquid crystals and can be viewed in high-brightness ambients. The liquid crystal cell, 6 to 12 µm thick, was addressed in real time by means of an electron beam using a wire-mosaic faceplate on a demountable cathode ray tube. Two different liquid crystal materials--anasylidene paraamino phenyl acetate (APAPA) and an RCA proprietary room temperature liquid crystal--were tested with similar results except for the difference in operating temperature (82 to 110° C and room temperature, respectively). The resolution of the 3.1-cm-square display was somewhat less than that required for commercial television (∼150 lines), limited by the mosaic structure but not the liquid crystal. Adequate resolution should be achievable in slightly larger displays. The contrast was not optimized (7.5 to 1) but several techniques are available to improve this value to 15 or 20 to 1 as has been demonstrated in static displays. The subjective evaluation of this display points out the attractiveness of reflective television panels; under suitable external illumination the panel has a wide viewing angle (±45° approximately). The current and voltage requirements to achieve the DSM in liquid crystals are such as to raise hopes that an integrated addressing approach suitable for flat television panels can be found.