Use Of Liquid Crystals Research Articles

Imaging of liquid crystals confined in carbon nanopipes

This letter investigates the unexplored phenomenon of the wetting of liquid crystals inside carbon nanopipes. Carbon nanopipes are similar to carbon nanotubes but are open ended with straight walls. Using environmental scanning electron microscopy, the authors have observed liquid crystal menisci inside the nanopipes. Raman spectroscopy was used to confirm the presence of liquid crystals confined in the nanopipes and verify the physical interaction between the two materials. In addition, observations from reflection polarized optical microscopy experiments suggest the use of liquid crystals inside carbon nanotubes to develop electro-optic switches.

Modal liquid crystal devices in optical tweezing: 3D control and oscillating potential wells

We investigate the use of liquid crystal (LC) adaptive optics elements to provide full 3 dimensional particle control in an optical tweezer. These devices are suitable for single controllable traps, and so are less versatile than many of the competing technologies which can be used to control multiple particles. However, they have the advantages of simplicity and light efficiency. Furthermore, compared to binary holographic optical traps they have increased positional accuracy. The transmissive LC devices could be retro-fitted to an existing microscope system. An adaptive modal LC lens is used to vary the z-focal position over a range of up to 100 mum and an adaptive LC beam-steering device is used to deflect the beam (and trapped particle) in the x-y plane within an available radius of 10 mum. Furthermore, by modifying the polarisation of the incident light, these LC components also offer the opportunity for the creation of dual optical traps of controllable depth and separation.

Propeller-like Hydrogen-Bonded Banana−Melamine Complexes Inducing Helical Supramolecular Organizations

The results of the study presented here show a new example of the use of liquid crystals and the interactions involved in the mesomorphic state to build up complex molecular organizations. We have pursued a design strategy in which hydrogen bonding allows the combination of the pi-stacking tendency of melamine and the lateral interaction capability of V-shaped molecules, which has been extensively demonstrated through nematic, smectic, or columnar mesophases. This combination addresses the formation of columnar arrangements with inherent helical organization. In this work, nonmesomorphic as well as mesomorphic V-shaped acids, with a structure similar to banana liquid crystals, have been complexed to a 2,4,6-triamino-1,3,5-triazine derivative in a proportion 3 to 1, respectively. Hydrogen-bonded supramolecules whose formation and stability in solution have been proven by infrared and NMR techniques have been thus obtained. DOSY experiments have allowed us to assess in solution the presence of the complexes and their tetrameric composition. All the complexes display mesogenic ability, and their mesomorphic organization has been studied by X-ray diffraction and CD spectroscopy. Results allow us to propose a helical columnar model for the mesophase originated from a propeller-like conformation of the supramolecular complexes.

The use of liquid crystals as electrorheological fluids in microsystems: model and measurements

Fluids with controllable flow properties have gained considerable interest in the past few years. Some of these fluids such as magnetorheologic fluids are now widely applied to active dampers and valves. Although these fluids show promising properties for microsystems, their applicability is limited to the microscale since particles suspended in these fluids tend to obstruct microchannels. This paper investigates the applicability of electrorheologic liquid crystals (LCs) in microsystems. Since LCs do not contain suspended particles, they show intrinsic advantages over classic rheologic fluids in micro-applications. This paper presents a novel physical model that describes the static and the dynamic behaviour of electrorheologic LCs. The developed model is validated by comparing simulations and measurements performed on a rectangular microchannel. This assessment shows that the model presented in this paper is able to simulate both static and dynamic properties accurately. Therefore, this model is useful for the understanding, simulation and optimization of devices using LCs as electrorheological fluid. In addition, measurements performed in this paper reveal remarkable properties of LCs, such as high bandwidths and high changes in flow resistance.

Comparison of optical properties of thermally reversible light scattering films consisting in dispersions of polystyrene/naphthalene domains or polystyrene/liquid crystal (EBBA) domains in epoxy matrices

Thermally reversible light scattering (TRLS) films are materials that can be reversibly switched from opaque to transparent states by varying temperature. TRLS films containing different types of dispersed domains in an epoxy matrix: a) naphthalene/polystyrene (NP/PS), and b) liquid crystal (EBBA/PS), were synthesized by polymerization-induced phase separation. The intensity of transmitted light was measured as a function of temperature in successive heating/cooling cycles. Transparent states were observed during heating cycles at temperatures close to the melting point of NP (81 °C) or the nematic-isotropic transition of EBBA (at about 70 °C). The NP crystallization from their polystyrene solutions in the course of cooling cycles required large undercooling (about 30 °C). In the temperature range comprised between melting and crystallization, optical properties depend on the cooling rate and the thermal history of the material. The situation was quite different for TRLS films based on EBBA. In this case, the generation of a nematic phase from the polystyrene solution required a small undercooling (about 2–3 °C), leading to a relatively sharp transition between opaque and transparent states, independent of the thermal history. This constitutes a significant advantage of the use of liquid crystals compared to organic crystals in the manufacture of TRLS films.

Using liquid crystals with giant diamagnetic anisotropy to study the domain structure in magnetic materials

This paper discusses the use of liquid crystals (LCs) to study the domain structure in magnetic materials. Domains on the surface of thin amorphous films made from cobalt-containing alloys have been studied by means of LCs that possess giant diamagnetic anisotropy. The results of using LCs are compared with the results obtained using magnetic emulsions. © 2005 Optical Society of America

Compound hyperbolic concentrator-type reflectors for liquid crystal display backlight systems

A novel compound hyperbolic concentrator-type reflector for use in liquid crystal display backlight systems is proposed. The three-dimensional reflector is simplified to a two-dimensional reflector first. The two-dimensional reflector is governed by reflector equations which are deduced from an edge-ray method and conservation of energy directly. A reflector for the three-LED backlight system is simulated. This reflector improves the x-directional illumination uniformity in an xy section of the light-guide plate as well as avoiding the LED backlight shadow phenomenon. The ray-tracing results show that the x-directional illumination uniformity is improved to about double.

Comparison of stereoscopic digital imaging and slide film photography in the identification of macular degeneration

Background: This study compared the sensitivity and specificity of stereoscopic digital photography of the retina through a dilated pupil with a 45° nonmydriatic camera and Joint Photographic Experts Group (JPEG) compression of the images with the sensitivity and specificity of 35-mm slide film photography in the identification of age-related macular degeneration (AMD). Methods: Consecutive patients with a diagnosis of AMD were enrolled. Stereoscopic retinal images of the disc,macula and temporal macula were captured with a digital 45° nonmydriatic camera (then compressed into JPEG format) and with a standard fundus camera and slide film. A single retinal specialist graded both image formats in masked fashion, at least 1 month apart, using a modified Age-Related Eye Disease Study (AREDS) severity scale.The digital images were displayed on a monitor and viewed with the use of liquid crystal display shutter glasses and stereo imaging software.The film images were mounted on a light box and graded with the use of a stereoviewer. Primary outcome measures included the presence or absence of AMD pathological features. Positive and negative predictive values (PPVs and NPVs), sensitivity, specificity and weighted κ w statistics were calculated. Results: We photographed 203 eyes (of 103 patients) with both digital and slide film cameras. Correlation of the 2 image formats was substantial in identifying AREDS level 3a or greater (κ w = 0.64, standard error = 0.08, PPV = 0.95, NPV = 0.66, sensitivity = 0.93, specificity = 0.74) and excellent in identifying level 4b or greater (κ w = 0.83, standard error = 0.05, PPV = 0.81, NPV = 0.98, sensitivity = 0.94, specificity = 0.94). Interpretation: High-resolution stereoscopic, mydriatic, 45° digital images captured with a nonmydriatic camera and JPEG compressed correlate well with stereoscopic slide film photographs in the identification of moderate to advanced AMD (AREDS level 3a or greater).

Homeotropic alignment on surface-initiated liquid crystalline polymer brushes

With the development of new types of liquid crystal displays and the use of liquid crystals as organic semiconductors, homeotropic alignment layers have become more and more important. Liquid crystalline polymer (LCP) brushes have been suggested as an alignment layer and are expected to have more interaction with the liquid crystal phase than other homeotropic alignment layers. Here we report the synthesis of side chain liquid crystalline polymer brushes by surface-initiated Atom Transfer Radical Polymerisation (ATRP) of an acrylate functionalized mesogen on glass and silicon substrates. Brush thicknesses up to 20 nm were achieved. Substrates coated with a brush layer of this polymer were used for alignment of the nematic liquid crystal 5-cyanobisphenyl (5CB). Nearly defectless homeotropic alignment over several square centimetres is demonstrated. Microcontact printing has been used to obtain a patterned alignment layer.

Technological evolution and performances improvements of a tunable phase‐shifter using liquid crystal

AbstractThe topic of this paper is the study of electrically tunable phase‐shifters based on the use of liquid crystals. The dielectric characterization of the commercial liquid crystal involved in this study allows us to determine its dielectric permittivities (with and without DC voltage applied) and then its dielectric anisotropy. In this way, two phase‐shifter configurations were studied. Every devices propose inverted microstrip line. The two circuits have been realized with coplanar accesses to be use in the millimeter‐frequency range. With the aim to improve the phase shift parameter (in °/GHz/cm), our work aims to reduce the transmission line. Instead of a rectilinear transmission line, this one is based on a symmetric stub line. The phase shift obtained is enhanced from 0.87°/GHz/cm to 2.77°/GHz/cm. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 43: 338–341, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20463

Molecular Orientation of Some Fluorescent Dichroic Dyes in Nematic Liquid Crystal

Highly fluorescent dyes, derivatives of 1,8-naphthalimide and 3-benzanthrone, have been studied for use in liquid crystal displays of the “guest-host” type. Special attention has been given to the evaluation of the orientation of the dyes with planar molecular structure in the nematic liquid crystalline phase. By means of polarized absorption and emission spectroscopy, the order parameters SA, SF and D of the dyes in the liquid crystal mixture ZLI 1695 have been determined. Moreover, the absorption transition moment directions with respect to the long molecular axis of the dyes have been assigned.

Media for studies of partially aligned states.

Measurement of residual dipolar couplings for proteins in nuclear magnetic resonance (NMR) requires a degree of molecular alignment. This may be achieved through the use of liquid crystals or compressed hydrated gels. Several media have been described in the literature, and this chapter describes five of the most commonly used systems. For two of these systems, bicelles and filamentous bacteriophage, the media can be purchased in a form ready for experiments. The remainder must be made by the investigator, yet they are generally straightforward to synthesize in the laboratory. Poly(ethylene glycol)/alcohol mixtures and Helfrich phases are made by simply mixing the ingredients in the correct manner, and strained gels use techniques familiar to all molecular biologists.

Liquid crystal-based optical space switches fordwdm networks

This paper addresses the issue of using conventional liquid crystal materials as reconfigurable holographic or diffractive phase elements within optical space switches and the resulting constraints in their choice and use in a telecommunication environment. We discuss the advantage of this technology for implementing holographic-based approaches with respect to other techniques. As an illustration we give two examples which illustrate two generic architectures and complementary use of liquid crystal to implement them. Finally, we give some trends on implementations of high channel capacity space switches based on this technology.

ZLI-1695 Liquid Crystal Anisotropy Characterization in the Near Infrared by Generalized Ellipsometry

One key point for an advantageous use of liquid crystals in optical devices for telecom applications is a proper design and simulation capability. For this purpose it is mandatory to know the optical behaviour of these materials at the wavelength of interest, i.e. the transparency windows of the optical fibers in the Near-IR region. To this aim, we have applied Generalized Ellipsometry, an extension of standard ellipsometry that allows optical characterizations of anisotropic thin films, for investigating liquid crystal samples. The optical anisotropy of the ZLI-1695 liquid crystal was measured as a function of the wavelength in the spectral range from 300 to 1700 nm. Comparing our experimental data with those reported by the manufacturer (Merck) at a single wavelength, we have also estimated the ordinary and extraordinary refractive indices spectra.

Liquid crystals for photonic applications

In this paper we describe application of liquid crystals in optical imaging and processing. Electrically and optically addressed liquid crystal spatial light modulators are key elements in real-time holographic devices. Their implementation for beam steering and hologram formation is briefly discussed. The Joint Fourier transform optical correlator for pattern recognition is presented as well as the use of liquid crystals for the adaptive optics purposes is discussed.

Application of liquid crystals in liquid chromatography. From low- to high-molecular-weight liquid crystals

The use of liquid crystals as stationary phases for liquid chromatography is reviewed. The synthetic pathways used for covalent immobilization of low-molecular-weight liquid crystals on the silica-gel surface are discussed. The techniques used to characterize stationary phase structure are also presented. Finally, trend for replacement of low-molecular-weight liquid crystals by liquid-crystalline polymers, either coated or bonded, is described. Examples are given of the separation of specific chemical compounds on liquid-crystalline stationary phases.

Establishing a degree of order: obtaining high-resolution NMR structures from molecular alignment

I would like to thank Eva de Alba for providing structural data as well as Ad Bax and James Baber for their critical comments.

STUDY OF LYOTROPIC LIQUID CRYSTAL IN LUBRICATION ON ALUMINUM ALLOY SURFACES

ABSTRACT The phase diagram of the system formed by diethanol ammonium dodecanoate ( DCD ) / n-butano1 / paraffin oil (LP) /water was determined The lyotropic liquid crystal region was confirmed and the use of liquid crystal as lubricant on aluminum alloy surfaces was studied. The effect of the liquid crystal structure on the tribological properties was also evaluated. Comparing the wear scar widths and the vertical loads of liquid crystal with that of a commercial grease. The lamellar liquid crystal showed a higher load carrying capacity relative to the commercial grease.

DSC studies on p-cyanophenyl p-( n-alkyl)benzoate liquid crystals: evidence for polymorphism and conformational change

Temperatures and enthalpy changes are reported for the melting, clearing, and nematic-to-solid transition of the p-cyanophenyl p-( n-alkyl)benzoate (RCBz) liquid crystals. The results reveal that several solid structures exist for the RCBzs. Evidence that the transitions between the various solid forms involve conformational changes is provided by exotherms and endotherms appearing in heating and cooling cycles, respectively. The study also demonstrates that as long as melted RCBzs are not allowed to solidify they undergo repeated, reversible nematic-to-isotropic and isotropic-to-nematic transitions making them suitable for use in liquid crystal displays (LCD).

Transition analysis by surface temperature mapping using liquid crystals

The determination of the transonic position on a two-dimensional model is an important subject of studies in the ONERA CERT T2 cryogenic wind tunnel. The use of liquid crystals with a specific methodology of calibration, acquisition and image processing allowed to go beyond the usual stage of qualitative imaging and to determine the temperature map on the surface of the model, the variation with time of this temperature and to know the precise location of the laminar/turbulent transition