In recent years thermochromic liquid crystals (TLC) have been successfully used in non-intrusive heat transfer and fluid mechanics studies. Thin coatings of TLC’s at surfaces are utilised to obtain detailed heat transfer data of steady or transient process. Application of TLC tracers allows instantaneous measurement of temperature and velocity fields for two-dimensional cross-section of flow. Computerised flow visualisation techniques allow automatic quantification of temperature of the analysed surface or the visualised flow cross-section. Here, we describe our experience in applying the method to selected problems studied in our laboratory. They include modelling of flow configurations in the differentially heated inclined cavity with vertical temperature gradient simulating up-slope flow as well as thermal convection under freezing surface. The main aim of these experimental models is to generate reliable experimental database on velocity and temperature fields for specific flow. The methods are based on computerised true-colour analysis of digital images for temperature measurements and modified particle image velocimetry and thermometry (PIVT) used to obtain the flow field velocity.
In this paper, relation between the diffraction efficiency in LC dye doped cell in two wave mixing system and the applied voltage parameters had been described. The goal of this work was increase in diffraction efficiency using low frequency AC voltage. The LC cells used in the experiments were filled with pure and dye-doped liquid crystal mixtures. In this system, we obtained diffraction efficiency increasing about five to eight times.
In the work, the use of antiferroelectric and V-shape liquid crystals for video projection using LCOS microdisplays has been explored. Antiferroelectric grey levels arise from a double symmetric hysteresis loop that can be stabilised by a constant holding voltage, thus an active matrix is not strictly required in this case. If used, then the voltage levels and the waveform must be adjusted to fulfil the voltage limitations dictated by the matrix. V-shape materials lack hysteresis; therefore the active matrix is mandatory to stabilise the levels. Voltage limitations, however, are less restrictive in this case, since V-shape smectics require just a few volts for full switching.
Realisation of complex high information density LCDs and
systematic optimisation of their electro-optical and ergonomic performance
would not be possible in the required time-frame without reliable numerical
modelling of the electro-optical performance of such display devices. In
this paper, we outline the history of numerical LCD modelling starting
with Berreman and van Doorn, finally arriving at modern state-of-the-art
LCD-modelling in two and three dimensions.
Numerical modelling of LCDs is carried out in two steps:
first, the effect of the electrical field on the orientation of the liquid
crystalline alignment has to be evaluated before the corresponding optical
properties can be computed. Starting from LC-elasticity theory we present
suitable numerical methods for computing various states of LC-deformation
(stable, metastable, bistable, etc.) in one-dimensional problems.
Light propagation in layered anisotropic absorbing media
is evaluated with the methods that are based on Maxwell's equations (Berreman
4´4-matrix approach). This approach can be simplified to yield methods
with reduced computing time and sufficient accuracy for many problems (e.g.
extended Jones 2x2-matrix formalism). A finite element method with automatic
mesh generation and refinement for computing accurate solutions in two-dimensional
problems is presented and its application illustrated with examples (e.g.
IPS-effect, VAN-cells, etc.).
In two- and three-dimensional problems, i.e., in cells
with lateral dimensions comparable to the cell thickness, a variety of
different director configurations are possible for a given geometry and
electrical driving and addressing, making the modelling more complicated.
Moreover, local defects can occur, which should be also considered in the
simulation. Suitable approaches for the director field calculation, i.e.
the vector and the tensor approach, are discussed.
The complexity of the problem increases considerably
when a third dimension is added, e.g. the geometry of the problem has to
be defined in three dimensions together with the respective boundary conditions
(anchoring geometry and elasticity) and electrodes. If strong deformations
or even distortions are present in the orientation of the LC-layer, the
applicability of known one-dimensional approaches for computing the optical
properties must be checked and new approaches eventually have to be developed.
The third dimension prohibits the use of some standard methods (e.g. FDTD),
solely because of the enormous memory requirements and the long calculation
times. Other approaches are presented and discussed.
This paper contains the results of theoretical considerations about light propagation through the real TN display working in reflective and negative mode. This mode provides us with a possibility to obtain a colour image. We have done mathematical and numerical analyses of a propagation of light wave through LC displays with antireflective layer, glass planes, conductive layers, liquid crystal layer and polarisers. We have taken into account real conditions of a display operation, i.e., spectral properties of all components, optical anisotropic and dichroic properties of LC layer, reflections from all phase borders and also spectral characteristics of light source and sensitivity of human eye.
Electrooptics of smectic liquid crystals has been drawing an increasing interest, especially for new classes of compounds, such as metal alkanoates which manifest as thermotropic as lyotropic smectic mesophases. In the present work, electric and electrooptic properties were studied in detail of pure and dye-doped (polymethine dye potassium kaproate, a water mixture of which is forming a lyotropic Sm A type liquid crystalline phase at room temperature. As follows from experimental data, for frequencies - f < 3´104 Hz, dispersion of the components of complex dielectric permittivity e’ and e” is observed. Appropriate relaxation process is described by the Debye equation. On the basis of the analysis of frequency dependencies of e’, e” for the frequencies f > 3´104 Hz the conductivity of samples on an alternating current was found. The introduction of dye in ionic lyotropic liquid crystals (ILLC) results in increase in conductivity (by 1.5 times) and reduction of relaxation time. The bleaching of the dye in ILLC under the action of a direct voltage (U >5 V) was observed. The mechanism explaining “threshold” character of such process was proposed.
The direct measurement of the refraction index profile in the nematic layer (NL) creates possibility to verify and exploit the non-linear solution of the Ericksen-Leslie equation (E-L). It has been done for NL of 6CHBT tuned in wide range of external voltage. The symmetrical case of the director field distribution has been analysed. The way for local values of the electric field estimation inside the lc layer is discussed.
Two binary mixtures have been studied of homologues from a series derived from 4-alkoxybiphenyl-4'-carboxylic acid with three chiral centres exhibiting ferroelectric (FE), antiferroelectric (AF) and tilted hexatic phases. One homologue exhibits a re-entrant SmC* (SmC*re) phase, which is miscible with the SmC* phase. The phase diagrams of the studied mixtures exhibit a close pocket of the SmC*A phase. The SmC*re phase is induced even in the mixtures composed of homologues without this phase. Dielectric spectroscopy reveals the Goldstone mode in both ferroelectric SmC* and the SmC*re phases, this mode being stronger when increasing concentration of the antiferroelectric homologue. In the SmC*A phase a relaxation mode is detected with the relaxation frequency three orders higher, which exhibits critical slowing down when approaching the SmC*re phase. This mode, which can be attributed to the anti-phase mode, is responsible for the appearance of the SmC*re phase. In mixtures exhibiting the SmC*A phase a helix twist inversion occurs in the vicinity of the low temperature border of the AF phase.
In this paper we present experimental results on the influence of an external light on phase grating recording process in nematic liquid crystal cell with a photoconducting polymeric layer (PVK doped with TNF). Depending on voltage applied to the cell, the external light can amplify or attenuate the diffracted, into first orde,r light power measured in degenerate two-wave mixing experiment. From the time delay between the opening of the external light and the moment of diffraction signal change, we deduce information about the effective charge carriers mobilities in PVK:TNF polymeric layer. We also discuss and present simple explanation of the observed effect.
Recently, there have been shown that in planar liquid crystalline waveguides light beams can form spatial soliton due to the reorientational nonlinearity. Such self-trapped beams require only a few tens of mW of light power and their stability is controlled by the state of light polarisation. In this paper, the collisions of previously observed solitons are analysed theoretically. Obtained results show that analysed self-trapped beams became unstable due to the interaction with other light beams.
Dynamic properties of some polymer nematics were investigated by Freedericksz transition methods in a magnetic field. The linear aromatic polyester, comblike combined side/chain-main/chain polymer and comblike copolymers with mesogenic and functional acid groups in side chains were studied. The temperature dependencies of the effective rotation viscosity g1* were obtained. At close molecular weight and temperature the g1*, value for linear polymer exceeds that for comblike combined polymer. Therefore the existence of side mesogenic groups decreases velocity of the orientational processes of polymer nematic. However, the highest values of effective rotation viscosity were obtained for the nematic phase of the comblike functionalised copolymers. Hence, the formation of hydrogen bonds in these copolymer leads to slowing of orientational processes. Influence of the intermolecular hydrogen bonds on dynamic properties of functionalised copolymers is stronger than that of intramolecular hydrogen bonds.
The analysis of the tuned, liquid crystalline Fabry-Perot interferometer (FPTI) has been done by means of 4´4 matrix. Wide-angle incidence has been analysed in terms of Trollinger-Chipman correction for extraordinary wave. Results have been applied to determine constraints in design of the monochromatic tuned FPTI. Main features of the FPTI device and liquid crystal determining FPTI parameters has been described in detail. Special attention has been paid to dispersion of the LC’ refraction indices as a factor of spectral FPFI’ properties.
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Liquid crystal display unit for reconfigurable instrument
for automotive applications
P.M. Knoll and B.B. Kosmowski
Nowadays, the car drivers are faced with a rapidly increasing flood of information. In addition to established information systems (car radio, vehicle monitoring, mobile phones), high class vehicles feature navigation systems almost as standard. In the current decade, driver assistance and collision avoidance systems will appear in vehicles. Hence, there is an increasing demand for supplying the driver with more information that helps him to drive safer and more economical. The price decline in the computer market and the availability of powerful graphic hard- and software concepts make it possible to enhance the classical functions of the instrument board to an interactive multifunctional information panel - an interface between information systems of the car and the driver. Therefore, the question of additional visual and cognitive stress, and a possible distraction of the driver by the large amount of information, and its complexity becomes predominant. Reconfigurable instruments, based on a microprocessor controlled active matrix colour display, provide a powerful alternative to the usual mechanical/electromechanical instrument clusters in vehicles. They will help to strengthen passive safety, they adapt to user and situation requirements, and they are easy to install, to configure, and to maintain. Reconfigurable instruments in future cars will have a high impact on traffic since they can provide the driver with much more information, presenting it in a way that is flexibly matched to the importance of particular data and to the ergonomic properties of the driver. The functions are manifold and span from classical driver information like speed to navigation prompts and ultimately to video and multimedia access.
A system LC - waveguide may be applied in different devices for physical values registration, and optical signal controlling devices. It is important to use a planar waveguide in combination with peculiarity of liquid crystal materials that are characterised by a high sensitivity to physical values of external influence liquid crystal materials. At present, time waveguides are widely used as elements for signals processing and transmission in optical systems that are realised on different optical effects. The using of planar waveguide as backplane of LC cell alleviate orientation process of liquid crystal layer in contrast to fibre waveguide.
Photorefractive properties and possibility of guest-host effect application for real-time holography have been described. Isothiocyanate nematic liquid mixtures with positive dielectric anisotropy as hosts while antraquinone dyes as guests were used. In dye-doped nematic liquid crystals high diffraction efficiency approaching 20% were obtained. Fast optical grating formation with the time constant t of a few ms have been observed. The dependence of grating formation on strength of applied dc and low frequency of ac electric field, configuration of light polarisation and nematic director orientation were reported.