Showing papers on "Liquid crystal published in 1969"

Piezoelectric Effects in Liquid Crystals

Abstract: 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.

Erratum: Conduction‐Induced Alignment of Nematic Liquid Crystals: Basic Model and Stability Considerations

Abstract: It is shown by way of model orientation patterns how conduction in nematic liquid crystals may cause a torque per unit volume that more than offsets the dielectric torque felt in the insulating state. This conduction‐induced torque arises from an anisotropy of the conductivity or of the dielectric constant. It is partly dielectric but its more important part is shear‐induced, the shear flow being a consequence of space charge which is generated by and interacts with the applied electric field. The stability of a uniform orientation pattern in electric fields parallel or perpendicular to the preferred axis is investigated, with respect to a complete set of sinusoidal perturbations and including distortional and (stabilizing) magnetic torques. The analysis shows that the shear‐induced torque cannot be treated like its dielectric counterpart. It depends in a peculiar way on the direction of the perturbational wave vector and may contain a nonconservative contribution. Formulas for the instability threshold, ...

Scarabaeid beetle exocuticle as an optical analogue of cholesteric liquid crystals

Abstract: Summary 1. A review is given of the optical and architectural analogies between cholesteric liquid crystals and certain insect cuticles (Coleoptera: Scarabaeidae). Earlier observations on the optical properties (reflexion of circularly polarized light and high form optical rotation) are confirmed and extended. Both cholesteric liquid crystals and lamellate cuticle have helicoidal structure (Fig. i). Even though their chemistry and physical states are very different, we are justified in making the analogy, since their optical properties depend primarily on the pitch of their helicoidal architecture. 2. The unusual optical properties were located for the first time in the outer 5 to 20 μ of the exocuticle. This layer is transparent and has regular spacings in the range required for interference colours according to Bragg's law. Among Scarabaeid beetles which show interference colours, we distinguish two types of outer exocuticle. (i) Optically active cuticles which reflect circularly polarized interference colours; show high angles of form optical rotation in transmitted light; and anomalous form birefringence perpendicular to the cuticle surface (reversible by deproteinization). (2) Optically inactive cuticles which show none of the above properties and in which the form birefringence is parallel to the cuticle surface. In the electron microscope the ultrastructure of these two types of outer exocuticle is clearly different. 3. All of the optically active species reflect left hand circularly polarized light, irrespective of the wavelength of the reflected colour. They therefore appear dark when viewed through a right hand circular analyser. The sense of reflected circularly polarized light does not reverse at higher wavelengths as recorded by previous workers. (A simple treatment is given for combinations of various wavelengths with retardation plates of varying values, as used in circular analysers.) We confirm earlier reports that the sense of reflected circularly polarized light is of the opposite sense to the transmitted light. 4. Using monochromatic light we have measured the anomalous dispersion with wavelength of the magnitude of optical rotation for various optically active cuticles. The dispersion curves change from negative values at lower wavelengths to positive values at higher wavelengths, and cross the zero optical rotation axis at a wavelength (AQ) corresponding to the interference colour of each sample. There is reasonable agreement between A0 and the interference colour calculated from ultrastructural evidence and by comparison with interference filters of known wavelength. A dispersion curve measured for a combined sample of two cuticles with different dispersion curves showed that the resultant is an algebraic summation of the two component curves. 5. We present the first experimental verification of existing mathematical treatments of anomalous form optical rotatory dispersion curves. Although these treatments were derived for cholesteric liquid crystals, they give a reasonable fit to our measured curves for cuticle. We have confirmed from our cuticle dispersion curves that a second zero value for optical rotation occurs at a wavelength higher than A0, as predicted by the theory of Chandrasekhar and Rao (1968). This has not yet been observed in any cholesteric liquid crystal system. 6. Our evidence shows that in optically active cuticle, interference colour is determined by helicoid pitch. In Lomaptera interference coloration follows the bilateral symmetry of the insect. Hence helicoidal pitch is controlled in a bilaterally symmetrical manner. However, the sense of helicoid rotation is the same all over the beetle and is therefore bilaterally asymmetrical. This supports the view that helicoid pitch is under the local control of the epidermal cells which secrete the cuticle, whereas its sense of rotation may be determined by an extracellular self-assembly process. In view of the self-assembling properties of cholesteric liquid crystals, it is tempting to suggest that helicoidal cuticle could be formed by the stabilization of a liquid crystal. 7. We discuss in detail the differences between optically active and inactive cuticles. The constructive interference colours arising from both types are then briefly compared with other multiple layer reflecting systems in other animals. 8. A detailed comparison is made between the optics of cuticle and cholesteric liquid crystals. The optical analogy provides a two-way contact between cuticle biophysicists and liquid crystal physical chemists.

On Molecular Structure and Physical Properties of Thermotropic Liquid Crystals

Abstract: Some models for the molecular structures of liquid crystals including a ferroelectric liquid are discussed. It is shown that a smectic liquid with a layered structure, in which the preferred orientation of the molecular axis is inclined against the layers, has some similarities with a nematic liquid. Such a tilted smectic liquid may also have a twisted modification corresponding to a twisted nematic or cholesteric liquid crystal. Attention is drawn to an optically isotropic texture of a cholesteric liquid. A proposal is made for its molecular structure in which a fairly regular, lattice-like arrangement of alignment singularities is assumed. Twist waves in nematic liquids are regarded and numerical estimations are made for their wave velocity and damping in p-azoxyanisole. It is concluded that the damping is too strong for an observation of such waves.

Guest-Host Interactions in Nematic Liquid Crystals

Abstract: new electro-optic effect based on guest-host interactions in nematic liquid crystals is described. The cooperative alignment of a nematic liquid crystal in an electric field is used to orient “guest” pleochroic dye molecules (molecules whose absorption spectrum is a function of the molecular orientation with respect to the polarization of the incident light). Electronic color switching was achieved using fields of the order of 10 kV/cm (dc through audio) with a variety of dyes. A detailed study of the optical density changes as a function of field strength and dye concentration for one guest-host system is also presented.

Influence of Electric Fields on the Molecular Alignment in the Liquid Crystal p-(Anisalamino)-phenyl Acetate

Abstract: The effects of electric fields on the molecular alignment in the liquid crystal p-(anisalamino)-phenyl acetate are discussed The behavior of this material is similar to that of p-azoxyanisole in that the ordering which is normally observed shows an alignment with the molecular axis preferring a direction parallel to the field at low audio frequencies, and perpendicular to the field for frequencies of a few hundred kHz Results comparing the effectiveness of electric and magnetic fields for producing molecular alignment are presented which support a theory presented earlier to explain the ordering in p-azoxyanisole at audio frequencies This theory involves the anisotropy associated with the electrical conductivity Results are also presented which show that the effectiveness of dc electric fields is comparable to that for low audio frequencies A few comments are made concerning recent work employing electric fields to produce ordering in liquid crystals for NMR studies

Quasielastic Rayleigh Scattering in Nematic Liquid Crystals

Abstract: Using a light-beat laser spectrometer, we observe the relaxation of the thermally excited fluctuations of anisotropy in the nematic phase of $p\ensuremath{-}\mathrm{azoxyanisole}$. We identify the two low-frequency purely dissipative modes recently predicted. A first study of the angular dependence of the width of these modes allows a determination of three among the six viscosity coefficients introduced by Leslie.

Encapsulated cholesteric liquid crystal display device

Abstract: The present disclosure is directed to articles of manufacture, e.g., display devices, having an encapsulated liquid-crystal member of enhanced color purity, color contrast and visual resolution due to an overlying essentially transparent top layer having a substantially smooth exterior surface. This essentially transparent layer has an index of refraction which approximates that of the capsule cell wall material and any polymeric or other binder used in conjunction with the encapsulated liquid crystals in the encapsulated liquid crystal layer or member.

Continuum Theory of Liquid Crystals of Nematic Type

Abstract: We describe continuum theory now being used to describe macroscopic behavior of liquid crystals of nematic type. Two types of predictions are discussed. The first concerns the propagation of orientation waves. The second area is viseometry, including effects of simple electromagnetic fields. We discuss some unusual size effects predicted when such fields are absent.

Long Range Order and Thermal Fluctuations in Liquid Crystals

Abstract: Some recent applications of the Oseen-Zocher-Frank elastic theory of nematic and cholesteric liquid crystals are reviewed (1) The effects of a static magnetic field H are calculated in detail In particular, for a cholesteric system, the spiral structure is distorted by the field and the period increases with H, up to a critical field Hc at which a nematic phase is expected Hc is inversely proportional to the pitch of the unperturbed helix, and should be observable in systems with long periods A cholesteric → nematic transition of this type has in fact been qualitatively observed (2) The optical striations which are observed when a cholesteric liquid is inserted in a region of variable thickness have been shown by Cano to correspond to discontinuities of the torsion The transition region between regions of different pitch has been analyzed in terms of a “disclination line” normal to the helical axis (3) In a nematic liquid crystal the long wavelength thermal fluctuations of orientation can

Magnetically Induced Cholesteric to Nematic Phase Transition in Liquid Crystals

Abstract: We optically observe the untwisting and alignment, along an applied magnetic field, of high-pitch cholesteric liquid crystals, mixtures of para-azoxyanisole and cholesteryl esters. The results agree quantitatively with de Gennes's predictions and allow a determination of the twist elastic constant of pure para-azoxyanisole.

Molecular Theory of Flow Alignment of Nematic Liquid Crystals

Abstract: A mechanism to explain flow alignment of nematic liquid crystals is proposed. It is based on the rodlike shape of the molecules of these substances. A model calculation is performed in which the molecules are assumed to be equally and rigidly oriented ellipsoids of revolution, colliding with each other like the molecules of a gas. The two coefficients governing the linear relationship between torque per unit volume and shear rate are derived. The theory is used to predict the degree of flow alignment (which is found to be incomplete and to be independent of the shear rate) and to establish a relationship between the viscosity and one of the shear–torque coefficients. Theory and experiment are compared for p‐azoxyanisole, and good agreement is found.

The Structure of p-Azoxybenzenes in Magnetic Fields

Abstract: -A systematic X-ray diffraction study of the 10 homologues of n-azoxybenzenes has been performed. The structures of nematic and smectic mesophases as a function of the strength of the magnetic field and the temperature have been established.

Continuum Theory of Cholesteric Liquid Crystals

Abstract: This paper discusses a continuum theory which has recently been proposed for liquid crystals of the cholesteric type. As in earlier continuum theories, the liquid crystal is regarded as an incompressible liquid with a preferred direction at each point, described by a unit vector. Solutions of the equations are investigated for shear flow between two flat plates, one at rest, and the second moving parallel to the first with a constant velocity. It is found that the theory predicts non-Newtonian behaviour, and that secondary flow and temperature variations occur. The predictions are compared with available experimental data, and one finds agreement in that a uniform apparent viscosity is predicted at large shear rates.

Orientational order in p-azoxyanisole, p-azoxyphenetole, and their mixtures in the nematic phase.

Abstract: The formula proposed recently by Vuks for the polarization field associated with strongly anisotropic organic molecules has been applied to evaluate the orientational order in p-azoxyanisole, p-azoxyphenetole and their mixtures in the nematic phase. It is found to yield accurate results as is confirmed by the internal consistency of the calculations. The degree of order S plotted against the relative temperature (T c - T) gives a set of nearly parallel curves, and for a given relative temperature, S is greater the higher the nematic liquid transition point T c . The degree of order in the mixture is expressible as p 1 S PAA + p 2 S PAP where p 1 , p 2 are the mole fractions of the pure compounds.

NMR Determination of Deuterium Quadrupole Coupling Constants in Nematic Solutions

Abstract: The analysis of the NMR spectra of suitable nematic liquid‐crystal solutions appears to offer a convenient and desirable method for the determination of small nuclear quadrupole coupling constants for molecules in the liquid state. Nuclear quadrupole coupling constants have been determined for deuterium in the C–H bond of a number of small molecules. Assuming that the field gradient has axial symmetry, values of e2qQ/h along the bond axis are as follows: CH3CD3167± 18 kHzC3D6184± 20C2D2198± 7CH3C2D199.4± 2.0DCN199.0± 3.0. The results are discussed in terms of various theoretical calculations of the field gradient at the deuteron, and a compilation of some of the accurate experimentally determined deuterium coupling constants is presented.

Electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal

Abstract: Optical rotatory power measurement for compensated cholesteric liquid crystal helical structure to observe thermally induced inversion and electric field perturbations

Proton Spin-Lattice Relaxation in Liquid Crystals

Abstract: Measurements of spin-lattice relaxation in nematic and smectic liquid crystals are contrasted to recent theoretical predictions for ${T}_{1}$ in these phases.

Liquid crystal color display

Abstract: A color display system capable of producing essentially all the colors of the visible spectrum comprises three cells each including a solution consisting of a pleochroic dye in a nematic liquid crystal composition. Each of the solutions can change its transmission of polarized white light in response to an electric field so as to change the color appearance of the solution. One solution of the system can change in appearance from color less to magenta, another from colorless to cyan, and a third from colorless to yellow. The system includes means for applying an electric field separately to each of the solutions and means for passing polarized white light successively through each solution.

A Boundary‐Layer Effect in Viscometry of Liquid Crystals

Abstract: We here explore the possibility that, in viscometry of liquid crystals of nematic type, non‐Newtonian effects stem from a competition between boundaries and flow, each striving to produce a different orientation.