Showing papers on "Liquid crystal published in 1972"
TL;DR: This article showed that elastic strain energy may account for the tendency of some nematic liquid crystals to lie parallel to the direction of rubbing on a solid surface that has been slightly deformed by rubbing, or perpendicular to a surface that is slightly rough in two dimensions.
Abstract: We show that elastic strain energy may account for the tendency of some nematic liquid crystals to lie parallel to the direction of rubbing on a solid surface that has been slightly deformed by rubbing, or perpendicular to a surface that is slightly rough in two dimensions.
782 citations
TL;DR: The role of the phase in the second order smectic A↔ nematic transition is similar to the role of phase functions in superconductors as mentioned in this paper, where twist and bend distortions correspond to magnetic fields.
705 citations
TL;DR: A comparison is made between certain fibrous and regularly twisted biological materials and certain ordered liquids commonly called 'esteric liquid crystals' and examples of twisted arrangements and typical textures of cholesteric mesophases.
Abstract: A comparison is made between certain fibrous and regularly twisted biological materials and certain ordered liquids commonly called 'esteric liquid crystals' Three examples of twisted arrangements (Arthropod cuticle, Ascidian tunica, Dinoflagellate chromosomes) and typical textures of cholesteric mesophases are studied for their optical properties and their defects. These materials are strongly different. Very long polymer chains occur in the organic matrix of skeletal tissues or in chromosomes. On the contrary, in a cholesteric liquid crystal, the molecules are free to move one along the other. However the geometry of such systems is similar. The objections to the twisted model are reviewed and evidence is presented to support a generalized twisted model. A list of the known biological cholesteric analogues is given.
662 citations
TL;DR: In this paper, a unified hydrodynamic theory is presented that is appropriate for crystals; smectic, cholesteric, and nematic liquid crystals; glasses; and normal fluids.
Abstract: A unified hydrodynamic theory is presented that is appropriate for crystals; smectic, cholesteric, and nematic liquid crystals; glasses; and normal fluids. In the theory, the increased spatial degeneracy as the system progresses from crystalline and mesomorphic phases to the isotropic fluid phase is marked by successive reductions in the number of firstorder elastic constants and in the number of transport coefficients. Distinction between local lattice dilations and local mass changes, and recognition of processes like vacancy diffusion that this difference makes possible, are crucial for understanding the connection between theories in different phases. Formulas are derived that give the number of hydrodynamic modes and the frequencies, lifetimes, and intensities of these modes in all of the above systems. In the nematic and cholesteric phases, the results agree with some found previously. In more complex systems, they are new. An attempt is made to explain the differences between the present hydrodynamic theory and other phenomenological proposals.
647 citations
TL;DR: Vacuum-deposited films have been used in substrate preparation for liquid-crystal cells as mentioned in this paper to provide an oriented surface for alignment of nematic liquid crystals far better than the well-known ''rubbed'' plate.
Abstract: Vacuum‐deposited films now replace the familiar ``rubbing'' process in substrate preparation for liquid‐crystal cells. Films deposited at an angle provide an oriented surface for alignment of nematic liquid crystals far better than the well‐known ``rubbed'' plate. The oblique deposit causes a film growth in a preferred direction, which results in a ``sympathetic alignment'' of the nematic liquid crystals when applied.
558 citations
TL;DR: The apparent heat capacities of the lipids in the liquid crystal state are, with the possible exception of dipalmitoyl lecithin, no more than 5 cal deg-1 (mole of lipid)-1 larger than in the gel state, indicating that the hydrocarbon chains have much less mobility in theLiquid crystalline statethan in the corresponding liquid normal paraffins.
453 citations
TL;DR: In this paper, X-ray scattering intensities from unoriented samples of cholesteryl nonanoate and myristate are reported for several temperatures in the smectic $A, cholesteric, and isotropic liquid phases.
Abstract: X-ray scattering intensities from unoriented samples of cholesteryl nonanoate and myristate are reported for several temperatures in the smectic $A$, cholesteric, and isotropic liquid phases. The measured Bragg-scattering intensities from the smectic planes are used to test a recent theoretical model of the smectic $A$ phase. Strong pretransition scattering (short-range-order or order-parameter fluctuations) are observed in the cholesteric phase and a Landau theory is constructed to describe this effect.
302 citations
TL;DR: In this article, the factors governing electrically induced phase transitions and other electro-optic effects in liquid crystals are discussed and experimental results presented, and it is shown that by suitably tailoring materials response times of the order of microseconds can be achieved.
258 citations
TL;DR: In this article, the problem of a nematic liquid crystal in an electric field is solved, and numerical solutions for various values of parameters are given for various parameters of the liquid.
Abstract: The problem of a nematic liquid crystal in an electric field is solved. Effects due to conductivity of the liquid have been neglected. Numerical solutions are given for various values of parameters. The solution is compared with experimental data taken from the literature. A computer program has been developed to do a non-linear least-squares fit of experimental data with the solution given in the present paper.
247 citations
TL;DR: In this article, a two-dimensional system of 170 long hard ellipses (whose axis ratio equals 6) was considered as a twodimensional model for a nematic liquid crystal.
Abstract: By Monte Carlo calculations in the canonical ensemble, we have studied some properties of a two‐dimensional system of 170 long hard ellipses (whose axis ratio equals 6). This system can be considered as a two‐dimensional model for a nematic liquid crystal. We have shown that this system exhibits two first‐order phase transitions: a solid‐nematic type phase transition at high density and a disorientation phase transition at a density 1.5 times smaller. This result suggests that excluded volume effects might play an essential role in the disorientation phase transition of nematic liquid crystals.
245 citations
TL;DR: In this paper, the Maier-Saupe theory of the nematic mesophase is extended to rod-like molecules interacting with a perfectly general anisotropic potential, and the resulting series expansion for the pseudo-potential accounts quantitatively for the observed temperature dependence of the orientational order, even when the expansion is restricted to just the first two terms.
Abstract: The Maier-Saupe theory of the nematic mesophase is extended to rod-like molecules interacting with a perfectly general anisotropic potential. The resulting series expansion for the pseudo-potential accounts quantitatively for the observed temperature dependence of the orientational order, even when the expansion is restricted to just the first two terms. However, the agreement can only be obtained if the expansion coefficients are assumed to depend on the inverse fourth power of the molar volume. The orientational molar enthalpy of transition calculated with the parameters determined by fitting the orientational order is qualitatively correct, although rather too large. The theory is also modified to allow for deviations from a spherically symmetric distribution of the inter-molecular vector in the mesophase. A comparison of this new expression for the pseudo-potential with that derived by Chandrasekhar and Madhusudana reveals that certain of their assumptions are invalid. Finally, calculations made with the new pseudo-potential indicate that a modification of the Maier-Saupe theory based solely on deviations from a spherically symmetric distribution cannot yield quantitative agreement with theory.
TL;DR: The dependence of the enthalpies of the cooperative transitions of aqueous dispersions of synthetic dimyristoyl and dipalmitoyl l-α-lecithin on the cholesterol content has been determined in an adiabatic differential scanning calorimeter.
TL;DR: In this paper, the theory of schlieren textures is discussed and forces between singularities are considered, and it is shown that for an isolated pair of singularities with the same absolute characteristic number but with opposite sign, the curves of equal molecular alignment are circles through the singularities.
Abstract: Observations on schlieren textures of nematic and smectic-C liquid crystals in thin layers are reported. The theory of schlieren textures is discussed and forces between singularities are considered. A singularity can be characterized by a positive or negative number which is proportional to the number of brushes it shows between crossed polarizers. In smectic-C, only singularities with four brushes are observed, nematics may in addition show singularities with two brushes. The latter singularities lead to conclusions on the molecular order and alignment: without the formation of inversion walls they can only occur in the absence of polarity and when the molecules align parallel to the surface. Singularities of opposite sign attract each other, those of equal sign repel each other. The sum of the characteristic numbers of the singularities in an extended layer tends to be zero. Under special assumptions about the elastic constants, it can be shown that the forces between singularities are additive and inversely proportional to the distances and that, for an isolated pair of singularities with the same absolute characteristic number but with opposite sign, the curves of equal molecular alignment are circles through the singularities.
TL;DR: In this paper, the authors present a theoretical treatment and experimental observations of the deformation that occurs in a nematic liquid crystal when electric or magnetic fields are applied to the liquid crystal.
Abstract: Abstract We present a theoretical treatment and give experimental observations of the deformation that occurs in a nematic liquid crystal when electric or magnetic fields are applied. We consider only normal deformations in the nematic material where fluid flow and other dynamic phenomena play no role. Three important sample geometries are considered in the magnetic field, and the experimentally observed deformations are in good agreement with theory. The normal deformation induced by electric fields is of interest from a device standpoint, and we give a solution for the deformation that is valid even for large dielectric anisotropics. This solution has been experimentally verified. We give a detailed comparison of the distortions produced by electric and magnetic fields and show that the deformations are of a similar form even though the field is nonuniform in the electric case. The change in birefringence and electrical capacitance as a function of distortion is discussed as a means of observing the deformation.
TL;DR: In this article, the bend and splay elastic constants of MBBA were measured over its nematic temperature range and the ratio of the two constants is 1.25± 0.05.
Abstract: The bend and splay elastic constants of MBBA were measured over its nematic temperature range. The ratio of the two constants is 1.25± 0.05. The temperature dependence of the elastic constants is found to be adequately described by the simple mean‐field models. A comparison is made between viscosity coefficients obtained from the elastic constants and light scattering spectra and from classical viscosity measurements. A method proposed for the measurement of twist elastic constants is evaluated and an upper limit of this constant of MBBA as a function of temperature is reported.
TL;DR: In this article, the authors measured the temperature dependence of the dielectric constants e, e⊥, and eis of some nematic liquid crystals with strong positive anisotropy (e ≃ 3 e ⊥) and determined the height of the potential barrier which hinders the reversal of the rodlike molecules in the nematic order.
Abstract: Measurements of the temperature dependence of the dielectric constants e‖, e⊥, and eis of some nematic liquid crystals with strong positive dielectric anisotropy (e‖ ≃ 3 e ⊥) are reported. The low frequency dispersion regions of the dielectric constants parallel to the unique axes were measured to be in the range from 10 kHz to 10 MHz. A correlation between the lengths of similar molecules and their relaxation frequency in the nematic state was found. The height of the potential barrier which hinders the reversal of the rodlike molecules in the nematic order was determined by measuring the temperature dependence of the low frequency relaxation and comparison with the temperature dependence of viscosity.
TL;DR: In this article, the temperature dependence of orientational order at constant molar volume in the nematic liquid crystal $p$-azoxyanisole was investigated and it was shown that both energetic and steric intermolecular interactions are present.
Abstract: Results on the temperature dependence of orientational order at constant molar volume in the nematic liquid crystal $p$-azoxyanisole are reported for the first time. These results clearly indicate the failure of theories of the nematic phase which do not include both energetic and steric intermolecular interactions.
TL;DR: The ratio of the elastic constants of nematic liquid crystals is calculated using a simple inner field model as mentioned in this paper, where the first three constants, which are the constants for splay, twist, and bend, respectively, are found to be positive, in contrast with the result of an earlier calculation which was based on the same model but neglected second order splay • bend contributions to the deformation energy.
Abstract: The ratio of the elastic constants of nematic liquid crystals is calculated using a simple inner field model. The result obtained is k11′: k22 : k33′: k24 : k13(2)=5:11:5:‐9:‐6. The first three constants, which are the constants for splay, twist, and bend, respectively, are found to be positive, in contrast with the result of an earlier calculation which was based on the same model but neglected second order splay‐bend contributions to the deformation energy.
TL;DR: In this article, the electron spin relaxation processes for oblate and prolate symmetric top paramagnetic probes dissolved in liquid crystals are studied by solving the diffusion equation for molecules subjected to an orienting potential, expressed as a series expansion of Legendre polynomials.
Abstract: The electron spin relaxation processes for oblate and prolate symmetric top paramagnetic probes dissolved in liquid crystals are studied by solving the diffusion equation for molecules subjected to an orienting potential, expressed as a series expansion of Legendre polynomials. The line shape variations upon ordering have been calculated for vanadyl complexes and nitroxide radicals, and the effect of the contributions of quadratic and quartic terms of the potential energy function, and of the diffusion tensor anisotropy have been examined.
TL;DR: In this article, the authors report field-induced realignment of a nematic liquid crystal, which has a negative dielectric anisotropy, is initially homeotropic.
Abstract: This paper reports field‐induced realignment of a nematic liquid crystal. A thin nematic film, which has a negative dielectric anisotropy, is initially homeotropic. Application of a threshold field parallel to the optic axis distorts the nematic ordering and increases the film's birefringence in certain light‐propagation directions. Experimental results are given for threshold voltage, threshold width, optical contrast ratio, frequency response, angular field of view, electrical power consumption, and transient response. Light‐valve applications are described.
TL;DR: In this paper, the adsorption of a monolayer of hexadecyltrimethylammonium bromide (CTAB) on glass-slides allows the preparation of monocrystal of p-n -methoxybenzilidene-p -butylaniline (MBBA) oriented either prependicularly to surfaces or in a direction parallel to surfaces (planar anchorage).
TL;DR: In this paper, the authors examined the possibility that plate-like molecules might give rise to biaxial liquid crystal order by considering a molecular field lattice model in which molecules are represented by rectangular plates.
Abstract: The possibility that the repulsive interaction between platelike molecules might give rise to biaxial liquid crystal order is examined by considering a molecular‐field lattice model in which molecules are represented by rectangular plates. We find that plates which are neither very square nor very rodlike in shape may exist in a phase characterized by biaxial long‐range order in the orientation of the plates. This biaxial phase occurs at high pressure. At some lower pressure the biaxial phase undergoes a second order phase transition to a uniaxial phase. At a still lower pressure the uniaxial phase undergoes a transition to an isotropic phase. This last phase transition is usually first order in that there are discontinuities in the density and the orientational order in going through the transition. However the size of the discontinuities depend strongly on the shape of the plates. For example for plates of length 6 and width approximately 2.22 and thickness 1, the discontinuities are zero, and the transition is accidentally second order. The discontinuities at this uniaxial‐isotropic transition are always very small for plates which have the potential to give rise to a biaxial phase. This last result is shown to have a direct physical basis which should transcend the limitations imposed by the approximate nature of our calculation. Thus we conclude that for real substances likely to have biaxial phases of the type we have studied, the value of the nematic order parameter at the nematic‐isotropic transition should be much lower than for other nematic liquid crystals.
TL;DR: In this paper, the authors achieved spatially uniform tunable birefringence with negative dielectric anisotropy materials using a unique previously unreported molecular configuration at zero field, where molecules align with their long axes at a small angle to the sample normal.
Abstract: Spatially uniform birefringences tunable over the range 0. 0–0. 15 for applied voltages less than 20 V rms and with a sharp threshold below 4 V rms are obtained for thin layers of nematic liquid crystals with negative dielectric anisotropy. The achievement of spatially uniform tunable birefringence with negative‐dielectric‐anisotropy materials requires a source of in‐plane anisotropy which is provided by a unique previously unreported molecular configuration at zero field. At V=0, the molecules align with their long axes at a small angle to the sample normal. The experimental results are analyzed in terms of a continuum model and experimental values obtained for the relevant elastic constants.
TL;DR: In this article, the dielectric anisotropy δϵ changes sign because of a relaxation of ϵ∥, and a new conduction regime is found in a liquid crystal.
TL;DR: In this paper, the angular dependence of the linewidths of a spin probe in the mesophase of a smectic liquid crystal has been studied and a theory for its angular dependence has been developed.
Abstract: A spin probe dissolved in the mesophase of a nematic or smectic liquid crystal behaves, in many ways, as if it were encorporated in a uniaxial single crystal. For example the positions of the lines in the electron resonance spectrum of the probe depend on the orientation of the liquid crystal with respect to the magnetic field. In addition the widths of the spectral lines might also be expected to be angular dependent. The form of this angular dependence is readily calculated provided the dominant spin relaxation process results from molecular reorientation with respect to the director in the mesophase. In this paper we develop a theory for the angular dependence of the linewidths and show how it could be used, with some advantage, to investigate molecular reorientation in a macroscopically anisotropic system. The observed angular dependence of the linewidths for a nitroxide spin probe dissolved in the mesophase of a smectic A liquid crystal confirms certain aspects of the theory.
TL;DR: The optical properties of a thin film of liquid crystal, with one of its surfaces bounded by the isotropic liquid of the same compound, reveal the existence of point defects in the liquid crystal as mentioned in this paper.
Abstract: The optical properties of a thin film of liquid crystal, with one of its surfaces bounded by the isotropic liquid of the same compound, reveal the existence of point defects in the liquid crystal. Particular sample geometries are examined in which the detailed structure of the optical patterns seen with polarized light can be analyzed to develop a qualitative picture of the structure of the liquid crystal layer near the defects. Changes in the optical pattern induced by an electric field are also analyzed. The observations indicate that the defects are point disclinations at the sirface of the liquid crystal, of the kind recently described by deGennes, rather than line disclinntions emerging from the liquid crystal.
Patent•
25 Sep 1972TL;DR: In this article, the authors proposed a three-dimensional stereo viewing system with two liquid crystal lenses, each comprising: (1) a thin layer of liquid crystal material, preferably nematic liquid crystal; (2) two plates of conductively-coated transparent material enclosing the liquid crystal, each plate being connectable to a source of voltage.
Abstract: An optical device, useful for three-dimensional stereo viewing not requir any mechanical shuttering, including two liquid crystal lenses, each comprising: (1) a thin layer of liquid crystal material, preferably nematic liquid crystal; (2) two plates of conductively-coated transparent material enclosing the liquid crystal, each plate being connectable to a source of voltage, and each plate being insulated from the other; (3) a front polarizing sheet forming the outside surface of the crystal lens; and (4) a rear polarizing sheet forming the inner surface of the crystal lens. The axes of polarization of the front and rear polarizing sheets are at right angles to each other, with the result that maximum light transmission through each of the lenses occurs when no voltage is applied across the conductively-coated plates, and minimum light transmission occurs when a voltage is applied across the coated plates.