Homeotropic Texture Research Articles

Application of Infrared ATR Spectroscopy to Liquid Crystals. I. Surface-Induced Orientation in Thin Films of Nematic MBBA

Abstract Infrared dichroism of nematic liquid N-(p-methoxybenzylidene)-p-butylaniline (MBBA) was measured using a conventional transmission method and an ATR method. In the former, KBr windows were used as substrates. The degree of orientation order of the homogeneous structure, which was achieved by a rubbing procedure, was obtained to be S=0.40. Using the measured dichroic ratios, the angles between the transition moments and the long axis of the molecule were calculated. The calculated values are in satisfactory agreement with the molecular conformation already suggested. In the ATR method, In2O3-coated glass plates were used as substrates. Rubbing of the In2O3 surface gave a uniform homogeneous orientation with an S value of 0.57. An In2O3 surface treated with dimethyldichlorosilane gave a homeotropic texture with an S value of 0.53. The directions of the transition moments relative to the long axis of the molecule closely agreed with those obtained from transmission spectra.

Periodic Ripples in a free Smectic A Surface Skin

Abstract A smectic A skin in a homeotropic texture on a liquid surface should form a periodic ripple when a horizontal compressive strain exceeds a critical value. The ripple wavelength is proportional to the fourth root of the skin thickness. In all practical cases the critical strain should be inversely proportional to the skin thickness. For a skin thickness approaching molecular dimensions, the critical strain has to be kept low by a small surface tension in order to avoid the formation of dislocations.

DEFECTS – APPLICATIONS TO DISPLAY DEVICESTHERMAL RELAXATION RECORDING DEVICE ON SMECTIC LIQUID CRYSTALS

We describe here a device using a relaxation process in smectic A liquid crystals. Two stable structures can exist in the smectic A phase : one is a quasi homeotropic texture (optically transparent) and the other is a focal conic structure (scattering texture). The transition is obtained by locally heating the material to a temperature closed to the smectic nematic transition temperature. This effect is used to duplicate an image on a liquid crystal cell.

Absorptions- und Fluoreszenzuntersuchungen in kompcnsierbaren cholesterischen Mesophasen Teil II: Elektroncnübergänge mit unterschiedlicher Polarisationsriehtung / Absorption and Fluorescence Measurements in Compensated Cholesteric Mesophases. Part II: Electronic Transitions of Different Polarization

Mixtures of cholesterogenic compounds of opposite helical twisting power at a definite temperature Tn behave like a nematic liquid crystal (compensated mixtures). Using lecithin treated quartz plates nematic mesophases form a homeotropic texture with the optical axes perpendicular to the quartz surface. Thus, in compensated cholesteric mixtures the long axes of the molecules at Tn switch by an angle of 90°. The orientation of the cholesteric solvent molecules is transferred to rodlike solute chromophores also switching at Tn . By measurements of the change of absorptivity or fluorescence intensity of the solute chromophores at Tn one can determine the direction of the transition moment with respect to the molecular axes. This effect has been tested in the case of chromophores with transitions of different polarization.

Mechanism of shear induced structural changes in liquid crystals-cholesteric-nematic mixtures

The reflection spectrum of a mixed nematic-cholesteric liquid crystal has been obtained as a function of mechanical shear. Data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal-conic texture for shear rates less than 103 sec−1. The correlated helical structure is shown to tilt under applied shear with slight distortion. At shear rates greater than 103 sec−1, homeotropic texture is generated. The results of this study are consistent with a theoretical model prepared earlier and suggest that the molecular forces maintaining the integrity of the helical structure in cholesteric systems in the Grandjean texture are significantly weakened by incorporation of nematic liquid crystalline materials.

Electrohydrodynamic Solutions for Nematic Liquid Crystals with Positive Dielectric Anisotropy

Abstract -The steady state electrohydrodynamic equations of motion have been solved for nematic liquid crystals which are characterized by a positive dielectric anisotropy and possess a parallel homogeneous texture when contained between transparent electrodes. It is found that a periodic distortion of the director is possible above a critical voltage which is a function of the material constants. The optical texture should be similar to that observed in the Williams domain mode. The wavelength of the distortion is a function of voltage, and the relationship is very similar to the dispersion relation calculated for a negative dielectric anisotropy material in the homeotropic texture. These calculations demonstrate that the numerous experimental observations of domain structures have a common explanation in terms of the continuum theory. A physical explanation is given for the double valued solutions observed in the parallel homogeneous negative dielectric anisotropy problem.

On the Mechanism of Shear Induced Texture Changes in Cholesterics-Electric Field Effects

Abstract Cholesteric systems are being used for display devices because they form several textures each having different optical properties. These textures are: Grandjean (highly reflective); focal conic (highly scattering); and homeotropic (practically transparent). In earlier papers, we have shown that shear induced transitions between these textures can be produced. These transitions are accompanied by color changes in the Grandjean texture and normal forces generated during shear. These effects can be explained in terms of the response of a correlated helical structure in the cholesteric mesophase to external stimuli. The shear induced transition associated with normal force generation is between the focal conic and homeotropic texture. In an effort to understand the responses of the cholesteric system to multiple stimuli, studies have been extended to include electric field effects. This work reports on the effects of an electric field on the dynamic focal conic to homeotropic transition examined as a function of a transverse electric field applied under constant shearing conditions. It is shown that the field decreases the amount of shear required to produce the transition. Calculations based on the helical correlation model show that helical distortion must take place during field application. It is postulated that these distortions facilitate the focal conic to homeotropic transition. Application of the model allows calculation of an average molecular interaction dipole operable in the transition region.

Birefringence of Compensated Cholesteric Liquid Crystals

AbstractThe temperature dependence of refractive indices of three compensated cholesteric mixtures has been measured using a commercial Abbe‐refractometer. At compensation temperature Tn a homeotropic nematic texture was achieved indicated by exchange of ordinary and extraordinary ray and strong increase of birefringence. A comparison is given between nematic and cholesteric birefringence yielding good agreement with the de Vries model of cholesteric structure.

Normal Stress Effects in Cholesteric Mesophases

Normal forces have been found in rheological studies on cholesteryl chloride-cholesteryl oleyl carbonate liquid crystal mixtures over the composition range 0–27 wt.% cholesteryl chloride. These normal forces appear to be associated with the homeotropic texture and are evident in the transition region for the shear induced focal conic to homeotropic transition. The Kotaka model for normal stresses arising from hydrodynamic interactions of rotating rods has been applied. The results are consistent with a model of the homeotropic texture of the cholesteric mesophase comprised of rodlike helical arrays of molecules rotating in an applied shear field. The calculated rod lengths are less than the characteristic pitch of the material.

Mechanism of Shear-Induced Structural Changes in Liquid Crystals-Cholesteric Mixtures

The reflection spectrum of the cholesteric Grandjean texture has been obtained as a function of mechanical shear. Data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal-conic texture for shear rates less than 103 sec−1. The correlated helical structure is shown to tilt under applied shear with little distortion. At shear rates greater than 103 sec−1, homeotropic texture is generated. A theoretical model is proposed which explains observed spectral intensity and wavelength data.

Shear-Induced Texture Changes in Cholesteric Liquid-Crystal Mixtures

Rheological studies on a 23/77 wt% cholesteryl chloride-cholesteryl oleyl carbonate liquid-crystal system show isothermal shear-induced texture changes. The shear-induced textures show non-Newtonian behavior at shear rates below \ensuremath{\sim} 5 ${\mathrm{sec}}^{\ensuremath{-}1}$ and Newtonian behavior above this value. These textures have been optically characterized as the Grandjean and focal conic. At shear rates in excess of \ensuremath{\sim} 1000 ${\mathrm{sec}}^{\ensuremath{-}1}$, non-Newtonian behavior is observed and identified with the homeotropic texture. The first normal-force measurements on a liquid-crystal system are reported.

The nature of the striated textures encountered with the middle (M 1) and inverse middle (M 2) liquid crystalline phases. Relationships between the M 1 phase of amphiphiles and the ordered solution phase of tobacco mosaic virus

The nature of the striated textures observed between crossed polaroids in the microscopy of the “middle” (M 1) and “inverse middle” (M 2) liquid crystalline phases of amphiphiles is discussed. It is indicated that in these textures the fibrous M 1 units lie parallel to slide and coverslip. This arrangement is analogous to the parallelism of the lamellar units to slide and coverslip in the homeotropic basal texture frequently encountered in slides of the “neat” (G) phase. In the striated texture of the M phases the fibrous units, though parallel to slide and coverslip and to one another, are collectively subject to fairly regular alternations of direction (zigzags). It is these alternations that lead to the striations. Resemblances between the striated texture of the M phases and a striated texture arising in the ordered phase of tobacco mosaic virus at high concentration are considered.