Permeation through a heterogeneous membrane: the effect of the dispersed phase

An Euler equation, consistent with Maxwell's equations, for describing the optically induced spatial reorientation of the director of a homeotropically oriented nematic liquid crystal is obtained for the case of normal incidence. The exact solution describing the orientation of the director is obtained. By examining the maximum deformation angle near the threshold, the threshold intensity and the criterion for the physical parameters that indicate whether the transition is first- or second-order at the threshold are obtained. The hysteresis accompanying the first-order Freedericksz transition is discussed. By choosing a compound (PAA, $p$-azoxyanisole) with suitable material parameters from known nematic liquid crystals, an experiment is proposed to observe, for the first time, a first-order Freedericksz transition in nematic liquid crystals. The Zel'dovich approach [Sov. Phys.-JETP 54, 32 (1981)] based on the geometrical-optics approximation is shown to be internally inconsistent and also inconsistent with the geometrical-optics approximation. The Euler equation using a self-consistent geometrical-optics approximation is also obtained, and turns out to be identical to our exact Euler equation, but different from the infinite-plane-wave approximation used by Durbin et al. [Phys. Rev. Lett. 47, 1411 (1981)]. Detailed comparisons between our approach and the Durbin approach are made. The dynamics of the transition are discussed and an approximate solution is given. The transient responses to the laser switch-on and switch-off are shown to have exponential time dependence. Finally, the effects of surface interactions on the transition are discussed and the exact solution is given. The procedure for determining the threshold, the saturation, and the parallel-state-maintenance intensities is given. We also discuss the first-order transition and propose experimental methods manifesting the effects of surface interaction. The criterion for the transition to be first order at any intensity is given.

Optically induced Freedericksz transition and bistability in a nematic liquid crystal

Triptycenes have general applicability for increasing the alignment of fluorescent and dichroic dyes in LC hosts. Dyes containing varying numbers of triptycenes were synthesized to study the effect of free-volume alignment of triptycenes on the alignment of dyes. These dyes were designed such that multiple triptycenes could be incorporated and the triptycene-free volume is coincident to the aspect ratio of the dye, allowing a cooperative effect to increase their overall average alignment. With increasing triptycene incorporation, a stepwise increase in the alignment parameters of each dye was seen. It was also found that the attachment of one triptycene group has a negligible effect on the optical switching response times of the dyes. This can be a powerful tool for designing dyes with higher alignments for a variety of applications including guest−host reflective LCDs and holographic data storage.

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Using “Internal Free Volume” to Increase Chromophore Alignment

It seems certain now that cholesteric liquid crystals (cholesterics, or CLC, for short) have been the first examples of mesomorphic state of matter known to humankind. Apart from the well-known discovery of the two-stage melting of cholesteryl benzoate made by Reinitzer in 18881 and cited in almost all the textbooks, one may recall some other papers where the authors, quite unaware of the real significance of their observations, presented undeniable evidence of the liquid crystalline state of matter. E.g., Professor Planer from the Lvov University as early as in 1861 did observe, putting it in the presentday terms, selective reflection from the planar texture of cholesteryl chloride forming a monotropic cholesteric mesophase on cooling.2 One should also note that the works of Fergason3,4 outlining the prospects of practical applications for CLC, did, in fact, precede similar endeavors by Heilmeier in the field of nematics. From the viewpoint of organic chemistry, a number of mesogenic cholesterol...

Cholesteric liquid Crystals: Physical Properties and Molecular-Statistical Theories

Polymer‐dispersed liquid crystal (PDLC) films are potentially useful in applications requiring electrically controllable light transmission. In these applications, both a high on‐state transmittance and a strong off‐state attenuation are often needed over a wide operating temperature range. These transmittance characteristics depend strongly on the refractive indices of the materials in the PDLC films. We have measured the temperature dependent refractive indices of typical PDLC film materials and the temperature dependent electro‐optic transmittance of a PDLC film composed of liquid crystal microdroplets dispersed in an epoxy matrix. We show that our refractive index measurements can account for all the features in the measured transmittance characteristics and discuss several methods for controlling refractive indices to optimize electro‐optic transmittance over an extended temperature range. We have also measured the room temperature refractive indices of mixtures of epoxy resins and hardeners as a fun...

Refractive indices of polymer‐dispersed liquid‐crystal film materials: Epoxy‐based systems

The temperature variation of refractive indices (ne, ne), birefringence (δn), density (ρ) and order parameter (S) of two liquid crystals, namely, N-(p-hexyloxybenzylidene)-p-toluidine (HBT) and N-(...

Refractive indices, density and order parameter of two liquid crystals HBT and OBT

Temperature variation of the refractive indices, birefringence, density and order parameter of liquid crystalline mixtures E7, E8, N10 and PCH-1132 are reported. The birefringence of PCH-1132 is found to be abnormally low compared to other mixtures.

Refractive indices, density and order parameter of some technologically important liquid crystalline mixtures

The ordering of a large number of pleochroic azo and anthraquinone dyes, has been investigated in two technologically important mesogenic mixtures E 1 and PCH-1132 at various temperatures. The effect of molecular geometry (i.e. length and breadth) of the dyes and the nature of the host on the order parameter of these dyes are examined.

Guest-Host Interaction of Pleochroic Dyes in Liquid Crystal Mixtures E 8 and PCH-1132

The temperature variation of the refractive indices ( n o, n e) and density are reported for two liquid crystals. The order parameter is evaluated by both Vuks’ and Neugebauer's approaches. The possible sources of error in evaluating the order parameter from refractive indices measurements have been discussed. The contradictory behavior of Δ n and Δχ during the nematic-smectic B phase transition of HBT has been explained on the basis of polydomain formation and the order parameter in the S B phase is estimated. The order parameter of two liquid crystals, HBT and OBT, is also evaluated by NMR measurements, confirming the polydomain formation in the S B phase of HBT.

Refractive Indices, Density and Order Parameter of Some Liquid Crystals

The dielectric behavior of a Schiff base N-(P-Hexyloxybenzylidene)-P-toluidine (HBT) and two technologically important mixtures, E8 and PCH-1132, are reported in the radio frequency region. The value of the mean dielectric constant increases by ∼5% on going from the nematic to the isotropic phase in the case of E8 and PCH-1132. This may perhaps be due to antiferroelectric arrangement of the molecules in the nematic phase. These mesogens show unusual dielectric behavior and exhibit positive anisotropy at lower frequencies and negative dielectric anisotropy above a critical frequency termed as isotropic frequency. The isotropic frequency is found to be temperature dependent. All these nematogens exhibit single relaxation in the radio frequency region which is due to hindered rotation of the molecules under nematic potential. The relaxation frequencies are 3.98 MHz (at 61.4°C), 2.27 MHz (at 34.3°C), 1.87 MHz (at 33°C) in case of HBT, E8 and PCH-1132 respectively. The activation energy is found to be...

R. K. Sarna

Papers

Refractive indices, density and order parameter of two liquid crystals HBT and OBT

Refractive indices, density and order parameter of some technologically important liquid crystalline mixtures

Guest-Host Interaction of Pleochroic Dyes in Liquid Crystal Mixtures E 8 and PCH-1132

Refractive Indices, Density and Order Parameter of Some Liquid Crystals

Dielectric studies of three liquid crystals HBT, E8 and PCH-1132