Wedge Cell Research Articles

Simple method for measuring the azimuthal anchoring strength of nematic liquid crystals

We present a simple method for measuring the azimuthal anchoring strength and the angular gliding of nematic liquid crystals at the surface of solid substrates. The method only needs symmetrical samples. It uses the averaging of the interference oscillations when using white light, which allows one to measure the azimuthal deviation angle at the surface δφ directly from the polarizers angles at the minimum of light. It is then easy to determine the azimuthal extrapolation length ξφ on varying the cell thickness, e.g., by means of wedge cells. As an example, we measure the azimuthal anchoring strength of 5CB onto self-assembled monolayers deposited onto polished indium–tin–oxide.

Polarizing Microscopy of a Chiral Discotic

We studied a liquid crystalline compound of disk-like molecules that has a mesophase in the temperature range 104–180°C. The compound exhibits schlieren textures typical to the discoidal nematic phase within this temperature range. Chemical modification of this compound with chiral groups allowed to synthesize a chiral discotic. Incorporating the chiral discotic as a dopant in the discotic compound we produced chiral structures. In the mixtures of chiral and non-chiral discotic, we observed textures typical to rod-like cholesterics: oily streaks, fingerprint textures, and Cano-Grandjean lines in wedge cells. The induced cholesteric pitch was estimated from fingerprint textures. We induced chiral structures with pitch between 10μm and 40μm for the concentration range 1–7 weight % of chiral discotic.

Synchrotron X-ray study of the q-fold quasicrystalline symmetry of the Smectic C Twist Grain Boundary phase (TGB )

We report the first X-ray scattering investigation of spatial variations of the q-fold quasicrystalline symmetry (so-called commensurability) of well-aligned TGBC samples. A spatial resolution of 20 × 40 μm2 was achieved using the ESRF microfocus beamline. The liquid crystal samples, contained between glass plates which were either parallel or in a wedge geometry, were scanned in order (i) to probe the mosaicity and (ii) to continuously change the balance between surface and volume effects. In the case of parallel plate cells, commensurability was observed everywhere throughout the sample, hence ruling out possible effects of mosaicity to explain the q-fold symmetry of the diffraction patterns previously reported when probed with a spatial resolution of 1 × 1 mm2 in rotating anode experiments. In the case of wedge cells, the evolution of X-ray patterns with thickness suggested that commensurate lockin occurs for sufficiently thick samples with a width that is statistical.

New Aspects of Organic Electric Materials in Calamitic Liquid Crystalline Photoconductors

ABSTRACTCalamitic liquid crystalline photoconductors such as 2-(4'-octylphenyl)-6-butyloxynaphthalene (8-PNP-O4) and 2-(4'-octylphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) exhibit fast ambipolar carrier transport in all the smectic mesophases up to 1.0 × 10−2 cm2/Vs. The structural defects of liquid crystals, i.e. domain boundaries and disclinations in domains, are electrically inactive and cause neither shallow nor deep defect states due to their fluid nature. It is illustrated that the carrier transport does not suffer from these structural defects in polydomain, by examining transient photo-currents in very thick liquid crystalline layers over 100μm. With an insight that this allowance of the polydomain structure and the fluidity of liquid crystalline photoconductors provides us new aspects in terms of electric property and application, e.g. multi-gap processing, the position detect sensor is demonstrated in a wedge cell filled of 8-PNP-O12, referring the position sensitive transit time of photo-induced transient current.The present result indicates that the photoconductive liquid crystalline material is available for multi-gap structured devices.

Measurement of Azimuthal Anchoring Energy of Nematic Liquid Crystal on Photoaligning Polymer Surface

A method to determine the surface azimuthal anchoring energy of a nematic liquid crystal is proposed. The technique implies the measurement of the director deviation on the cell substrate as a function of strength and direction of the applied magnetic field. As an example, the dependence of the azimuthal anchoring coefficient on the exposure time is measured at the interface between the nematic K 15 and polyvinylcinnamate film exposed by UV light. The analogous measurements performed in a wedge cell show that the method with magnetic field is more precise.

TGBAphase in a series of propionates containing two stereocenters

Abstract A homologous series of chiral materials, (R)-2-pentyl (S)-2-(6-(4-(4′-alkoxyphenyl)benzoyloxy)-2-naphthyl)propionates (R,S)PmPBNP (m = 7-14), derived from a naphthalene ring as part of the core structure in conjunction with a chiral tail containing two stereocentres has been synthesized for the investigation. The twist grain boundary smectic A (TGBA) and smectic C (TGB*C) phases were characterized by the microscopic textures of compounds packed in two untreated glass slides, Cano wedge cell, or homogeneously aligned cell. It was found that the occurrence of these phases depend remarkably on the nature of alkyl chain length m; in the case of shorter alkyl chain length (m = 7-10), the TGBA phase behaves as a metastable-like phase mediated between N and SA phases in a short temperature range, whereas in the longer chain length (m' 11-14), both the TGBA and TGBC phases become thermodynamically stable phases with a wide temperature range. Consequently, this series of chiral materials resulted in two d...

Cross-sectional observations of the cholesteric texture in a Cano wedge cell

Abstract A novel procedure for an observation of a liquid crystal texture is proposed by using a UV curable liquid crystal. Optical observations from a cross-section of the Cano wedge cell are demonstrated and layered structures of cholesteric LC are visualized clearly. The structures near the Grandjean lines are then revealed in detail from images. From the observations, it is confirmed that the texture near the Grandjean line is similar to fingerprint texture

Complex index-of-refraction measurements for RP-1 liquid rocket fuel

Complex index-of-refraction values of RP-1 liquid rocket fuel are reported at laser wavelengths of 0.193 μm (ArF excimer), 0.4765 μm (argon ion), 0.488 μm (argon ion), 0.5145 μm (argon ion), 0.532 μm (Nd-YAG, frequency doubled), 0.6328 μm (He-Ne), 1.064 μm (Nd-YAG), and 10.5915 μm (CO₂). The imaginary part of the index of refraction (<i>k</i>) is determined by traditional transmission methods. The real part (<i>n_r</i>) at the specific laser lines is determined using reflectance measurements, critical-angle measurements, Mueller matrix elements, and Michelson interferometric measurements. Reflectance measurements are used to obtain <i>n_r</i> at a wavelength of 0.193 μm. The critical-angle method is used to determine <i>n_r</i> at 0.4765, 0.488, 0.5145, and 0.532 μm: the real part of the refractive index is obtained from Snell's law by measuring the critical angle. The real part of the refractive index at 0.6328 and 1.064 μm is derived from elements of the Mueller matrix, which are obtained using a TMA Technologies scatterometer. A Michelson interferometer is used to obtain phase shifts in a wedge cell, which are then used to calculate <i>n_r</i> at 10.5915 μm. The need for many methods to measure the complex index of refraction is a result of the large changes in <i>k</i> over the wavelengths of interest.

A rheo-optical study of polydimethylsiloxane melt

Results from a rheo-optical study of polydimethylsiloxane (PDMS) melt flowing through a converging wedge cell at room temperature for pressure drops of 276, 483, and 689 kPa are described. Birefringence measurements in extensional flow confirmed that stress and polarizability tensors were linearly related over a range of strain rate that extended well into the non-Newtonian region. Local velocity measurements were made using Laser Doppler Anemometry (LDA) in order to compute the rate-of-strain tensor. Along the cell centerline, extensional flow behavior of the polymer was studied via birefringence measurements in the linear stress-optical region. Also, constitutive equations were tested in combined shear and extension flow by comparing their predicted orientation angles with experimental values. For this study, the two constitutive equations tested were the Power-law model and the Goddard-Miller model (a quasilinear corotational model). The orientation angles computed using a two-term version of the Goddard-Miller model with a single Maxwell-type relaxation time constant of 0.009 s and a zero-shear viscosity of 300 Pa s agreed quite well with experimental results.

High-birefringence materials using metal-containing liquid crystals

In this paper we report the measurement of the birefringence of a range of metallo-mesogens, the metals incorporated being palladium, silver and iridium. The birefringences are determined by two methods: approximate values are determined from a wedge cell, and accurate values are measured using a modulated optical interference method. The materials investigated were found to have high birefringences, in some cases greater than 0.3, and supercooling to lower temperatures could lead to materials of even higher optical anisotropy.

Fate mapping the avian neural plate with quail/chick chimeras: Origin of prospective median wedge cells

The origin of prospective M cells, which are median neuroepithelial cells that become wedge-shaped during bending of the neural plate and eventually form the midline floor of the neural tube, was determined by constructing quail/chick chimeras and using the quail nucleolar marker to identify quail donor cells in chick host blastoderms. Two possible sites of prospective M-cell origin in the epiblast were examined: a single, midline rudiment located just rostral to Hensen's node and paired rudiments flanking the cranial part of the primitive streak. Our results suggest that M cells arise exclusively from the midline, prenodal rudiment. From this rudiment, M cells extend caudally throughout the entire length of the neuroepithelium. This new information on the origin of prospective M cells will aid in the analysis of their role in neurulation.

Hybrid aligned cholesteric: A novel liquid-crystal alignment

A hybrid aligned cholesteric (HAC) liquid-crystal (LC) structure is reported. The LC director tilt varies from a small angle at one surface to almost 90° at the other, while incorporating a helical twist. Cells having one surface treated with rubbed polyvinyl alcohol and the other with the organosilane octadecyltriethoxysilane, adopt the HAC structure when filled with a long pitch chiral mixture. The presence of the HAC structure was confirmed by studies of the variation of the optical rotation, first with thickness in a wedge cell and second with temperature for a compensated cholesteric mixture. Use of the HAC structure in a wedge cell is characterized by an absence of the pitch breaks seen with conventional alignments.

Layering transitions in colloidal crystals as observed by diffraction and direct-lattice imaging.

Layering transitions of colloidal crystals confined between two smooth glass surfaces have been studied by transmission diffraction of light as well as direct-lattice imaging with an optical microscope. The polystyrene spheres we studied have diameter \ensuremath{\Phi}=0.305 \ensuremath{\mu}m and a surface charge of \ensuremath{\sim}${10}^{4}$ electronic charges, and form three-dimensional (3D) colloidal crystals in completely deionized water at a volume concentration above \ensuremath{\approxeq}0.3%. A 3D suspension of these spheres in completely deionized water forms both body-centered-cubic (bcc) and face-centered-cubic (fcc) crystalline structures as a function of colloid density n\ensuremath{\equiv}(1/${a}_{s}$${)}^{3}$, with lattice constants ranging between 0.7 and 1.5 \ensuremath{\mu}m. When the colloid is confined between glass plates separated by distances D\ensuremath{\sim}0.5--1 \ensuremath{\mu}m, the spheres form a single-layer 2D fluid. For D near 1 \ensuremath{\mu}m, a transition occurs to a single-layer 2D hexagonal crystal. As D increases, the evolution from two- to three-dimensional crystals is observed as a series of structural transitions distinguished by the number of crystal planes between the plates and by the preferred crystal symmetry parallel to the glass boundaries. We present here a study of colloidal crystals confined in a wedge cell that allows diffraction and imaging from the same crystallite. We present diffraction and imaging measurements of structural phases of one- through seven-layer colloidal crystals confined between two smooth glass surfaces, for a range of densities such that 2${a}_{s}$/\ensuremath{\Phi}6. The sequence of structural phases we observe for this range of densities for clean samples is similar but not identical to that observed for ${a}_{s}$/\ensuremath{\Phi}2 by Pieranski, Strzlecki, and Pansu, which was modeled as a hard-sphere system. We also observe differences between the clean thin crystalline phases and ``dirtier'' thin phases to which a stray electrolyte has been added. The use of both diffraction and imaging was found necessary to fully characterize the system.

A wedge cell characterization method for the bistable cholesteric twist cell

The bistable cholesteric twist cell (BCTC) described here is a liquid-crystal device which can be in either of two stable laminar configurations while a particular holding voltage is applied. The major shortcoming of previous implementations of this device was the narrow thickness tolerance required for proper bistable operation. In order to ascertain the effect which material parameters might have on increasing this thickness tolerance an adequate method is needed for characterizing the behavior of the cell. In this paper we describe a wedge cell method for characterizing the behavior of a given BCTC system. The characterization can be conveniently done within a single cell. The data obtained include both the thickness to pitch range for pratical bistable operation and the switching properties. All of these features can be interpreted in terms of static free energy curves calculated for the system. We have also used this wedge cell method to determine the effect of the bend to splay elastic constant ratio on the thickness range of bistable operation in the BCTC. The range is observed to increase with increasing ratio of bend to splay elastic constant. A material has been identified with a bistable thickness range in excess of 10%.

Lens and Wedge Absorption Cells for FT-IR Spectroscopy

New cell geometries for extending the dynamic range of FT-IR absorption spectroscopy are described. These are variants of the stepped multiple thickness cells described before, in which the cell thickness varies continuously with location. Either homogeneous wedge or lenslike cells are used. The resulting distortion of the cell's photometric linearity can be straightened out computationally. However, the change in the measured energy levels drastically increases the dynamic range of the measurement for a given tolerable relative error in absorbance. The theoretical behavior of the cells is analyzed, and their photometric distortion and dynamic range calculated. The experimental realization of one of them is used to show the usefulness and limitations of the procedure.

Analytic ultracentrifuge calibration and determination of lipoprotein-specific refractive increments.

Accurate quantification of the major classes and subfractions of human serum lipoproteins is an important analytical need in the characterization and evaluation of therapy of lipid and lipoprotein abnormalities. For calibrating the analytic ultracentrifuge (AnUC), we routinely use a Beckman calibration wedge cell with parallel scribed lines 1 cm apart. Such a cell gives a rectangular pattern in the schlieren diagram, which determines magnification and also provides an area corresponding to an invariant refractive increment. We have independently validated this wedge calibration cell using a special boundary-forming cell in which 1.174% sucrose is overlayered with distilled water. Comparing wedge cell area with extrapolated zero time boundary area refractive increment gives agreement to within less than 1%, corresponding to a refractive increment error of +/- 0.00002 delta n. Complete calibration for AnUC analysis of lipoproteins also requires accurate determination of the specific refractive increments (SRI) of the major lipoprotein classes, namely low density lipoprotein (LDL) and high density lipoprotein (HDL). These are measured in the density in which they are analyzed, i.e., 1.061 g/ml for LDL and 1.200 g/ml for HDL. Five fresh serum samples were fractionated for total LDL and total HDL and their SRI determined. Total lipoprotein mass was determined using precise CHN elemental analysis and compositional analyses. The results yielded corrected SRI of 0.00142 and 0.00135 delta n/g/100 ml for LDL and HDL. Thus, our current values using 0.00154 and 0.00149 delta n/g/100 ml underestimate LDL and HDL by 9% and 11%. Corrections of all previous LDL and HDL AnUC data can be made using appropriate factors of 1.087 and 1.106.

Simple Method For Measuring The Refractive Index Of A Liquid Or Glass Wedge

This paper describes a simple method of measuring the refractive index of a liquid. The basic element is a thin wedge cell, and the source of light is a low power He-Ne laser. Conditions are derived to obtain the best possible accuracy using this setup. The same setup can also be used for the determination of the refractive index of a solid medium such as glass.

Laser measurement of extinction coefficients of highly absorbing liquids

A coaxial dual-channel laser system has been developed for the measurement of extinction coefficients of highly absorbing liquids. An empty wedge-shaped sample cell is first translated laterally through a He-Ne laser beam to measure the differential thickness using interference fringes in reflection. The wedge cell is carefully filled with the oil sample and translated through the coaxially positioned dye laser beam for the differential attenuation or extinction measurement. Optional use of the instrumentation as a single-channel extinction measurement system and also as a refractometer is detailed. The system and calibration techniques were applied to the measurement of two crude oils whose extinction values were required to complete the analysis of airborne laser data gathered over four controlled spills.

Simple Method for Measuring the Refractive Index of a Liquid

This paper describes a simple method of measuring refractive indices of liquids. The basic element is a thin wedge cell and the source of light is a low power HeNe laser. Conditions are derived, to obtain the best possible accuracy using this setup. Some applications of this method are suggested.

The closure of the neural tube in the golden hamster.

AbstractThe sequence of events leading to the closure of the neural tube has been investigated in hamster embryos. This consisted of: (1) approach and fusion of the neural folds at the level of the presumptive neural‐crest cells; (2) active proliferation of the neural‐crest cells at the area of fusion and formation of a distinct wedge by them; (3) fusion and closure of the surface ectoderm; (4) reduction of the number of neural‐crest wedge cells due to their active migration from it; (5) fusion of the neurectoderm beginning at the ependymal surface, progressing dorsally until the neural tube was completely closed; and (6) fusion and closure of the mesoderm between the surface ectoderm and the closed neural tube. The time required to complete this sequence of events, the amount of embryonic tissue involved in it at a given time, and the number of neural‐crest cells formed at the area of fusion varied from one region of the embryo to another. It is postulated that the mesoderm appeared to play a double role in the process of closure—a direct one late in the closure process (see 6) and an indirect one, perhaps as a buttress supporting the other cellular elements during the entire closure process.