Birefringence Colors Research Articles

Investigating the effect of SiO2 thin films with sculptured topographies on the alignment properties of liquid crystals

In the present work, sculptured thin films (STF) based on silicon dioxide (SiO2) induced on the indium tin oxide (ITO) coated substrates via glancing angle deposition (GLAD) technique using the physical vapor deposition (PVD) process. Our main motive to perform this work is to address the issues we have faced with the traditional alignment method and give a try to overcome those problems. Keeping these challenges in our mind, we are giving here an alternative to traditional method and explored this different alignment technique with GLAD process. We have fabricated a liquid crystal device (LCD) in a way of electrically-controlled birefringence (ECB) and placed the engineered substrates in anti-parallel alignment. Morphological, optical and electro-optic behaviour of resulting device has been studied in the ∼5 μm thick device. Effect of applied voltage on the morphology and electro-optic behaviour of liquid crystal has been established. Interestingly, LC material demonstrates electrically switchable birefringence colours viz. brown, green, pink, purple and mauve colour, under applied electric field in room temperature at 10X magnification under polarizing optical microscope. A very nice extinction and contrast has been obtained with fabricated ECB device, which elucidate the device quality with impeccable alignment without any disclination defects. Besides, the electro-optic characteristics it also shows a significant impact on the liquid crystal properties. The obtained results demonstrate nice optical retardation, a lower Frederick's threshold voltage i.e. 1.5 V and 19.03 dB higher contrast respectively. An impressive switching response time τon of 5.4 ms and τoff = 600 µs at 20 V has attained with respective device as well. Such change in parameters may correspond to the higher electrical torque experienced by the LC molecules on the application of applied field. Anchoring energy and pretilt angle of respective system has been calculated and correlated with observed behaviour. Thus, engineered device with this superior alignment method show remarkable electro-optic results which can open a new way for the development of fast LCD for advanced optical device applications.

Full-color Window Display Using Birefringence Colors by OPP Films Which Enable to Make Images Invisible from Back Side

Full-color Window Display Using Birefringence Colors by OPP Films Which Enable to Make Images Invisible from Back Side

Petrographic features of Late Cretaceous biotite granites in the Kurtoğlu region (Elazığ/Turkey)

The study area is located in a local area around Kurtoğlu village of Elazığ province, located within the Southeast Anatolian Orogenic Belt. The studied biotite granites belong to the Elazig Magmatic Complex. The Elazığ Magmatic Complex is grouped into volcanic, sub-volcanic and plutonic rocks and mafic and felsic. Felsic rocks belonging to the Elazığ Magmatic Complex are represented by granite, granodiorite, tonalite, quartz monzonite, monzodiorite, and mafic rocks are represented by diorite, quartz diorite and gabbros. The biotite granites are porphyric biotite minerals with an average size of one cm. Petrographically, they are composed of K-feldspar, plagioclase, quartz, biotite and opaque minerals. K-feldspars have low birefringence colours, grey tones, and earthy colours in single nicol. Plagioclases; anhedral, euhedral and mostly subhedral crystals. They typically indicate albite, albite+karlsbad and polysynthetic twinning. In some samples, sericitization and carbonation are observed mostly in the middle parts of the plagioclase resulting from alteration. Biotites; It is generally in the form of subhedral platy-prismatic, rod-like crystals. Pleochroism is seen in brown tones in single nicol. It has high birefringence colours and indicates vivid interference colours in yellowish, blue and green, especially brown tones in double nicol. Porphyritic texture in which phenocryst and smaller crystals are generally observed in granites.

Improved saturation and wide-viewing angle color filters based on multi-twist retarders.

Birefringent color filters serve a critical role in next-generation display systems, including augmented-/virtual-/mixed-reality headsets, and many types of optical remote sensing. Most prior polarization interference filters (PIFs) employ many individually aligned plates that enable only relatively thick color filters (≥100s of µm), are usually limited to small clear apertures (few cm), and offer poor off-axis performance. Here, we report on a family of monolithic, thin-film, birefringent PIFs formed using liquid crystal polymer (LCP) network materials, also known as reactive mesogens. These multi-twist retarders (MTRs) are only a few µm thick and have a single alignment surface. They offer high color saturation with a notch-type pass/stopband, analogous to Solc PIFs and stable off-axis performance. Here, we apply simplifying assumptions inspired by Solc PIFs, and develop a design method resulting in MTRs with an alternating achiral/chiral architecture. We theoretically and experimentally presented three types of MTR color filters (blue-yellow, green-magenta, and cyan-red), which manifest strong color filtering behavior and improved angular performance (up to ±20°) with larger color space coverage and high total light efficiency compared to their Solc filters counterparts. Such high-saturated and wide-viewing MTR color filters can be promising elements to maintain the system field of view (FOV) in the next-generation displays or spectral imaging applications.

Osmotic-Pressure-Induced Nematic Ordering in Suspensions of Laponite and Carboxy Methyl Cellulose.

Laponite is a synthetic clay that is known to form gels in aqueous suspensions at low concentrations (0.01 g/cm3). Although it is expected to form lyotropic liquid crystals, such phases usually do not form, as a consequence of laponite's tendency to form gels at concentrations below the threshold for liquid crystal formation. Here we show that macroscopic, birefringent phases of laponite can be prepared through osmotic compression of a laponite solution by an aqueous solution of carboxy methyl cellulose (CMC). We present polarization imaging studies showing how the initially dilute, isotropic laponite phase shrinks while developing typical birefringence colors between crossed polarizers. Using the Michel-Lévy interference charts, we were able to extract the refractive index and orientation of the laponite nanodisks in the compressed region. Our observations allow us to propose a tentative state diagram, indicating the concentration regions for which we obtain optically anisotropic gels.

Incubation temperature affects the development of calipash related traits and collagen deposition in embryos of soft-shelled turtle Pelodiscus sinensis

Chinese soft-shelled turtle Pelodiscus sinensis possesses a distinctive soft and pliable apron around its dorsal carapace termed calipash, of which the formation and its relation to extracellular matrix remain largely unclear. Herein, we investigated calipash quality traits and collagen deposition of turtle embryos from eggs incubated at two different temperatures (26 °C and 32 °C) to evaluate the involvement of collagen in calipash development during embryogenesis. We found that the calipash related traits including embryo mass, embryo length and calipash mass showed an overall increasing tendency as embryos developed. The positive correlation between collagen content and the above morphologic traits in calipash was also found. Embryos incubated at 32 °C incubation temperature had significant bigger embryo size, heavier calipash and higher collagen content at most developmental stages as compared with those embryos incubated at 26 °C. Strong birefringence color was observed from the starting of developmental stage 23 and there were much more collagen fibers presented polarized orange or red colors from embryos incubating at 32 °C. The percentage of type I collagen areas in turtle calipash tissue was found to increase overall and with significantly higher value in embryos incubated at 32 °C. The observed promoted mRNA level of type I collagen gene of turtle eggs incubated at 32 °C in developmental stage 23 indicated that the biosynthesis of collagen in P. sinensis embryos might be induced. The present work indicated that 32 °C incubation temperature is conducive to more collagen deposition and related more preferable calipash traits during the embryonic development of P. sinensis. The data provided the possible manipulation of calipash trait development by incubation temperature changing. High incubation temperature is suggested when attempting to develop large calipash tissue in P. sinensis aquaculture.

Glass: Home of the Periodic Table.

Glass is the most common material around us, and humankind uses it every day for more than 5000 years. However, from the chemical point of view, glass is the only material that could represent almost all elements of the Periodic Table inside itself, showing the effect of the Periodic Law on properties of the final material. In this paper, we show the most remarkable examples demonstrating that glass can rightfully be called “home” for all chemical elements providing different properties depending on its composition. We gave a new look at the Periodic Table and described how a small number of glass-forming components creates unique glass structure which could enclose almost all remaining elements including transition and noble metals, lanthanides and actinides as modifying components providing an inconceivable number of discoveries in material science. Moreover, we reviewed a series of studies on the direct femtosecond laser writing in glasses which paves the way for a redistribution of chemical elements in the spatially confined nanosized zone in glass volume providing unique properties of laser-induced structures. Finally, for the first time, we reproduce the Periodic Table in birefringence colors in the bulk of silica glass using a direct laser writing technique. This image of 3.6 × 2.4 mm size can withstand temperature up to 900°C, humidity, electromagnetic fields, powerful cosmic and reactor radiation and other environmental factors and demonstrates both the art of direct laser writing and symbolic role of glass as the safest and eternal home for the Periodic Table.

Electrically switchable birefringent self-assembled nanocomposites: ferroelectric liquid crystal doped with the multiwall carbon nanotubes

ABSTRACT Functional nanocomposites are designed via doping ferroelectric liquid crystal (FLC) matrix by 0.03wt% and 0.1wt% of functionalised multiwall carbon nanotubes (MWCNTs). Mesomorphic, optical, thermo-optic and electro-optic behaviour of resulting nanocomposites have been studied in the ~5μm thick cells. Effect of applied voltage on the mesomorphic and electro-optic behaviour of ferroelectric liquid crystal has been established. Interestingly, FLC material demonstrates electrically switchable birefringence colours viz. yellow, reddish, violet and green colour, under applied electric field at different temperatures. No considerable perturbation of FLC smectic order has been observed with doping of MWCNTs. However, the MWCNT doping strongly modifies the optical contrast, birefringence, transmission and contrast ratio of the FLC material. The most significant results are obtained for nanocomposite with 0.03wt% MWCNTs concentration. Doping by MWCNTs leads to the generation of new colours in the nanocomposite systems and also effectively reduce the driving voltage for the colour appearance. For the designed nanocomposites, a decrease in the spontaneous polarisation and fastening of the switching time has been detected. Such electrically switchable birefringent MWCNTs/FLC nanocomposites can be effectively utilised for practical applications in the field-sequential colour displays, FLC switches, optoelectronic and photonic devices.

Fast copolymer network liquid crystals for tunable birefringence colors.

A fast-switching, tunable color filter was found in a copolymer network liquid crystal (LC), which was in situ generated in a conventional LC test cell with parallel aligned glass plates and investigated with polarized light. Polarization filters were used to convert the tunable optical phase retardance of the test cells to birefringence colors as is always possible in a LC test cell with carefully adjusted cell gap and effective birefringence. The cell gap of the samples could be adjusted to a value of 9 μm, which is not easily possible in a polymer LC composite without creating defects. In these samples, the typical pastel colors seen frequently in birefringent samples could be avoided. The transmittance spectra were recorded and converted to CIE 1931 color coordinates, which showed that the colors seen had a reasonable distance to the white point. The electro-optic switching times of the samples were investigated: Fast responses of t on +t off <5 ms were found, which is an impressive speed for tunable birefringence colors in LCs and LC composites. Upon increasing addressing voltages, a blueshift of the peak seen in the transmittance spectra was observed. The samples consisted of copolymer network LC, generated from a reactive mixture with mesogenic monomer and nonmesogenic comonomer. The tunable color was seen selectively in samples with dodecyl acrylate as comonomer. The experiments show how even a straightforward electro-optic experiment still can result in unexpended findings, which may expand the use of LC composites in nondisplay applications. The polymer morphology in samples with a larger cell gap was investigated with scanning electron microscopy, and interdefect distances of ≈40 μm were found. The appearance of defects in test cells with a cell gap of 9 μm could be avoided because the cell gap was much smaller than the measured interdefect distances in test cells with a larger cell gap.

Using birefringence colors to evaluate a tunable liquid-crystal q-plate

Q-plates are geometrical phase elements that enable the realization of vector beams in simple and compact optical setups. In this work, we consider a tunable liquid-crystal commercial q-plate operative in the visible and near-IR range and study its spectral and color birefringence properties under broadband illumination. We first characterize the spectral retardance function of the device in a wide range from 400 to 1600 nm and determine how it changes upon applied voltage. Then we evaluate the color transmission characteristics when inserting the q-plate between crossed and parallel linear polarizers. These color properties agree with the trajectory in the CIExy chromaticity diagram as the applied voltage changes. Finally, we demonstrate that a simple visual inspection of the transmitted birefringence color perceived when placing the q-plate between crossed polarizers can be used to obtain a rapid estimation of the q-plate retardance at given wavelength ranges.

Nano-assembly of intertwining lamellae of opposite bending senses in poly(ethylene oxide) co-crystallizing with poly(p-vinyl phenol)

Strongly interacting amorphous poly(p-vinyl phenol) (PVPh) was added into semi-crystalline poly(ethylene oxide) (PEO) at 20 wt.% to generate dendritic lamellae of alternating birefringence colors, which intertwine roughly in the radial directions as periodic stripes. Optical and atomic-force microscopy analyses are performed on revealing lamellae assembly in highly dendritic PEO spherulites by crystallizing PEO/PVPh (80/20) at crystallization temperature (T c ) equals to 45 °C. The lamellae of two opposite colors in the same PEO spherulite quadrant are exactly of same geometry (of elongated plates, each being 0.2 μm in width and 2–5 μm in length), but they fan out +/−45o, respectively, as they grow away from the radial direction. The opposite colored-lamellae are radiating out in radial directions, interweaving each other to display alternating colors in radial alignment. However, such alternating colors can also be similarly seen in ring-banded spherulites in many polymers. The results presented in this study have demonstrated that the lamellae of alternating birefringent colors cannot only be seen in banded spherulites, but also in spherulites with radial dendrites.

Chiral nematic liquid crystal microlenses

Nematic liquid crystals (NLCs) of achiral molecules and racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and immersed in water. On addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns and radial variation of the birefringence color. Both are related to a biconvex shape of the chiral liquid crystal film; the rings are due to interference. The curvature radii of the biconvex lens array are in the range of a few millimeters. This curvature leads to a radial variation of the optical axis along the plane of the film. Such a Pancharatnam-type phase lens dominates the imaging and explains the measured focal length of about one millimeter. To our knowledge, these are the first spontaneously formed Pancharatnam devices. The unwinding of the helical structure at the grid walls drives the lens shape. The relation between the lens curvature and material properties such as helical pitch, the twist elastic constant, and the interfacial tensions, is derived. This simple, novel method for spontaneously forming microlens arrays can also be used for various sensors.

Transitional Ring Bands Constructed by Discrete Positive- and Negative-Birefringence Lamellae Packed in Poly(1,6-hexamethylene adipate) Spherulites

Neat poly(1,6-hexamethylene adipate) (PHA) is one polyester that never forms ring banded spherulite when crystallized at any crystallization temperatures (Tcs). However, PHA spherulites readily transform from negative-type at low Tcs (32–38 °C) to positive-type at high Tcs (46–48 °C). By utilizing such morphological traits, “artificial” ring-banded spherulites can be constructed by step-crystallization cycles at 38 and 46 °C, respectively for controlled times. Such artificially constructed ring bands are analyzed in details using polarized-light optical microscopy (POM), atomic force microscopy (AFM), and electron diffraction (ED) in transmission electron microscopy. For the classical continuous spiral twisting to work for ring-banded spherulites, it requires two assumptions that may be far from reality and hard to prove: (I) lamellae in the spherulites remain single-crystal-like plates with no secondary/ternary branches at all from nucleation to completion of crystallization; and (II) strong forces are required for holding continuous spirals of all thousands of single-crystal lamellae in precise synchronizing paces of twists. On the other hand, as proven in this novel approach in constructing artificial ring-banded PHA spherulites, the ridge band and valley band are two discrete layers of lamellae species with interfacing discontinuity. No forces are required to hold the crystals in these two discrete layers of lamellae that have different morphology and optical axes. As a result of opposed crystal axes (radial vs tangential), they show optically alternating positive–negative birefringence rings in the artificial PHA spherulites just like the natural ring-banded spherulites in other polymers. Furthermore, AFM and ED patterns provide clear evidence that the crystals in two discrete layers of opposed optical birefringence colors are all polycrystals, with multiple branches as spherulites evolving from nucleation to growth completion, which describes the real assembly in polymer spherulites.

Crepidolithus cantabriensis nov. sp., a new calcareous nannofossil (Prymnesiophyceae) from the Lower Jurassic of northern Spain

A new calcareous nannofossil, Crepidolithus cantabriensis nov. sp., is described from the Lower Jurassic marls and marly limestones of the Cantabrian Range (Northern Spain). The new species is characterized by medium-sized, normal to narrowly elliptical coccoliths with open central-areas. In the light microscope, under crossed nicols and in distal view, it displays a characteristic bicyclic extinction pattern. Its inner rim edge shows maximum birefringence colours, which vary from white to yellow, whereas the faintly outer rim margin exhibits low birefringence, tending to grey, and they are separated by a dark and sigmoidal isogyre. C. cantabriensis nov. sp. seems closely related to Crepidolithus crassus on the basis of shared morphological characters. Both of them have a distal shield composed of around thirty sub-vertical, dextrally imbricating laths. The central-area width of C. cantabriensis nov. sp. is approximately half of the coccolith width, whilst it is reduced to one or two tiny lenticular slits in C. crassus. Nevertheless, the phyletic relationships between C. cantabriensis nov. sp. and Crepidolithus cavus are not clear, since they show different rim structures, and C. cavus has a delicate bar aligned along the minor axis of its central area. The first specimens of C. cantabriensis nov. sp. have been recognized within the Raricostatum AZ in the Tudanca section. This species seems to be more abundant in the Reinosa area, which occupied a more distal position with respect to the Iberian Massif during the studied time interval. It is hypothesized here that C. cantabriensis nov. sp. could have inhabited the lower-photic zone.

Voltage Sensor with wide Frequency Range using Deformed Helix Ferroelectric Liquid Crystal

In this paper we propose a new approach to fiber optic voltage sensors via voltage-controlled Liquid Crystals (LC), which would allow direct measurement of up to 400 kV/m electric fields at multiple points. In addition, the electro-optical behavior of deformed helix ferroelectric (DHF) liquid crystal in reflective mode is described and tested. The electrically controlled reflectance has been measured at sub-kilohertz driving voltage frequency for different polarizations of the incident light. Full Text: PDF References Y.V. Izdebskaya, V.G. Shvedov, A.S. Desyatnikov, W. Krolikowski and Y.S. Kivshar, "Soliton bending and routing induced by interaction with curved surfaces in nematic liquid crystals", Opt. Lett. 35, 1692-1694 (2010). CrossRef Z. Ge, S. Gauza, M. Jiao, H. Xianyu and S.T. Wu, "Electro-optics of polymer-stabilized blue phase liquid crystal displays", Appl. Phys. Lett. 94, 101104 (2009). CrossRef Z. Brodzeli, F. Ladouceur, L. Silvestri, A. Michie, V. Chigrinov, G. Q. Guo, E. P. Pozhidaev and A. D. Kiselev, "Liquid crystal-based hydrophone arrays", APOS (2012). CrossRef T. Tanigawa, Y. Sakakibara, S. Fang, T. Sekikawa and M. Yamashita, "Spatial light modulator of 648 pixels with liquid crystal transparent from ultraviolet to near-infrared and its chirp compensation application", Opt. Lett. 34, 1696-1698 (2009). CrossRef K. H. Yang and T. C. Chieu, "Transport Properties of Ions in Ferroelectric Liquid Crystal Cells", Jpn. J. Appl. Phys. 28, 2240-2246 (1989) CrossRef I.C. Khoo and S.T. Wu, "Optics and Nonlinear Optics of Liquid Crystals", World Scientific, Singapore (1993). V.G. Chigrinov, "Liquid Crystal Devices: Physics and Applications", Artech House, Boston: London, (1999). A.D. Kiselev, E.P. Pozhidaev, V.G. Chigrinov and H.S. Kwok, "Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch", Phys. Rev. E 83, 31703 (2011). CrossRef N. A. Clark and S. T. Lagerwall, "Submicrosecond bistable electro-optic switching in liquid crystals", Appl. Phys. Lett. 36, 899-901 (1980). CrossRef L.A. Beresnev, V.G. Chigrinov, D.I. Dergachev, E.P. Pozhidaev, J. Funfshilling and M. Schadt, "Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals", Liq. Cryst. 5, 1171-1177 (1989). CrossRef Z. Brodzeli, H.K. Bal, V.G. Chigrinov, A. Murauski, and F. Ladouceur, "Electrical energy harvesting device for current/voltage fibre-based sensors", Optical Fibre Technology (ACOFT), 2010 35th Australian Conference on Fibre Technology, 1-2, (2010). DirectLink X. Zhao, F. Boussaid, A. Bermak and V.G. Chigrinov, "High-resolution thin ?guest-host? micropolarizer arrays for visible imaging polarimetry", Opt. Exp. 19, 5565-5573 (2011). CrossRef X.H. Li, A. Murauski, A. Muravsky, P.Z. Xu, H.L. Cheung, and V. Chigrinov, "Grayscale Generation and Stabilization in Ferroelectric Liquid Crystal Display," J. Display Technol. 3, 273-279 (2007). CrossRef E.P. Pozhidaev, S. Torgova, M. Minchenko, C. Yednak, A. Strigazzi and E. Miraldi, "Phase modulation and ellipticity of the light transmitted through a smectic C* layer with short helix pitch", Liq. Cryst. 37, 1067-1081, (2010). CrossRef E.P. Pozhidaev and V.G. Chigrinov, "Bistable and multistable states in ferroelectric liquid crystals", Crystallography Reports. 51, 1030-1040 (2006). CrossRef Q. Guo, F. Fan, T. Du, and V.G. Chigrinov, "Electrooptic Response of FLC: Effect of Alignment Materials", SID'11 Conference, P-144, Los Angeles, USA, May, (2011). CrossRef G. Hegde, P. Xu, E.P. Pozhidaev, V.G. Chigrinov, and H.S. Kwok, "Electrically controlled birefringence colours in deformed helix ferroelectric liquid crystals", Liq. Cryst. 35, 1137-1144 (2008). CrossRef

Thermo-responsive discotic nematic hydrogels

We synthesized a novel thermo-responsive discotic nematic hydrogel. This liquid crystalline polymer nanocomposite was fabricated using extremely high-aspect-ratio (ξ = diameter/thickness ∼ 103) zirconium phosphate (ZrP) nano-sheets embedded into a polymeric network. These nano-sheets were immersed into a solution of constituent monomers of N-isopropylacrylamide (NIPAm) and acrylamide (AAm) with a subsequent polymerization in situ to obtain a hydrogel embedded with the nematic liquid crystal of the nano-sheets. We analyzed the effect of initiator (APS) and catalyst (TEMED) concentration on the nematic domains at ZrP nano-sheet volume fractions (ϕZrP) near the isotropic–nematic (I–N) transition. The mixing during hydrogel preparation allowed the aligning of the nano-sheets into highly ordered domains, which were eventually disturbed by the effect of APS and TEMED on the polymer matrix. The effect of ZrP nano-sheet concentration and temperature on the nematic hydrogels was also studied. By increasing the temperature above the LCST of PNIPAm, the hydrogels entered into a hydrophilic-to-hydrophobic transition. Interestingly, hydrogels containing ZrP nano-sheets presented increased hydrogel water retention. In addition, we observed that the variation of the nematic ordering (S2) and ϕZrP within the system manipulated the birefringent color patterns of the hydrogels. Finally, a nematic hydrogel placed into a thin-rectangular capillary from 298 K to 333 K presented edge-wrinkling. An isotropic-to-nematic transition guided by the increment in the ZrP nano-sheet nematic ordering was observed at a nano-sheet concentration slightly below the isotropic concentration (ϕI).

Collagen in histologic stages of oral submucous fibrosis: A polarizing microscopic study

Background:Oral submucous fibrosis (OSF) is a precancerous condition in which there is excessive deposition of collagen in connective tissue. The purpose of present study was to compare changes in birefringence of collagen fibers in histopathological stages of OSF.Materials and Methods:Collagen in connective tissue of 91 cases of OSF was studied by staining 5 μm thick sections with picrosirius red. The polarization colors of thin (0.8 μm or less) and thick (1.6-2 μm) collagen fibers were recorded.Results:The birefringence of thin collagen fibers showed no difference in both histopathological connective tissue stages and degrees of epithelial dysplasia. The polarization colors of thick collagen fibers showed a gradual change from predominantly yellow-orange to greenish-yellow in advancing connective tissue stages and degrees of epithelial dysplasia.Conclusion:The results of present study show a significant change in birefringence of collagen between connective tissue stages and between mild, moderate to severe degree of epithelial dysplasia. This change in birefringence colors and arrangement of collagen fibers may give an implication of impending neoplastic change in OSF.

Melilite-group minerals at Oldoinyo Lengai, Tanzania

Oldoinyo Lengai and the volcanic centres of the Lake Natron–Engaruka province contain melilite as a widespread mineral. Extraordinarily Na–Al-rich melilites (up to 6 wt.% Na 2O and 9 wt.% Al 2O 3) from recent explosive eruptions are among the most Na-rich ever reported. Their unusual mineral composition leads to optical properties with vivid birefringence colours of 2nd order. The continuous variation in mineral composition from common åkermanite to Na–Al–melilite ( alumoåkermanite) is documented and reflects the whole peralkaline trend of Oldoinyo Lengai. The data presented allow the volcano's evolution from primitive olivine melilitites to highly evolved and peralkaline combeite–wollastonite nephelinites to be traced. Melilite compositions of Oldoinyo Lengai extend the magmatic field in the Ca 2Fe(Si 2O 7)–Ca 2Mg(Si 2O 7)–(CaNa)Al(Si 2O 7) end-member ternary compositional diagram.

Electrically controlled birefringence colours in deformed helix ferroelectric liquid crystals

A short helix pitch (p 0 = 0.45 µm) ferroelectric liquid crystal (FLC) mixture was found to provide four electrically switched birefringence colours: blue, green, yellow and red. The contrast ratios of the birefringence colours were measured as functions of both driving voltage amplitude and frequency. The FLC mixture, which had a relatively high birefringence, Δn⩾0.2, was controlled by a voltage less than 5 V. A simple model is proposed, which was confirmed by experiments. A possible application is discussed in field‐sequential colour displays, where fast switching times are required.

Electrically Tunable Color by Using Mixtures of Bent‐Core and Rod‐Shaped Molecules

Seminal findings in the liquid-crystal properties of bent-core molecules, such as the observation of ferroelectricity and spontaneous breaking of chiral symmetry in smectic phases composed of achiral molecules, have broad implications for the general field of soft condensed matter. However, their practical applications are limited because they appear only at high temperatures (>70 8C) owing to the bends in the molecules, which lead to locking into layered smectic structures. To overcome this difficulty, one needs to prohibit the locking mechanism either by molecular design or creating mixtures, or by combining both. Mixtures of bent-core and rod-shaped molecules have shown interesting properties, such as enhancement of the chirality in cholesterics, induction of antiferroelectric order in smectics, or a complete miscibility of smectic bent-core and nematic rod-shape substances. Although there are few examples in which bent-core materials do not crystallize at room temperature, they are glassy and could not be switched at room temperature. Here we show that by mixing suitable bent and rod-shape molecules one can form fluid liquid crystals at room temperature that change birefringence color at electric fields, thus opening up a path towards possible practical applications.