Oily Streaks Research Articles

Surface-frustrated periodic textures of smectic-Aliquid crystals on crystalline surfaces

Using polarizing optical microscopy we studied thin films and droplets of smectic- A 4-cyano- 4;{'} -n-octylbiphenyl (8CB) liquid crystal deposited in air on crystalline surfaces of muscovite mica that induce monostable planar anchoring. The competition with the homeotropic anchoring at the 8CB-air interface leads to the formation of one-dimensional (1D) patterns composed of straight, parallel defect domains that are organized in periodic arrays over areas as large as several mm;{2} . We have developed a simple model which identifies the arrays with self-assembled "oily streaks," comprising straight disclination lines and curvature walls. The model reproduces the observed monotonic increase of the period p with the film thickness h in the range p=1-4 microm and h=0.8-17 microm. For higher values of h we observed a sharp transition to a 2D lattice of fragmented focal conic domains. Despite the apparent generality of our model for hybrid planar-homeotropic anchoring conditions, periodic arrays of straight oily streaks have been observed so far only for 8CB on crystalline surfaces such as mica or MoS2 . Our model indicates that this specificity is due to a particularly strong anchoring of the liquid crystal on such surfaces.

Improvement in Planar Orientation of the Polymer-Stabilized Cholesteric Liquid Crystal Cells

We investigate the effects of thermal annealing on the improvement in the planar orientation of the polymer-stabilized cholesteric liquid crystal (CLC) cell. The obtained results reveal that annealing the CLC/monomer mixture at the CLC–isotropic transition temperature (Tci) effectively reduces the viscosity of the CLC/monomer mixture, accelerating the annihilation of oily streaks. Increasing the annealing time effectively removes the oily streaks, giving the polymer-stabilized CLC cell a high transmittance and a sharp stop band edge. An 8-µm-thick polymer-stabilized CLC cell with a defect-free planar texture is achieved when the annealing time of the cell exceeds 10 min.

Synthesis, thermal and lyotropic liquid crystalline properties of protic ionic salts

A series of protic ionic salts were synthesised by a simple acid–base reaction from various pyridine derivatives and dodecylbenzenesulfonic acid in a common organic solvent and characterised in terms of their thermal and lyotropic liquid crystalline properties using various experimental techniques. All of them exhibited lyotropic liquid crystalline phases in toluene, methanol, acetonitrile, dimethyl sulfoxide and water. Their critical concentrations for the formation of biphasic solutions and concentrations for the formation of lyotropic solutions were quite broad depending on the dielectric constants of the solvents. Their lyotropic phases were identified as lamellar phases, since their textures exhibited bâtonnets, oily streaks and mosaic textures. They can potentially be used for many organic transformations, which may have implications in green chemistry.

Synthesis and characterization of liquid crystalline copolymers with dual photochromic pendant groups

In order to study the photoreactivity and the optical properties of liquid crystalline copolymers with multiple photochromic groups, a series of novel liquid crystalline binary and ternary polyacrylates consisting of one (CC or NN) or dual (CC and NN) photochromic segments were synthesized and characterized considering their liquid crystalline, optical, and photochromic properties and their thermal stability. Achiral homopolymer P1 shows a smectic A phase (fan-shaped texture), and all chiral copolymers CP1–CP6 exhibit chiral nematic phases (cholesteric, oily streaks textures). The polymers show excellent solubility in common organic solvents such as chloroform, toluene, and THF. These polymers also exhibit good thermal stability, with decomposition temperatures (Tds) greater than 373°C at 5% weight loss, and beyond 440°C at 50% weight loss under nitrogen atmosphere. UV irradiation caused E/Z photoisomerization at NN and CC segments of the synthesized photochromic copolymers leading to reversible and irreversible isomerizations, respectively. The synthesized liquid crystalline ternary copolymer CP6, containing two different photochromic NN and CC groups, is sensitive to different UV wavelengths and is notably interesting from the viewpoint of photochromic copolymers.

Synthesis and mesomorphic properties of cholesteric elastomers based on chiral mesogenic crosslinking agent

AbstractSix chiral monomers (M1−M6), and their corresponding polymers (P1, P4) and elastomers (P2, P3, P5, P6) derived from chiral mesogenic crosslinking agent were synthesized. The chemical structures of M1−M6 were confirmed by FTIR and 1H NMR spectroscopy. The structure‐property relationships of M1−M6 and P1−P6 were discussed. Their mesomorphic properties and phase behavior were investigated by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), polarizing optical microscopy (POM), and X‐ray diffractometer (XRD) measurements. All monomers obtained, except M2 and M5, showed typical oily streaks texture and focal conic texture of cholesteric phase on heating and cooling cycles. The selective reflection of cholesteric monomers and elastomers shifted to the short wavelength region with increasing temperature. The elastomers P2 and P5 did not reveal the mesomorphic properties, and P3 and P6 showed cholesteric phase. TGA showed that P1−P6 had a high thermal stability. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Optical trapping of director structures and defects in liquid crystals using laser tweezers

We demonstrate optical manipulation of structures and defects in liquid crystals (LCs). The effective refractive index depends on the LC molecular orientations and the laser beam's polarization. We use the orientation-mediated refractive index contrast for the laser trapping in LCs with a homogeneous composition, but with spatially-varying patterns of molecular orientations. Tightly-focused polarized beams allow for optical trapping of disclinations and their clusters, dislocations and oily streaks, cholesteric fingers and focal conic domains, etc. We calculate the optical gradient forces for typical structures and explain the trapping properties at low laser powers. We also show that when a high-power beam causes local molecular realignment, the laser trapping properties change for two reasons: (1) the refractive index pattern and optical gradient forces are modified; (2) additional elastic structural forces arise to minimize the elastic free energy.

Rheology of the lamellar liquid-crystalline phase in polyethoxylated alcohol/water/heptane systems.

Linear viscoelastic tests as well as transient and steady flow experiments were carried out on lamellar liquid crystalline samples of poly (oxyethylene) alcohol/water/heptane systems. The effect of surfactant and heptane concentrations on the rheological properties of the lamellar mesophase was investigated. The mechanical spectrum inside the linear viscoelastic regime shows, in all cases, a well-developed plateau region in the whole frequency range studied. The values of the dynamic functions were higher for intermediate surfactant or heptane concentrations indicative of a major development of the elastic network in the midrange of existence of the lamellar phase. Transient and steady flow experiments point out a shear-induced evolution of the lamellar microstructure. Different power law regions with different values of the flow index were detected in the viscosity versus shear rate plots. These shear-induced structural modifications were confirmed by using polarizing microscopy in an optical shearing cell. Structural modifications appear to be highly influenced by shear rate. In general, applying relatively high constant shear rates, the alignment of the bilayers followed by the appearance of the “oily streaks” structure was observed. Appearance of shear-induced vesicles occurs at high heptane content, as indicates the texture of close-packed monodisperse spherulites detected by polarizing microscopy.

Phase Structures and Self-assembled Helical Suprastructures via Hydrogen Bonding in a Series of Achiral 4-Biphenyl Carboxylic Acid Compounds

A series of achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH) connected with alkoxyl chains having various carbon numbers (n = 6−10) and terminated by phenyl groups with meta-positioned hydroxyl groups was synthesized. Different phase structures including nematic, smectic A (SmA), smectic C (SmC), and highly ordered smectic liquid-crystalline phases along with crystalline phases were identified based on wide-angle X-ray diffraction and electron diffraction experiments. It was found via infrared spectroscopy that the hydrogen (H)-bonds were formed between the carboxylic acids to construct head-to-head dimers as the building blocks for these ordered structural formations. H-bonds formed via the meta-positioned hydroxyl groups also played an important role in forming ordered layers in these structures. The morphology of this series of BPCA-Cn-PmOH as observed under polarized light microscopy showed an oily streak (cylinder) texture with Myelin-figure in the SmA phase. When temperature cools to enter the SmC phase, these streaks (cylinders) started to twist into helical suprastructures, which were not only by the birefringence changes but also by the three-dimensional helical geometry observed in other microscopic techniques. The dynamic conformational changes of the aromatic and aliphatic parts in this series of BPCA-Cn-PmOH at different temperatures correspond well with the thermal transitions via solid-state carbon-13 nuclear magnetic resonance experiments. Computer simulation indicated that the head-to-head dimers possess a twisted rather than a bent conformation. It was deduced that the twisted conformation of the dimers and the terminal meta-substituted phenyl groups at both ends of the dimers are critically important in forming the helical suprastructures.

Amphotropic Ionic Liquid Crystals with Low Order Parameters

Methyl-tri-n-decylphosphonium salts (1P10A, where the counterion A is NO 3 - or the hydrate of bromide, Br - .H 2 O) have been shown to exhibit unique liquid-crystalline properties as neat compounds and with high concentrations (up to 20 wt %) of a small organic molecule, acetonitrile. These results indicate dual thermotropic-lyotropic (i.e., amphotropic) behavior. Optical micrographs of salts with even small amounts of acetonitrile are oily streak patterns. The low-angle peaks in X-ray diffractograms characteristic of the lamellar spacings in the smectic A phases of the neat salts are broadened but do not change their positions appreciably as the concentration of added acetonitrile is increased. The liquid-crystalline phases of both salts become oriented in strong magnetic fields. As a result, NMR experiments with the nitrate salt exhibited only one type of orientation for acetonitrile at concentrations up to ca. 10 wt %. However, two types of orientations with opposite signs of the order parameter were detected in the Br - .H 2 O salt at < 1 wt % acetonitrile. The acetonitrile proton line width decreases by a factor of 2 between 0.7 and 13 wt % in the nitrate salt. The dual thermotropic-lyotropic behavior and relatively low order parameter of the solute suggest that these amphiphilic salts might be useful for structural studies using NMR spectroscopy.

In Situ Sensing of Salinity in Oriented Lipid Multilayers by Surface X-ray Scattering

The influence of LiCl solutions on liposomal and surface-supported phosphatidylcholine/water systems (dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), respectively) has been studied by small-angle X-ray techniques. In liposomal dispersions of DPPC, an osmotically stressed liquid-crystalline phase, denoted as Lalpha osm, forms readily after rapid mixing with salt solutions. The transition from Lalpha -->Lalpha osm proceeds in two steps. The first step takes place within seconds and is due to water diffusion from the liposome into the bulk solution. The second, slower process (minutes) can be attributed to the relaxation of initially deformed intermediate liposomes into spherical ones. In experiments with aligned lipid bilayers supported on silicon wafers, it was possible to reproducibly exchange different concentrations of LiCl solutions on a single sample and to determine the lattice changes by time-resolved X-ray scattering at grazing incidence. Independently of the deposition technique (spray- or spin-coating, respectively), none of the investigated POPC samples displayed an osmotically stressed liquid-crystalline phase. While liposomes can be considered nearly defect-free, supported bilayer stacks show a high abundance of defects, such as oily streaks typical of the Lalpha phase. Thus, the alkali ions are free to diffuse into the interbilayer water regions and to cause a slight increase of the bilayer separation (about 1 Angstroms). It is concluded that low to medium concentrations of Li+ ions partially screen the attractive van der Waals force between adjacent membrane layers. However, upon annealing the defect regions or regions of high curvature in the oriented lipid matrix, e.g. by low amounts of oleyl alcohol (OA), the system is able to sense osmotic stress upon addition of a salt solution.

Fractal analysis of lyotropic lamellar liquid crystal textures

We apply fractal analysis to study the birefringence textures of lyotropic lamellar liquid crystal system (water/cethylpyridinium chloride/decanol). Birefringence texture morphologies are important as they provide information on the molecular ordering as well as defect structures and therefore has been adopted as a standard method in characterizing different phases of liquid crystals. The system under consideration shows a gradual morphological transition from mosaic to oily streak structures and then to maltese cross texture when the water content is increased. Since these textures are the characteristic fingerprints for the lyotropic lamellar phases, it is necessary to have robust techniques to obtain image quantifiers that can characterize the morphological structure of the textures. For this purpose we employ three different approaches namely the Fourier power spectrum for monofractal analysis, the generalized box-counting method for multifractal analysis and multifractal segmentation technique for estimating the space-varying local Hurst exponents. The relationships between estimated image parameters such as the spectral exponent, the Hurst exponent and the fractal dimension with respect to patterns observed in the birefringence textures are discussed.

Evolution of disclinations in cholesteric liquid crystals.

We studied the evolution of defect lines (oily streaks) in cholesteric liquid crystals. In contrast to nematic liquid crystals, there were no interactions between the defect lines in cholesteric liquid crystals. We observed experimentally that individual open defect segments shrank with time with a linear relation, and individual defect loops shrank with time with a square root relation. We also observed that the defects in cholesteric liquid crystals were mainly interconnected loops, and the total length of the defect lines decreased with time logarithmically because of the distribution of loop size. The decay constant depended on the helical pitch of the liquid crystal and temperature.

Synthesis and properties of photochromic cholesteric liquid crystalline polysiloxane containing chiral mesogens and azobenzene photochromic groups

AbstractA series of novel thermotropic side‐chain liquid crystalline polymers(P0–P12) were synthesized by grafting copolymerization of mesogenic monomer cholesteryl undecylenate (M1) and photochromic monomer 4‐allyoxy‐4′‐nitroazobenzene (M2) on polymethylhydrosiloxane. The chemical structures of polymers were characterized by infrared (IR) and ultraviolet (UV) spectroscopy. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to measure the thermal properties of those polymers, and the mesogenic properties were characterized by polarized optical micrograph, DSC, wide‐angle X‐ray diffraction and small‐angle X‐ray scattering. The glass transition temperatures (Tgs) of the polymers increased from P0 to P4 and decreased from P5 to P12. The clearing point temperatures (Tis) of the polymers P1–P12 were lower than that of P0, but increased from P1 to P4 and decreased from P5 to P12. They showed thermotropic liquid crystalline properties in a broad mesogenic region at temperatures &gt;100°C. The polymers P0–P8 exhibited a cholesteric mesophase with oily streaks and lined texture, and polymers P9–P12 showed a chiral smectic mesogenic phase with a layered texture. All of the polymers were thermally stable to ∼ 320°C. The UV‐induced trans–cis photoisomerization was investigated for the azo monomer and polymers P8 and P12. The solution of the azo monomer and liquid crystalline polymers P8 and P12 can undergo photoisomerization, and the environments of the azo group were responsible for the aforementioned photochemical process. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2155–2162, 2002

Textures of homologous 4-n-alkyloxybenzoic acids: spontaneous chirality and surface memory

By using polarizing microscopy analysis we have found that several achiral homologues of the 4-n-alkyloxybenzoic acids, displaying only the nematic phase, exhibit the optical properties of a chiral liquid crystal system. These acids possess a mesophase due to the formation of dimers via hydrogen bonding. The microtextural analysis was carried out in the temperature ranges of the isotropic, nematic and crystal phases. The nucleation of a chiral texture in small domains emerging on cooling in the isotropic phase was observed. These small domains are characterized by a conoscopic cross which presents an azimuth of 45° with respect to the polarizer axis, contrary to the usual nematic drops, for which the conoscopic cross is not rotated. On further cooling, these domains coalesce in the nematic phase close to the clearing point, thus building large chiral monodomains. Such coalesced droplets exhibit very thin stripe lines, as in the case of pure cholesterics with a tilted helix axis. Moreover, left- and right-handed chiral domains were observed, combined in regions partially separated by 'oily streaks', also typical of pure cholesterics. On cooling, the chiral nematic (N*) phase transformed through a pronounced texture transition into a normal nematic phase. However, the small chiral grains that formed from the isotropic phase are retained close to the surface, acting as 'memorizing centres'. With suitable boundary conditions, they can provide a macroscopic twist driven by the surface. Moreover, a twisted smectic B not present in the bulk phase diagram was found and interpreted as induced by the surface. Also in the crystal phase a strong memorization of the chiral N* texture was observed.

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.

Rheology of lyotropic and thermotropic lamellar phases

We investigate the steady-state rheological behaviour of the lamellar phase of a lyotropic system (CpCl, hexanol, brine) and of a thermotropic system (8CB). Power laws characterize the behaviour of the imposed stress as a function of the measured shear rate and similarities are observed for both systems; the same regime γ˙∼σm with m≈1.7 is obtained at low shear stresses corresponding to a texture of oily streaks oriented in the direction of the flow, as shown by microscopic observations. The “onion state” only exists in the case of dilute samples of the lyotropic lamellar phase; the stress then varies as γ˙∼σm with m≈4.8, as already observed by other groups with different systems. Rheological measurements at different temperatures allow determination of different activation energies relating to the still badly understood processes involved in the different rheological regimes. We propose a model which reproduces the experimental power laws and which is based on an analogy with the theory of high-temperature creep in metals and alloys.

Lyotropic liquid crystal behavior of polystyrene-block-polyisoprene diblock copolymers in toluene as a truly neutral solvent

We present our experimental observations concerning lyotropic liquid crystal behavior and textures of the solutions of the two polystyrene-block-polyisoprene block copolymers in a truly neutral solvent (toluene). The block copolymers used either lamellar or cylindrical microdomains in bulk and also in solutions above the critical concentration (cc). The optical textures, such as fan-shaped texture, parabolic focal conic texture and oily streaks, which have been often observed in smectics or columnar phases of lyotropic low-molecular-weight liquid crystals, were observed in the solutions above cc enclosed in between two glass slides. This demonstrates that the smectic or columnar lyotropic phases are formed in the ordered solutions. Further, we discuss the origin of the formation of the lyotropic phases of the block copolymer solutions in relation to the microdomain structures of the solutions.

The lyotropic polymorphism of two pharmacologically active molecules

The two drugs considered are amiodarone hydrochloride (AC), used in heart therapy and palmitoyl timolol malonate (PTM), a prodrug of timolol and a medicament for glaucoma. These crystalline compounds are poorly soluble in water at room temperature, but form transparent phases, which are sols or gels, much more concentrated than the critical micellar concentration (CMC), when the crystals are heated in the presence of water to temperatures from 50 to 80 degrees C and cooled to room temperature. These transparent pseudo-solutions were also obtained by dissolution in chloroform, followed by rapid solvent evaporation and addition of water. A lamellar phase was also observed, with its classical textures: myelin forms, oily streaks and polygonal fields. Two transparent and isotropic phases, one more concentrated than the lamellar phase and the other one less concentrated, are presumed to be inverse sponges. The water-rich phase, mainly studied with amiodarone, formed a milky cloud on further dilution and a model has been proposed. From these two examples, some remarks of a geometrical kind are presented on the rich polymorphism of certain water-lipid systems.

Rheo-Optics Studies in the Development of Hybrid Liquid Crystalline Based Mwir Polarizers

ABSTRACTWe present here a rheo-optical study of the flow behavior of two cholesteric liquid crystals, one with a large pitch and one with a small pitch. The large pitch compound has been investigated as a possible fixed wavelength polarizer in the mid-wavelength infra-red region (3-5 micron). The investigation of these compounds is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thickness. In our attempts to obtain consistent thin films with reproducible contrast ratios, we explored the defect textures of both compounds under a polarizing optical microscope. These materials were sheared at various strain rates and at various temperatures in an attempt to determine the best processing window for defect free films. The pitch lengths of the two materials investigated were 160 and 1330 nm. The flow behavior of the large pitch material resembles a pure nematic with defect refinement taking place under flow. The short pitch material exhibited the typical Grandjean oily streaks upon shearing followed by coarsening. Observations made during this rheological study were used to identify a processing technique for the large pitch materials. Upon application of a conventional buffed alignment layer, films with consistent quality were routinely made. The measured polarization contrast of &gt;70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength regime.

Cholesteric cellular patterns with electric-field-controlled line tension

A field-induced instability of wall defects, commonly called oily streaks, is observed in cholesteric liquid crystals. The electric field favors reorientation of cholesteric layers and drives the line tension of the oily streaks negative. Defects with negative line tension elongate, preserving their width. The electrically controlled line tension is a nonmonotonic function of the width; streaks that are too narrow or too wide remain straight. The phenomenon is explained within a coarse-grain model that accounts for the surface anchoring.