Liquid Crystal Region Research Articles

Tunable single photonic defect-mode in cholesteric liquid crystals with laser-induced local modifications of helix

The authors demonstrate a tunable single photonic defect-mode in a single cholesteric liquid crystal material based on a structural defect introduced by local modification of the helix. An unpolymerized region of cholesteric liquid crystal acting as the defect was left between two polymerized regions via a two-photon excitation laser-lithography process. Upon polymerization, the cholesteric liquid crystal helix elongated and became thermally stable, and a single photonic defect mode was exhibited due to the contrast in the helix pitch at the defect. The defect mode showed tunability upon heating, and a 36nm redshift was seen over a temperature range of 30°C.

Tunable refraction and reflection of self-confined light beams

Light filaments or optical spatial solitons are self-confined (non-spreading) beams that originate from the balance between diffraction and self-focusing in nonlinear optical media (those with a response dependent on the level of excitation)1,2,3. Owing to their ability to self-trap as well as to guide weaker signals (even if differing in colour or modulation format) within the waveguides or ‘light-pipes’ they induce, optical spatial solitons could form the basis of future all-optical processing networks4,5. One of the most interesting challenges in soliton propagation and engineering concerns light filaments incident on linear/nonlinear or nonlinear/nonlinear interfaces. Here we report the robust propagation, refraction and reflection of optical spatial solitons at the interface between two regions of a nematic liquid crystal. The ability to independently tune the optical properties of each region enables us to steer the beams by refraction and total internal reflection by as much as −18 and +22 degrees, respectively. Moreover, the extended (nonlocal) and anisotropic response of our system supports polarization healing of the solitons across the interface as well as non-specular filament reflection. Finally, exploiting the inherent and all-optically tunable birefringence, we demonstrate unprecedented nonlinear Goos–Hanchen lateral shifts in excess of 0.5 mm.

Studies on the phase properties of lyotropic liquid crystals of Brij35/sodium oleate/oleic acid/water system: By means of polarizing microscope, SAXS, 2H-NMR and rheological methods

The pseudo-quaternary phase diagram of Brij35/sodium oleate/oleic acid/water systems has been investigated, and the liquid crystal area has been identified, which covers about two thirds of the whole phase diagram. The liquid crystal structure and behavior have been also studied by using polarizing texture, small angle X-ray scattering, 2H-NMR and rheometer etc. The result shows that when the composition of the system changes along the line of AA’ in this large liquid crystal region, the structural change is cubic→cubic/lamellar→lamellar→lamellar/hexagonal→hexagonal. Meanwhile, we made the first attempt of systematic study of the rheological properties of the above system. The lattice constants of cubic and hexagonal liquid crystals are 10.53 and 5.68 nm, respectively.

Concentration Fluctuations Induced by Orientation Fluctuations in Polymer−Liquid Crystal Mixture

Polymer−liquid crystal (LC) binary mixtures have two modes of concentration fluctuations characterized by the Ornstein−Zernike−Debye form with correlation length ξ (OZD mode) and Debye−Buche form with correlation length l (DB mode) (l > ξ) in their one-phase region and isotropic state of LC. The temperature dependence of OZD mode exhibits the critical phenomena similar to common binary mixtures while the DB mode is almost independent of temperature. Under the condition where LC molecules oriented by electric field, the OZD mode is insensitive to the orientation while the DB mode is enhanced perpendicular to the orientation direction. These results indicate that the DB mode is induced by the orientation of LC molecules.

Study on the Doping and Interactions of Metal Nanoparticles in the Lyotropic Liquid Crystals

Abstract The hydrophilic silver and hydrophobic gold nanoparticles are doped simultaneously or separately into different regions of a lyotropic liquid crystal (LLC) with long-range structural order built with the AOT/isooctane/ water ternary system. A stable lamellar hybrid is produced under suitable conditions. Polarized optical microscopy (POM) and small angle X-ray scattering (SAXS) are used to compare the phase changes upon various doping manners. The interactions between doped particles and surfactant bilayers are analyzed and compared in detail. Factors affecting the stability of inorganic/organic hybrids are also discussed. The doped hydrophobic particles affect the equilibrium of van der Waals and Helfrich interactions between them and the membranes. When hydrophilic particles are incorporated into LLC aqueous region, electrostatic force will be dominant. The results suggest that it is the balance of electrostatic, van der Waals, and Helfrich forces in the lyotropic liquid crystal that determines the hybrid structure.

Defect-free switchable phase grating

Liquid-crystal-filled polymer structure devices offer a very low cost switchable spatial phase modulator. The phase profile set by the polymer structure may be varied or switched on/off with an applied field. Defects have been observed in some devices giving rise to spurious diffraction peaks. Computational modeling of the liquid-crystal director profile suggests that these defects might be suppressed if the dimensions of the liquid-crystal region are small. Experimental measurements confirm that this approach is effective in controlling the defects. This provides a route to fabrication of low-cost switchable complex diffractive devices.

Ionic Liquid Crystal Precursors for Inorganic Particles: Phase Diagram and Thermal Properties of a CuCl Nanoplatelet Precursor

We have recently reported (Angew. Chem. Int. Ed. 2004, 43 (40), 5380) the formation of CuCl nanoplatelets from an ionic liquid crystal precursor (ILCP) consisting of a 50/50 (wt/wt) mixture of bis(dodecylpyridinium) tetrachlorocuprate 1 and 6-O-palmitoyl ascorbic acid 2. Here we present the full ILCP phase diagram and the thermal behavior from a mixing ratio of 1/0 (i.e., pure 1) to 0/1 (i.e., pure 2). The ILCP exhibits a crystalline-smectic-isotropic phase transition at all mixing ratios, and the liquid crystal region is up to 90 degrees C wide. DSC shows a broad exothermic peak between ca. 70 and 170 degrees C, which is associated with the thermally induced CuCl formation. The reaction enthalpies reach -150 kJ/mol at around 50/50 (wt/wt) mixtures of the two components, and the activation energy for CuCl formation is ca. 190 kJ/mol. Thermogravimetric analysis shows that the samples degrade above ca. 200 degrees C.

Defect suppression in liquid crystal filled polymer devices

Liquid crystal filled polymer structure (LiCFiPS) devices consist of a polymer structure, which performs the desired spatial phase modulation of the incident light, the spaces within which are filled with liquid crystal to permit modulation of this optical function. Potentially, fabrication of this type of device may be carried out at very low cost using established polymer hot rolling or embossing techniques. In previous work on coarse gratings liquid crystal defects have been observed when the alignment direction of the liquid crystal is perpendicular to the grating ruling direction. These give rise to spurious diffraction peaks. The models of this type of device suggest that these defects might be suppressed if the dimensions of the liquid crystal region are small. Measured diffraction patterns show that for devices with smaller liquid crystal region dimensions, diffraction free of spuriae is in fact observed. Being fixed function, such devices might not offer the ultimate versatility of matrix addressed SLM devices; they do however offer electronic control of diffractive optical devices at very low cost.

INFLUENCE OF CONFINING GEOMETRY ON THE DIELECTRIC BEHAVIOR OF A FLC SAMPLE HAVING TGBA PHASE

We report on the dielectric permittivities in 1 kHz to 4 MHz frequency region of the ferroelectric liquid crystal (FLC) material CB500 having a TGBA phase. Molecular and collective relaxation processes have been studied in bulk and in confined environment. In the bulk sample, a Goldstone mode (GM) and a molecular mode were observed in the SmC* phase and the softmode (SM) was observed in the TGBA phase. The Goldstone mode was also present in the FLC sample confined in 0.2 μm Anopore membranes. The relaxation frequency of this mode was shifted to the higher-frequency region in comparison to that observed in the bulk sample. In the Anopore confined FLC sample, another relaxation process (AP) was observed, which possibly arises due to some collective movement of the molecules attached to the cavity walls. The relaxation frequency of this process is almost independent of temperature and this process is only observed in the SmC* phase.

Phase diagram of α‐sulfonate methyl esters derived from palm stearin/alcohol/water systems

AbstractPhase diagrams of α‐sulfonate methyl ester derived from palm stearin (α‐SMEPS)/alcohol/water systems were mapped at 30±0.1°C. Two main regions—an isotropic and a lamellar liquid crystalline—were the focal points in this system. An increase in alcohol chain length decreased the isotropic region and increased the lamellar liquid crystal region. In the isotropic region, self‐assembly of α‐SMEPS at different alcohol chain lengths was determined by using conductivity and viscosity measurements. The phase boundaries of micelle to bicontinuous structure and bicontinuous structure to inverse micelle transitions in the isotropic regions are proposed from these analytical methods. The increase in alcohol chain length shifted the micelle ⇆ bicontinuous structure to the water‐rich corner, and the bicontinuous structure ⇆ inverse micelle transition moved toward the alcohol‐rich corner.

The effect of N-(4- n-butyl benzilidine) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide on the nematic-to-isotropic phase transition of 4- n-pentyl-4′-cyanobiphenyl

The nematic-to-isotropic phase transitions of ‘pure’ 4- n-pentyl-4′-cyanobiphenyl (5CB) and nine mixtures of the radical N-(4- n-butyl benzilidine) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide (BBTMPO) at very low mole fractions in 5CB were investigated by a ‘visual’ method and the coexistence lines of the phase diagram obtained were found to be virtually linear as quadratic least-squares fits had only slightly higher correlation coefficients than the linear ones. Thus theoretically predicted curvature in the coexistence lines remains unobserved experimentally. The result for one of the mixtures was confirmed by two methods: the density approach and an EPR method that made use of a setup which allows the control of the temperature of the sample to ±0.01 °C. In the EPR study BBTMPO acted as both probe and impurity, thereby eliminating the effect of the addition of a probe to the mixture that is being investigated. The agreement between the results of the EPR, density and visual measurements was very good. The study shows that BBTMPO is a suitable EPR probe and that a nitroxide probe need not be deuterated for investigating the two-phase region of an impure liquid crystal.

Phase behavior and structural properties for the triton X‐100/n‐CnH2n+1COOH/H2O system

AbstractIn the Triton X‐100/n‐CnH2n+1COOH/H2O system, n‐CnH2n+1COOH can be used as a cosurfactant. As its chain length increases, the regions of the microemulsions showing oil‐in‐water (O/W), water‐in‐oil (W/O), and bicontinuous structures decrease and at the same time, the region of the lamellar liquid crystal increases. In the O/W region, the distribution coefficient K of n‐CnH2n+1COOH between Triton X‐100 micellar phase and water phase increases with the chain length of saturated unbranched monocarboxylic acid. The relationship between the standard solubilization Gibbs free energy of saturated unbranched monocarboxylic acid and the number of methylene groups in the saturated unbranched monocarboxylic acid is given by the equation: ΔGm0=−2.364−2.818 n(CH2) kJ·mol−1 in the Triton X‐100 micellar system. In the lamellar liquid crystal region, small‐angle X‐ray diffraction shows that the thickness of the bilayer d0 is independent of the weight ratio of n‐CnH2n+1COOH to Triton X‐100, but the volume of the solvent penetrating from the solvent layer to the amphiphilic bilayer increases with the weight ratio of n‐CnH2n+1COOH to Triton X‐100. Furthermore, the d0 value increases with the chain length of saturated unbranched monocarboxylic acid, which will contribute to the formation and stabilization of the lamellar liquid crystal.

Phase behaviour of mixtures of colloidal spheres and excluded-volume polymerchains

We study the phase behaviour of mixtures of colloidal spheres and polymers thathave an excluded-volume interaction dispersed in a (background) solvent using theconcept of free volume theory. The depletion layer thickness is calculated from thenegative adsorption of polymer segments around a sphere. The correlation lengthand thermodynamic properties of the excluded-volume interacting polymer chainsin solution are taken into account by using results from the renormalizationgroup theory. For small polymer–colloid size ratios the difference from anideal description of the polymers is small, while for larger size ratios thegas–liquid coexistence region shifts in the direction of higher polymerconcentrations and at the same time the liquid–crystal coexistence region becomesmore extended. Both the gas–liquid region and the gas–liquid–crystalregion become less extended. These features are compared to experiment.

Surface Evanescent Field Characterisation of Antiferroelectric Liquid Crystals

In this paper we report a study of the dynamics of antiferroelectric liquid crystals by using simultaneous total internal reflection and transmission. In devices having both substrates coated with a polyimide, but only one rubbed, we found that the optic axis of the liquid crystal region close to the unrubbed substrate deviates from the rubbing direction less than the bulk. In switching experiments we find that this region is also slower then the bulk and it is virtually free from the undesired pretransitional effect. We also found an asymmetry in the switching of the surface region and we use a simple model to account for it.

Effect of Adding an Amphiphilic Solubilization Improver, Sucrose Distearate, on the Solubilization Capacity of Nonionic Microemulsions

We have studied the effect of adding sucrose distearate (2C18SE) on the solubilization capacity of microemulsions formed in the water/C12EO6/n-decane system. Upon addition of 2C18SE to the binary water/C12EO6 system, a lamellar liquid crystal region developed. This suggests that the rigidity of the surfactant layer is strengthened. The solubilization of water and n-decane in the bicontinuous microemulsions increases about three times upon replacing 10% C12EO6 with 2C18SE; besides, the HLB temperature is not greatly affected by 2C18SE. On the other hand, sucrose monostearate (C18SE) does not have such a function. The effect of added 2C18SE on the solubilization capacity of the discrete droplet-type o/w or w/o microemulsions was also studied. The efficiency of the solubilization-improving effect is reduced when the system is far from the HLB temperature.

Highly Concentrated Cubic-Phase Emulsions: Basic Study on D-Phase Emulsification using Isotropic Gels.

In a hydrophilic octaethylene glycol dodecyl ether (C12EO8)-water system, a micellar cubic phase is formed below 20°C. Upon addition of decane, the maximum temperature of micellar cubic phase (I1) is increased, and, moreover, the hexagonal liquid crystal region changes to the I1 phase. Beyond the solubilization limit of decane in the I1 phase, highly viscous and gel-like O/I1-emulsions are formed in the two-phase region of I1+O. In the case of a surfactant mixture (C12EO4.8) containing C12EO8+C12EO3, a single I1 phase region is not present in a pseudo ternary phase diagram of water/C12EO4.8/decane system, but an I1+O region appears in the oil-rich region. C12EO4.8 forms lamellar liquid crystal in the aqueous system but C12EO3 in the mixture is dissolved in oil and the mixing fraction of C12EO8 in the surfactant aggregate increases as oil is added. Finally, a two-phase region of I1+O appears. It is considered that the O/I1-emulsion or gel based on the cubic phase corresponds to the gel appeared in the so-called D-phase emulsification process. Upon addition of alkyl polyol such as glycerol or 1,3-butanediol, the transparency of the emulsion in the I1+O region increases because the difference in refractive index between the cubic phase and excess oil is decreased. The maximum temperature of the I1 phase decreases with increasing polyol content. Consequently, the addition of alky polyols is not crucial to form O/I1 gels in an oil-rich region.

Pyroelectric investigation of the surface polarization in the isotropic phase of a nematic liquid crystal

Measurements of the temperature dependence of the pyroelectric coefficients in cells with homeotropic and planar boundary conditions were performed in order to study the surface polarization and determine its nature in the isotropic phase of a nematic liquid crystal. Analysis of these data established that the ordering polarization arising as a result of the nonuniformity of the order parameter in the near-boundary region of the liquid crystal and the polar monolayer due to the bifilar properties of the molecules of the liquid crystal at the boundary with the solid wall of the cell are responsible for the surface polarization in the isotropic phase of a nematic liquid crystal. The values of the pyroelectric coefficients of monolayers and the coefficients of the ordering polarization for planar and homeotropic orientations are estimated.

Novel intermediate phases in an ionic fluorocarbon surfactant/water system

This small angle X-ray scattering study of the lyotropic phases in the binary tetramethylammonium perfluorodecanoate/water system shows that there are no classical lyotropic mesophases present. Much of the liquid crystal region is taken up with a random mesh intermediate phase, Mh1(0) and a phase with rhombohedral symmetry which is probably a rhombohedral mesh intermediate phase, Mh1(R3m). This behaviour is unusual since previously these mesh phases have been associated with hydrocarbon surfactants or diblock copolymer melts. All the mesophases found have non-uniform interfacial curvature and a sufficiently strong inter-layer interaction to ensure the long range correlation of structures in some phases.

Periodic Property of Domain in Nematic Liquid Crystal Induced by Elastic Wave

Periodic domain patterns have been observed in a nematic liquid crystal cell induced by an elastic wave which propagates on a glass plate over a wide frequency range. The frequency dependence of the periodic length of the domain pattern is measured and a numerical analysis of elastic wave propagation is carried out in a sandwiched glass-nematic-glass medium. One of the three modes existing in the liquid crystal cell region is related to the longitudinal wave velocity in the liquid crystal. The longitudinal wave velocity in the nematic liquid crystal N-(p-methoxybenzylidene)-p'butylaniline)] (MBBA) is evaluated in the relation to the periodicity of the domain pattern.

Hydrotrope Action of Vitamin C (VC) and Its Application in a Sunscreen

The solubility regions of the sunscreen 2-ethyIhexyl p-methoxycinnamate (E557) in the CTAB/ n-CsHnOH/ H2 O system with and without the hydrotrope agent vitamin C (VC) are determined. E357 was only sohibilized in the bicontinuous microemulsion (BI), the W/ O microemulsion area and the lamellar liquid crystal region (LLC). The addition of VC in the system greatly enlarged the solubility amount of E557in the bicontinuous region, but reduced h in the lamellar liquid crystal area. Small angle X-ray diffraction measurement was used to determine the location of E557 in the lamellar liquid crystal showing the suncreen molecule penetrating between the hydrocarbon chains in the liquid crystal structure.The UV absorption spectra of E557 in various media was measured, surprisingly showing a dependence on the colloidal structure.