Measurement of the Kerr effect in cholesteric blue phases

The electric field induced birefringence within the blue phases and the isotropic phase of several different liquid crystals has been measured. The temperature dependences within the blue phases are found to be strong, characteristically different for each phase, and unlike what is predicted by simple theoretical arguments.     

Ultrasonic absorption studies in the cholesteric and isotropic phases of cholesteryl valerate

Ultrasonic absorption has been measured in the cholesteric and isotropic phases of cholesteryl valerate as a function of both temperature and frequency. The anomalous absorption observed in the cholesteric phase has been interpreted by considering the coupling of the sound wave to the director fluctuations. In the isotropic phase coupling of the sound wave to the tensor order parameter is considered to analyse the absorption results.     

The effect of an electric field on the selective reflection bands of liquid crystalline blue phases

The effect of an electric field on the Bragg reflection of the liquid-crystalline cubic blue phases BPI and II has been studied. The results can be explained by a reorientation of the cubic BP lattice in the electric field. A BP II/I phase transition cannot be induced by the field in the systems under examination.     

Optical and mesomorphic properties characterisation of chiral liquid crystalline copolysiloxane exhibiting cholesteric and blue phases

A series of liquid crystalline polymers (LCPs) have been synthesised by two cholesteric monomers M₁, M₂ and a nematic monomer M₃. The chemical structures and liquid crystalline properties of the monomers and polymers have been characterised by FTIR, ¹H-NMR, differential scanning calorimetry, thermogravimetric analyses, X-ray diffraction measurements and polarising optical microscopy. All LCPs show a high thermal stability with wide mesophase temperature ranges. For polymer P₁ bearing only cholesteric LC monomers component, it shows a cholesteric phase, whereas others display a blue phase besides a cholesteric phase. The formation of the blue phase is based on the structures of the polymers and the produced biaxial helix. The glass transition temperature and isotropic temperature of the polymers decrease on heating cycle with increasing the content of M₃ in the polymers. The specific rotation values of the polymers are temperature-sensitive. The reflection spectra of polymers P₁–P₆ show that the maximum reflected wavelengths shift to long wavelength with increasing the content of M₃ in the polymer systems. The frequency and intensity of the bands change sharply at the temperature where cholesteric phase changes to blue phase, but they show a weak dependence on temperature in the blue phase.     

Selective reflection in the cholesteric and blue phases of a chiral-racemic mixture of CE6

Abstract

The reflection spectrum for visible light is examined for the cholesteric and blue phases of chiral CE6. Pronounced side band oscillations are observed. The Bragg wavelength for total reflection diverges towards the smectic phase with an exponent v = 0·71±0·05. Going from the cholesteric phase to BPI, the lattice parameter increases by (2)^1/2. Evidence is given for the existence of a long-lived supercooled blue phase (BPS).     

Selective reflection in the cholesteric and blue phases of a chiral-racemic mixture of CE6

The reflection spectrum for visible light is examined for the cholesteric and blue phases of chiral CE6. Pronounced side band oscillations are observed. The Bragg wavelength for total reflection diverges towards the smectic phase with an exponent v = 0·71±0·05. Going from the cholesteric phase to BPI, the lattice parameter increases by (2)^1/2. Evidence is given for the existence of a long-lived supercooled blue phase (BPS).     

Scattering of circularly polarized light from the isotropic to the cholesteric phase of cholesteryl oleyl carbonate

The Rayleigh intensity and linewidth of circularly polarized incident light have been measured in the isotropic phases and blue phases of cholesteryl oleyl carbonate. In the isotropic phase the intensity data show mean field behaviour. In the blue phase region the scattering of right handed or left handed circularly polarized light shows a different q dependence for the intensity as well as for the linewidth. The correlation function of the intensity fluctuations can be fitted to two exponentials. The experimental results are compared with the predictions of the theory proposed by Hornreich and Shtrikman.     

Scattering of circularly polarized light from the isotropic to the cholesteric phase of cholesteryl oleyl carbonate

The Rayleigh intensity and linewidth of circularly polarized incident light have been measured in the isotropic phases and blue phases of cholesteryl oleyl carbonate. In the isotropic phase the intensity data show mean field behaviour. In the blue phase region the scattering of right handed or left handed circularly polarized light shows a different q dependence for the intensity as well as for the linewidth. The correlation function of the intensity fluctuations can be fitted to two exponentials. The experimental results are compared with the predictions of the theory proposed by Hornreich and Shtrikman.     

Synthesis and characterization of side chain liquid crystalline polymers exhibiting cholesteric and blue phases

A series of cyclosiloxane-based cholesteric liquid crystalline (LC) polymers were synthesized from a cholesteric LC monomer cholest-5-en-3-yl(3β) 4-(2-propenyloxy)benzoate and a nematic LC monomer butyl 4-[4-(2-propenyloxy)benzoxy]benzoate. All the polymers exhibit thermotropic LC properties and show cholesteric phases. Most of the polymers display four types of phase transition behaviour corresponding to glass transition, melting point, cholesteric phase-blue phase transition and clearing point. The mesophase temperature range of the blue phases are as broad as 20°C. The blue phase was confirmed by the apperance of planar textures and cubic packings. With an increase of non-chiral component in the polymers, the clearing point decreases slightly, while the glass transition and melting temperatures change little. In the reflection spectra of the polymer series the reflected wavelength broadens and shifts to longer wavelength with increase of the non-chiral component in the polymer systems, suggesting that the helical pitch P lengthens.     

Synthesis and liquid crystal properties of new cyclic carbonate monomers functionalised with cholesteryl moiety

ABSTRACT

Four new liquid crystal cyclic carbonate monomers M₁–M₄, with cholesteryl moiety and flexible spacer of different lengths, were synthesised through coupling reaction. The chemical structures, mesophase properties and thermal behaviour of the monomers were characterised with Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy (¹H NMR), polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Based on these results, the relationship between the number of methylene groups into flexible chain and the mesomorphism of the monomers was investigated. It was found that all the monomers showed a focal conic texture of a smectic A phase and exhibited an interdigitated molecular arrangement. Moreover, the glass transition temperature and the isotropisation temperature of the monomers except M₁ decreased, and the mesophase range narrowed as the number of methylene units into the flexible chain increased.     

Influence of interfacial rheology on the viscosity of concentrated emulsions

New models are developed for the viscosity of concentrated emulsions taking into consideration the effects of interfacial rheology and Marangoni phenomenon. The interface is assumed to be viscous with non-zero surface-shear and surface-dilational viscosities. The Marangoni effect is accounted for through non-zero Gibbs elasticity of the interface. The experimental viscosity data for a number of emulsion systems are interpreted in terms of the proposed models.     

Comparison of structures and properties of lyotropic cholesteric phases induced by center and axial chiral compounds

We studied the phase chirality in disklike lyotropic cholesteric (Ch_D) phases which were obtained by adding center and axial chiral dopants to achiral lyotropic nematic (N_D) host phases. In a lyotropic nematic matrix of the N_D phase in the hexadecyldimethylethyl-ammonium bromide/water/n-decanol ternary system, a Ch_D phase was induced by adding center chiral sterols (cholesterol, prednisolon, taurocholic acid) and the axial optically active compound R(−)-1,1′- binaphthalene-2,2′-diyl-hydrogen phosphate (BDP). The helical twisting power (HTP) of BDP is generally lower than the HTP of inducing substances with center chirality, such as cholesterol, prednisolon, etc. At constant composition of the N_D phase, the helix lengths were determined from the ordered fingerprint texture, the so-called “spaghetti-like texture” seen in polarizing microscopy. The reciprocal helix lengths change linearly with the BDP concentration. The properties of the Ch_D phase (textures, helix lengths, micelle parameters) induced by the chiral compounds and changed by the composition of host phases give information on the mechanism of chirality transfer from the molecular level to that of the micellar aggregates and, eventually, to the liquid-crystalline superstructure. The structure in the Ch_D phase was described in the form of micelle parameters. For helix formation a minimum concentration of the chiral compounds is necessary. During the helix formation the number of micelles per helix length changes as a function of the concentration of the center and axial chiral molecules. The first step during the formation of the Ch_D phase is the solubilization of dopants into the micelles. Interaction between the optically active molecules then leads to the formation of hydrogen bridges between adjacent optically active molecules in the helical stack. Received: 20 December 1999/Accepted: 22 May 2000     

Effect of lysozyme adsorption on the interfacial rheology of DPPC and cholesteryl myristate films

A model tear film lipid layer composed of a binary mixture of cholesteryl myristate (CM) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was characterized using surface tension measurements, Brewster angle microscopy (BAM) and interfacial stress rheology (ISR). Isotherms showed that films containing >or=90 mol % CM have a 17-fold greater % area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Lipid films with or=50 mol % CM became elastic at higher surface pressures. Increasing CM content reduced the surface pressure at which the mixed film became elastic. Lysozyme adsorption into a CM film increased the compressibility and resulted in a more expanded film. Lysozyme increased the ductility of the CM/DPPC films with no film breakdown occurring up to the highest pressure measured (40 mN/m). In summary, CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption increased the ductility and decreased the isotherm hysteresis for CM/DPPC films.     

Investigation of the blue phase in a compensable cholesteric mixed system

Pitch dependence of cholesteric blue phase stability has been investigated in a compensable mixed system of cholesteryl nonanoate/chloride. Two critical pitches above which no blue phase exists were found depending on the structure of the liquid crystal. The clearing enthalpy does not exhibit discontinuities over the whole mixing range.     

DNA cholesteric phases: the role of DNA molecular chirality and DNA-DNA electrostatic interactions

DNA molecules form dense liquid-crystalline twisted phases both in vivo and in vitro. How the microscopic DNA chirality is transferred into intermolecular twist in these mesophases and what is the role of chiral DNA-DNA electrostatic interactions is still not completely clear. In this paper, we first give an extended overview of experimental observations on DNA cholesteric phases and discuss the factors affecting their stability. Then, we consider the effects of steric and electrostatic interactions of grooved helical molecules on the sign of cholesteric twist. We present some theoretical results on the strength of DNA-DNA chiral electrostatic interactions, on DNA-DNA azimuthal correlations in cholesteric phases, on the value of DNA cholesteric pitch, and on the regions of existence of DNA chiral phases stabilized by electrostatic interactions. We suggest for instance that 146 bp long DNA fragments with stronger affinities for the nucleosome formation can form less chiral cholesteric phases, with a larger left-handed cholesteric pitch. Also, the value of left-handed pitch formed in assemblies of homologous DNA fragments is predicted to be smaller than that of randomly sequenced DNAs. We expect also the cholesteric assemblies of several-kbp-long DNAs to require higher external osmotic pressures for their stability than twisted phases of short nucleosomal DNA fragments at the same DNA lattice density.     

Experimental study of the growth of cholesteric fingers subjected to an AC electric field and a temperature gradient

We study the behaviour of cholesteric fingers of the first and second types (CF1 and CF2, respectively) which form in cholesteric samples treated for homeotropic anchoring when they are subjected to the combined action of an AC electric field and a temperature gradient. We investigate under which conditions each type of finger can drift and form a spiral. We then show that the ends of the growing CF1 follow circular trajectories, the curvatures of which depend linearly on the temperature gradient. This observation opens the possibility of measuring the thermal Lehmann coefficient in any cholesteric liquid crystal at all temperatures.     

Chirality,twist and structures of micellar lyotropic cholesteric liquid crystals in comparison to the properties of chiralic thermotropic phases

Phase chirality in disk-like lyotropic cholesteric phases (Ch(D)) was investigated, which was induced by addition of center and axial chiral dopants to achiral lyotropic nematic host phases (N(D)). In a lyotropic nematic matrix of the disk-like N(D) phase in the ternary system hexadecyldimethylethyl ammonium bromide (C16Me2EABr)/water/n-decanol, a disk-like lyotropic cholesteric phase Ch(D) was induced by addition of the axial optically active compound R(-)-1,1'-binaphthalene-2,2'-diyl-hydrogen-phosphate (BDP). The helical twisting power (HTP) of the BDP is generally lower than the HTP value of inducing substances with center chirality as cholesterol, prednisolone and taurocholic acid. At constant composition of the N(D) phase, the helix lengths were determined in dependence on the BDP and steroid concentration by means of evaluation of the 'spaghetti-like' texture using polarizing microscopy. The reciprocal helix lengths are changing linearly with rising BDP concentration. The properties of the Ch(D) phase (textures, helix lengths, structural parameters of the micelles) induced by the chiral compounds and changed by the composition of host phases can give information to the mechanism of chirality transfer from the molecular level to that of the micellar aggregates and finally, to the liquid crystalline superstructure. Furthermore, the matrix influence of the N(D) phase on the helix formation was examined at constant BDP and steroid concentration. The structure in the Ch(D) phase was described in terms of micelle parameters. Finally, the inducing properties of a center chiral optically active compound such as cholesterol, prednisolone and taurocholic acid were compared with those of the axial chiral compound BDP. Last but not least, the situation of the theoretical and structural background for helix formation in liquid crystals, e.g. the explanation of chiralic transfer between micelles is analyzed and discussed. Two main conditions are necessary to build up the helix in the Ch(D) phase: the formation of H-bridges; and the existence of a specific chiralic interaction energy between neighboring micelles in the cholesteric superstructure.     

The effect of interfacial viscosity on the droplet dynamics under flow field

The effects of interfacial viscosity on the droplet dynamics in simple shear flow and planar hyperbolic flow are investigated by numerical simulation with diffuse interface model. The change of interfacial viscosity results in an apparent slip of interfacial velocity. Interfacial viscosity has been found to have different influence on droplet deformation and coalescence. Smaller interfacial viscosity can stabilize droplet shape in flow field, while larger interfacial viscosity will increase droplet deformation, or even make droplet breakup faster. Different behavior is found in droplet coalescence, where smaller interfacial viscosity speeds up film drainage and droplet coalescence, but larger interfacial viscosity postpones the film drainage process. This is due to the change of film shape from flat‐like for smaller interfacial viscosity to dimple‐like for larger interfacial viscosity. The film drainage time still scales as Ca⁰ at smaller capillary number (Ca), and Ca^1.5 at higher capillary number when the interfacial viscosity changes. The interfacial viscosity only affects the transition between these limiting scaling relationships. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1505–1514, 2008     

External electric field effect on electronic properties and charge transfer in CoI2/NiI2 spinterface

Electronic structure and spin-related properties of CoI₂/NiI₂ heterostructure were studied by means of density functional theory. It was shown that the electronic structure at the Fermi level can be characterized by a band gap. The effect of the external electric field on charge transfer and electronic properties of the CoI₂/NiI₂ interface was investigated, and it was found that band gap width depends on the strength of the applied electric field, switching its nature from semiconducting to a half-metallic one. An easy control of the electronic properties and promising spin-polarized nature of the CoI₂/NiI₂ spinterface allows the heterostructure to be used in spin-related applications.