Electrically-induced optical storage effects in smectic polysiloxanes A variable transmission device

Changes in the optical transmission induced by electric fields have been studied in a smectic side-chain polysiloxane liquid crystal. The transmission varied from effectively zero, for a highly scattering (opaque) texture, to total, for a homeotropic texture, on increasing the applied voltage across the material. The field-induced textures were durably stored in the smectic phase, enabling the order parameter of the side groups to be studied by dielectric relaxation measurements. For the highly transparent texture, a side group order parameter of 0-92 has been observed.     

Electroclinic effects in a homogeneously aligned smectic A cell

The electroclinic effect, in a material having a first order S_A to S_C* transition, is studied using the half leaky guided mode geometry. Using an approximately 1 μm thick, homogeneously aligned cell, the voltage induced director twist is characterized at a few temperatures in the S_A phase. The mean field theory readily explains the data recorded at low fields where a linear dependence on voltage is found. However, at higher fields, an unexpected saturation occurs which is most likely caused by the influence of strong surface anchoring forces.     

Electro-optic effects in smectic C liquid crystals

Electro-optic effects are observed in the smectic C states of 4-n-heptyloxybenzoic acid, 4-n-octyloxybenzoic acid. 4,4′-bis-n-heptyloxyazoxybenzene, and 4-n-decyloxy cinnamic acid under the application of an external ac electric field. The stripe domain pattern is observed for all the compounds studied above the first threshold. In the smectic C state there exists a second threshold at which the direction of the domain lines changes.     

Side chain polysiloxanes with fluorinated mesogenic groups. The role of the fluorine content on the properties of the smectic A state

A series of liquid crystalline polymers has been synthesized in which fluorinated or semi-fluorinated mesogenic moieties are used as side groups linked to a polysiloxane backbone. Clear-cut differences in the structure of the smectic A phase are evidenced between fluorinated polymers and their classical non-fluorinated homologues. In a comparative analysis, we discuss the relevance of the steric hindrance of perfluorinated sequences and of the microsegregation of the backbone in response to these modifications as related to the fluorine content in the side group and to the proportion of mesogenic groups.     

Observation of a chevron hybrid structure in the smectic A phase of a liquid crystal device

Small angle X-ray diffraction experiments show the emergence of a chevron structure on cooling from the nematic phase into the smectic A phase of the commercial mixture S3 (Merck Ltd, UK), in a display device. The chevron angle increases from 0 at the nematicsmectic transition temperature, T NS, to a maximum value of 8.5 , which is reached when the sample is 15 C below T NS. Between 5 C and 15 C below the transition temperature a quasi bookshelf structure emerges, in addition to the prevailing chevron structure; such a structure has not been hitherto reported. There is no further resolvable change in the device structure on cooling lower than T NS -15 C. The chevron structure is due to the combination of layer thinning and fixed surface layers, as confirmed by a comparison of layer spacing calculated from the Bragg angle equation and from the layer thinning equation ( d = d nematic cos delta, where delta is the chevron angle).     

A re-entrant smectic C phase in a sequence smectic C-oblique columnar-smectic C-nematic

A new six-ring double-swallow-tailed compound is presented which exhibits a re-entrant smectic C phase in the sequence smectic C-oblique columnar-smectic C-nematic. This unusual sequence detected by observation of the optical textures and by X-ray investigations is also confirmed by the phase diagrams with homologous compounds.     

An unusual chevron structure in an achiral smectic C liquid crystal device

The layer structure that occurs in an achiral smectic C liquid crystal device has been investigated as a function of temperature using the small angle X-ray scattering facility at the Synchrotron Radiation Source, Daresbury UK. The material studied shows a direct phase transition from the nematic to the smectic C phase. The layer structure proposed on the basis of the diffraction data is relatively complex, containing regions with chevron, quasi-bookshelf and curved structures. A rationale for the formation of the structure is presented, relying on both the phase transition characteristics of the system and the anisotropic layer elasticity in the smectic C phase. Qualitative analysis indicates that the layer constant A is greater than A 21, i.e. layer flexing is easier perpendicular to the plane of the director than parallel to it. It is also demonstrated that the surface chevron angle is several degrees different from the tilt angle of the smectic C phase at temperatures well below the smectic C to nematic phase transition. 12     

Analysis of threshold field effects in smectic C phases

A qualitative analysis of the threshold behaviour of plane smectic C layers in external fields is made. All possible relative orientations between the liquid crystal, the boundaries and the field vector are considered. Arbitrary material constants are assumed in the uniaxial approximation. The conditions for first- and second-order transitions are given.     

A new memory effect: smectic liquid crystalline memory having both conductive and optical memory

In order to compensate for light leakage in the oblique viewing directions of the dark state of the horizontal-switching mode such as the in-plane switching and fringe field switching modes. The viewing angle performance of the horizontal-switching mode with an compensation film has been evaluated on a case by case basis according to the retardation values of the films. Consequently, we found that a much better dark state can be achieved by using an optimised compensation film.     

Some flow effects in continuum theory for smectic liquid crystals

This paper presents a constrained theory for smectic C liquid crystals that may be useful for the analysis of some effects in these materials. The theory is based on two simplifying assumptions, namely that the layers although deformed remain of constant thickness, and also that the tilt with respect to the layer normal remains fixed. The equilibrium version of the theory proves to be a non-linear generalization of the earlier Orsay theory, and promises to model a number of static effects satisfactorily. Here our aim is to examine preliminary predictions based on the corresponding dynamic theory, where some progress proves possible for shear flow, and also for a shear wave reflection-refraction experiment useful for the measurement of some viscous coefficients.     

Preliminary communication Dispersion of the optical axes in smectic C* liquid crystals

Optical axes dispersion denotes the dependence of the orientation of the indicatrix and the optical axes on the wavelength of light. Theory predicts optical axes dispersion in optically biaxial phases with low crystallographic symmetry, like the C 2h-symmetry of SmC or the C2 symmetry of SmC* phases. The magnitude of this axes dispersion was measured electrooptically for two liquid crystal materials exhibiting SmC* phases using several wavelengths of light. Far below the phase transition temperature, the wavelength dispersion of the optical tilt is about 1-2 (5-10% of the total tilt) over the range of visible wavelengths.     

Layer-thinning transitions in free-standing smectic A films

In the present paper a discrete mean-field model for thin smectic A liquid crystal films with two boundary surfaces is offered. The model accounts for the recently observed phenomenon of layer-thinning transitions in free-standing smectic A films upon heating. In particular, the model predicts the observed multiple layer jumps, as well as, for films thinner than 13 layers, the observed power law dependence of the layer-thinning transition temperatures on film thickness.     

Time resolved X-ray diffraction studies of electric field induced layer motion in a chevron geometry smectic A liquid crystal device

Small angle time-resolved X-ray diffraction was used to monitor the behaviour of the smectic layers during the electric field induced planar to homeotropic transition in a smectic A cell possessing a chevron layer geometry. The liquid crystal material used was S3, from Merck Ltd, and was sandwiched in a 15 mum parallel plate device. The main features of the transition are the cooperative rotation of layers and the creation of an asymmetric chevron structure during the early stages of switching. The time scale for the planar-to-homeotropic transition in the device is approximately 5 s, at a temperature of 3 C below the nematic-to-smectic A phase transition and for an applied electric field of 2 V mum -1 (rms).     

A novel fiber optical device for ultraviolet disinfection of water

Since there are several problems in traditional UV disinfection techniques, a highly efficient, reliable and economical method, using quartz optical fibers to deliver UV light is proposed. The principle of the experimental setup is that ultraviolet rays are gathered by a reflector and converge on a light point, the diameter of approximately 5mm. In this way UV light can be transferred into water to kill the bacteria in the water. This paper presents preliminary results on water disinfection using this new UV disinfection setup. Its suitability for application could be shown in experiments with E. coli (ATCC8099) as test microorganisms. We have optimized the distribution of the optical fibers in the water in bench-scale study. This result can provide guidance for pilot-scale and field-scale study of this new technique. The results show that the new technique had a good performance under different conditions as follows: (a) turbidity level=10.2 NTU, (b) ferric ion concentration=0.3 mg/L, and (c) humic acid concentration=5 mg/L. The new technique provides a promising approach to disinfection treatment of drinking water.     

A backflow effect in smectic C liquid crystals

This paper discusses the influence upon a smectic C liquid crystal cell of backflow induced by the relaxation of alignment following the removal of a strong electric or magnetic field. Our study, based upon a recently proposed continuum theory, concentrates upon the homeotropic configuration in which the smectic layers are parallel to the boundary plates, but some consideration is also given to the bookshelf geometry. Although the governing equations prove to be rather complex, some progress is possible analytically by repeating an approximation made in the corresponding problem for a nematic.     

Strong thermal optical nonlinearity caused by CdSe nanoparticles synthesised in smectic ionic liquid crystal

Spectral and nonlinear optical properties of cadmium octanoate composites containing CdSe nanoparticles (NPs) have been studied by using optical absorption spectroscopy and laser scanning technique (Z-scan). CdSe NPs are chemically synthesised in thermotropic ionic liquid crystal (ILC) phase of cadmium octanoate which is used as nanoreactor. Anisotropic glassy nanocomposites are obtained by the rapid cooling of the ILC phase of nanocomposites to room temperature. The sizes of the CdSe NPs are determined from the absorption spectra. The thermo-optical nonlinearity of the new nanocomposites is characterised by extremely large value of the nonlinear refractive index, n₂, under relatively low-powered CW laser irradiation. This nonlinearity is caused by (1) the efficient light-induced heating due to the CdSe NPs strong exciton absorption, and (2) consequent thermal dissipation, which in turn, produces the photoelastic tensions in the glassy smectic matrix.     

A microfluidic device based on droplet storage for screening solubility diagrams

This work describes a new microfluidic device developed for the rapid screening of solubility diagrams. In several parallel channels, hundreds of nanolitre volume droplets of a given solution are first stored with a gradual variation in the solute concentration. Then, the application of a temperature gradient along these channels enables us to read directly and quantitatively phase diagrams, concentration vs. temperature. We show, using a solution of adipic acid, that we can measure ten points of the solubility curve in less than 1 hr and with only 250 microL of solution.     

Induction of the smectic Ad phase in polar systems II. Role of steric effects in the smectic Ad phase induction

Binary mixtures in which one component is nCBB or its chiral analogue, and the second component is nCB, nOCB, n.CN, or one of their chiral analogues are studied by thermomicroscopy. The branched compounds induce the smectic A_d phase more strongly than unbranched compounds with the same alkyl chain length. The observed behaviour is discussed from the point of view of dimer formation. In all chiral systems, the TGB_A phase appears.