The linear electro‐optic effect in short‐pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro‐optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well‐controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero transmission. In order to do this the perturbation from the quadratic dielectric coupling has to be ruled out or minimized, which requires materials with essentially zero dielectric anisotropy. This has been done, and it has been found that the bare flexoelectric‐induced tilt has a surprising range of linearity: The linear response in tilt could be followed up to about 30° after which the high electric field caused breakdown. The response time is typically about 100 μs and below.

Linear electro‐optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy

The formation and properties of dispersions of double stranded natural and synthetic nucleic acids are described. Evidence is given for the liquid-crystalline state of the nucleic acid within the dispersed droplets in these phases. The exploitation of nucleic acid dispersions in biosensors is discussed.

Invited Article Liquid-crystalline dispersions of nucleic acids

The CD spectra of dispersions of DNA, in the form of cholesteric liquid crystalline droplets, in an aqueous continuum have been studied. Calculated curves have been fitted to experimental spectra. The amplitude and the sign of the intense absorption band of the purine and pyrimidine bases vary with the droplet size, the pitch and the twist sense of the cholesteric phase. The CD spectra of dispersions of the complex formed by DNA and a coloured intercalating antibiotic have been similarly studied. A general satisfactory level of fitting between observed and calculated CD spectra was found.

Comparison of calculated and observed CD spectra of liquid crystalline dispersions formed from double-stranded DNA and from DNA complexes with coloured compounds

Phase transitions of aqueous suspensions of droplets of smectogens have been studied using the collective Cotton-Mouton (CM) technique. Two compounds were examined: NPOOB and 8OCB. A statistical model (previously developed for suspensions of nematogens) has been adapted to describe the collective CM effect in these systems. It was found that both the phase transitions and the changes of architecture within the droplets cause distinct changes in the CM effect. An attempt was made to investigate suspensions of theliquid crystalline germs created in the isotropic melts of nematic and smectic mesogens mixed with a chiral additive. In such mixtures, nematoids (the liquid crystal analogues of the whiskers of crystalline solids) are formed. It is usually found that nematoids arise as an extension of the tail of a monopole droplet or grow from a focal conic multipole droplet but “free” nematoids (without droplets at their ends) can also occur. The aspect ratio of nematoids varies with the composition, ra...

Phase Transitions in Freely Suspended Smectic Droplets. Cotton-Mouton Technique, Architecture of Droplets and Formation of Nematoids

We study the dynamics of filaments formed when certain materials undergo the isotropic-smectic A phase transition. The basis of this work is a solidification model derived from first principles, taking into account elastic, hydrodynamic and thermodynamic effects. The permeation process, which is solely responsible for the growth of the filament, is analyzed in detail. Various characteristics of the filament, such as growth rates, velocity profile inside the filament and transport along the filament, are calculated by studying the asymptotic limit of a slender body problem. We also derive, in this limit, the equation governing the dynamics of the centerline of the filament. Courant Institute of Mathematical Sciences, New York University, New York, NY 10012. Research supported by Air Force grant F49620-95-1-0065 and NSF grant DMS-9623137 Liquid Crystal Institute, Kent State University, OH 44242. Research supported by NSF under ALCOM grant 89-DMR20147

/pdf/dynamics-of-filaments-during-the-isotropic-smectic-a-phase-rddepbgrei.pdf

Dynamics of Filaments during the Isotropic—Smectic A Phase Transition

New three-component liquid crystal systems creating spontaneously nematoid-type filaments at the transition from the isotropic melt and formation of the nematoid-type filaments at the nematic - smectic A transition in dilute solutions of smectogen in silicon oil are reported. We have observed over 50 new three-component systems, composed of smectogen, nematogen and chiral dopant, in which the formation of the nematoids occurs. The plate-type nucleation of the mesomorphic phases has been observed at the transition from the isotropic melt for pure smectogens and also for three-component systems. In addition, creation of the nematoid-type filaments in dilute solutions of mesogen (NPOOB) in sicon oil has been observed here. Early stages of the plate-type nucleation have been examined using Nomarski contrast. The refractive profiles of the filaments have been examined using double-refracting microinterferometers.

Nematoid Filaments: New Mixtures, Cluster Morphology and Comparative Microinterferometry

Plate-to-nematoid transition of self-organized layers in liquid crystal systems

A review is given of our present general classification of the liquid crystals. The liquid crystal structures are divided into two main groups: nematic phases with parallel ordering of the long molecular axes and smectic phases having additional layered structure. An interesting phenomenon is that the liquid crystal phases when formed by optical active molecules develop chiral modifications: chiral nematics and chiral smectics. There are four fundamental smectic phases: A, B, L and E having skewed analogues: C for smectic A and F, G, H (with long molecular axes tilted to the side of the hexagon) and I, J, K (tilted to the apex of the hexagon) for smectics B, L and E, respectively. Chiral nematics N* and chiral modifications of smectics with weak interlayer correlation (C*, I* and F*) form long-range helicoidal structures. We also briefly discuss the main topological defects in liquid crystal structures: dislocations in layered (smectic) or pseudolayered (chiral nematic) phases and disclinations (...

Basic Liquid Crystal Structures, Polymorphisms and Defects

The stable structural inhomogeneities in nematics/optical active substance solutions have been experimentally investigated. The MBBA or MBBA/EBBA eutectic and abietic acid (AA) as convenient system was chosen. In the solutions of concentrations above some threshold value the creation of separated cholesteric filament regions immersed in nematics were observed, i. e. the distribution of the AA is not homogeneous in nematic liquid. Attempts have been made to interpret teh filament structure in terms of teh lowest energy configuration of the mixtures. The regular band systems of the filament and the lamellar structures play the role of permanent liquid-crystalline diffraction gratings of phase type. The grating constants lie in the range from 0.3 to 3 microns and more. The diffraction is of the Bragg type below the boundary value of the diffraction grating constant and is of the Raman-Nath type above this value. The boundary value was estimated.

The Stable Inhomogeneities of Mixtures and the Permanent Liquid-Crystalline Diffraction Gratings

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