Continuum theory of tilted chiral smectic phases

We demonstrate that the sequence of distorted commensurate phases observed in tilted chiral smectics is explained by the gain in electrostatic energy due to the lock-in of the unit cell to a number of layers which is the integer closest to the ratio pitch over thickness of the subjacent Sm-C$^*_\alpha$ phase. We also explain the sign change of the helicity in the middle of the sequence by a balance between two twist sources one intrinsic and another due to the distortion of the Sm-C$^*_\alpha$

Nonequilibrium Fluctuations, Travelling Waves, and Instabilities in Active Membranes

The stability of a flexible fluid membrane containing a distribution of mobile, active proteins (e.g. proton pumps) is shown to depend on the structure and functional asymmetry of the proteins. A stable active membrane is in a nonequilibrium steady state with height fluctuations whose statistical properties are governed by the protein activity. Disturbances are predicted to travel as waves at sufficiently long wavelength, with speed set by the normal velocity of the pumps. The unstable case involves a spontaneous, pump-driven undulation of the membrane, with clumping of the proteins in regions of high activity.Comment: 4 two-column pages, two .eps figures included, revtex, uses eps

Extension of the Hamaneh - Taylor model using the macroscopic polarization for the description of chiral smectic liquid crystals

Chiral smectic liquid crystals exhibit a series of phases, including ferroelectric, antiferroelectric and ferrielectric commensurate structures as well as an incommensurate SmCalpha phase. We carried out an extension of the phenomenological model, recently presented by M. B. Hamaneh and P. L. Taylor, based upon the distorted clock model

Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell

A. Mazzulla, F. Ciuchi, and J. Roy Sambles, Physical Review E, Vol. 64, article 021708 (2001). "Copyright © 2001 by the American Physical Society."We present an optical study of the influence of both the flexoelectric effect and surface polarization on a hybrid-aligned nematic cell using the half-leaky guided mode technique. Tilt angle profiles, obtained from fits of experimental data (reflectivity curves) taken under applied voltages, are compared with the ones derived by a complete theoretical model. Measurements with an applied alternating voltage allow the evaluation of the anchoring energy by solving the torque balance equation at the planar surface. From measurements with static fields, the sum of flexoelectric coefficients and the surface polarization are determined by numerical solution of Euler-Lagrange equations

Generalization of Meyer's ferroelectricity to cholesteric phase : electroclinic coupling and prediction of a new ferrocholesteric phase

We present a generalization of Meyer's theory of the trilinear coupling between the smectic layers, the tilt angle of the optical axis with respect to the layer normal and the electric polarization which accounts for the spontaneous ferroelectricity in smectic C* liquid crystals. This coupling allows for the description of the electroclinic effect, i.e. the appearance of a polarization and a tilt angle induced by an external electric field in smectic C*, smectic A and N* (cholesteric) phases. A new “ferrocholesteric” phase is expected between C* and N* phases when the local order is smectic C*-like with a small but non-zero polarization. Eventually, a comparison is made with experimental data available in N* phase.
Nous présentons une généralisation de la théorie de Meyer du couplage trilinéaire entre les couches smectiques, l'inclinaison de l'axe optique par rapport à la normale aux couches et la polarisation électrique qui est à l'origine de l'existence d'une phase smectique C* ferroélectrique ainsi que de l'effet électroclinique. Celui-ci décrit l'apparition d'une polarisation et d'un angle d'inclinaison en présence d'un champ extérieur dans les phases smectique C*, smectique A et cholestérique. Nous prévoyons une nouvelle phase “ferrocholestérique” entre le smectique C* et le cholestérique quand l'ordre local est de type C* avec une polarisation macroscopique non nulle. Nous comparons enfin nos prédictions avec les résultats expérimentaux disponibles en phase cholestérique

Relaxational behavior of flexoelectricity

We give what we believe to be the first clear evidence of the Meyer’s contribution to flexoelectricity

Biaxial nematic and smectic-A boundaries in thin planar samples of 8OCB aligned by rubbed polyimide

The birefringence of planar 8OCB samples has been measured as a function of temperature at thicknesses decreasing from 7.2 to $\rm 2.6 \,\mu m$. The first curve ($\rm 7.2 \,\mu m$) is perfectly superimposable to the result of conoscopic measurements done with a $\rm 100\,\mu m$ homeotropic uniaxial sample, from a minimum of 0.08 close to the isotropic phase up to 0.19 in the smectic-A phase. For thinner planar samples, one gets an increasing birefringence (up to 35% more in both phases with a new plateau at 0.26). This means that the bulk uniaxial phase is surrounded by biaxial boundaries (a few $\rm \mu m$ thick) where the extreme indices of refraction lie parallel to the plates. A mean-field model of long-range interaction is proposed that takes into account the observed features

Scanning conoscopy measurement of theoptical properties of chiral smectic liquid crystals

We report on a new scanning conoscopic method which,by rotating the sample and analyzing the ellipticity oftransmittedlight,providesanaccuratetooltomeasurethetemperaturedependenceofthe two indices ne, no and of the optical activity for uniaxial liquid crystals.Their determination is useful to give informations about the tilt angle y and the macroscopic helicity in the different phases and the non the structures of the liquid crystal phases. We tested the method with the reference compound (99%S, 1% R)MHPOBC

Linear electroclinic coupling in ferroelectric liquid crystals

We report a study of the electroclinic coupling (soft mode), close to the smectic A to smectic C$^\star$ phase transition in several chiral compounds. The main characteristic of the technique used is to measure both optical and dielectric quantities on a single sample. This way one may derive the first coefficients of the landau expansion describing the phase transition. The simplest expectations of this theory are found to hold well without unusual temperature dependences of the coefficients. Furthermore, the relaxation frequency of the electroclinic coupling, (which is essential for applications), is found to lie in the 10 kHz to 1 MHz range.Nous avons étudié le couplage électroclinique (mode mou) près de la transition de phase smectique A-smectique C$^\star$ pour différents composés chiraux. La principale originalité de notre montage est de permettre la mesure des quantités optiques et diélectriques dans un même échantillon. Nous pouvons ainsi déterminer les premiers coefficients du developpement de Landau associé à la transition de phase. Les prédictions les plus simples de cette théorie sont vérifiées sans dépendances thermiques anormales des coefficients. D'un autre point de vue, celui des applications, la fréquence de relaxation de l'effet électroclinique est de l'ordre de 10 kHz à 1 MHz pour les composés étudiés