24 research outputs found
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 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
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 . The first curve () is perfectly
superimposable to the result of conoscopic measurements done with a
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 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
The influence of some atomic processes on the compression of microballoons
One-dimensional computer simulations have been performed to investigate the effect that preheating of the filling gas by x rays coming from the shell has on the compression of inertial confinement fusion microballoons, and the dependence of this effect on the equation of state describing the filling gas. The targets considered here are glass microballoons filled with D-T mixtures at an initial pressure of 10 bars. Nonlocal thermal equilibrium ionization is explicitly calculated and along with inverse bremsstrahlung, photoionization is taken into account, both as an absorption mechanism and for its effect on the ionization state. The results clearly indicate that radiative preheating energy has a non-negligible adverse effect on compression, as evidenced by the Lawson criterion, and the effect of photoionization and out of equilibrium ionization proves important. © 1991 American Institute of Physics