Front propagation into unstable and metastable states in Smectic C* liquid crystals: linear and nonlinear marginal stability analysis

We discuss the front propagation in ferroelectric chiral smectics (SmC*) subjected to electric and magnetic fields applied parallel to smectic layers. The reversal of the electric field induces the motion of domain walls or fronts that propagate into either an unstable or a metastable state. In both regimes, the front velocity is calculated exactly. Depending on the field, the speed of a front propagating into the unstable state is given either by the so-called linear marginal stability velocity or by the nonlinear marginal stability expression. The cross-over between these two regimes can be tuned by a magnetic field. The influence of initial conditions on the velocity selection problem can also be studied in such experiments. SmC$^*$ therefore offers a unique opportunity to study different aspects of front propagation in an experimental system

STUDY OF FERROELECTRIC DOMAINS IN GADOLINIUM MOLYBDATE

On a observé les domaines ferroélectriques dans des cristaux de molybdate de gadolinium par une méthode topographique aux rayons X par transmission. Lorsque les plans réflecteurs sont perpendiculaires aux parois des domaines (leur géométrie est analogue à celle des domaines pour les cristaux de KDP), on observe entre les domaines des lignes alternativement noires et blanches. La mesure de leur largeur permet de calculer l'angle de cisaillement de la maille. Le contraste des topographies permet d'envisager un modèle pour la structure en domaines tenant compte des contraintes mécaniques aux limites entre zones.Ferroelectric domains are observed in gadolinium molybdate crystals by X-Ray transmission topographic method. When the reflecting planes are normal to the domain walls, the domains are separated by alternatively black and white images from the width of which one can measure the shear angle of the cell (the geometry of domains is similar to that in KDP crystals). From the contrast of domains, one proposes a model for the domain structure taking the mechanical matching conditions into account

Anchoring strength of a nematic liquid crystal on a ferroelectric crystal interface

Using a magneto-optical set-up, we study the birefringence of a wedge cell filled with room temperature nematic MBBA (methoxy benzylidene butyl aniline). The cell is constituted by a TGS (Triglycine sulfate) cleaved plate and a glass plate. The TGS is silane treated to give a homeotropic orientation. The antagonistic plate gives initially a planar alignment, and after some time a homeotropic one. In both cases, the homeotropic anchoring on TGS is found weak, with an extrapolation length L = 2.0 ± 0.2 μm. The anchoring follows the simple Rapini-Papoular form. No polar effect is found, neither from a possible surface polarization, nor from the change of orientation of the ferroelectric domains of the TGS plate.En utilisant un montage magnéto-optique, nous étudions la biréfringence d'une cellule coin remplie du nématique à l'ambiante MBBA (methoxybenzylidenebutylaniline). La cellule est formée d'une lame clivée de TGS (sulfate de triglycine) et d'une lame de verre. Le TGS est traité au silane pour donner une orientation homéotrope. La lame opposée donne dans un premier temps une orientation planaire, puis homéotrope. Dans les deux cas, l'ancrage homéotrope mesuré sur le TGS est faible, avec une longueur d'extrapolation L = 2.0 ± 0.2 μm. L'ancrage suit la loi simple de Rapini-Papoular. Aucun effet polaire n'apparaît, ni d'une éventuelle polarisation de surface, ni du changement d'orientation des domaines ferroélectriques du TGS

Translational Self-Diffusion in the Synclinic to Anticlinic Phases of a Ferroelectric Liquid Crystal

In this work, the first direct measurement of inter and intra-layer molecular self-diffusion in the smectic phases of a chiral smectogen (S)-2-methylbutyl-[4'-(4 ''-heptyloxyphenyl)-benzoyl-4-oxy-(S)-2-((S)-2')benzoyl)-propionyl)]-propionate (ZLL7/*) by means of pulsed field gradient NMR is reported. Self-diffusion coefficients as low as 10(-13) m(2) s(-1) have been measured by combining magic echo spin decoupling with a stimulated echo sequence. The transitions among synclinic-anticlinic smectic phases are associated with a change in the self-diffusion coefficients