Biaxial nematic and smectic phases of parallel particles with different cross sections

We have calculated the phase diagrams of one--component fluids made of five types of biaxial particles differing in their cross sections. The orientation of the principal particle axis is fixed in space, while the second axis is allowed to freely rotate. We have constructed a free-energy density functional based on fundamental--measure theory to study the relative stability of nematic and smectic phases with uniaxial, biaxial and tetratic symmetries. Minimization of the density functional allows us to study the phase behavior of the biaxial particles as a function of the cross-section geometry. For low values of the aspect ratio of the particle cross section, we obtain smectic phases with tetratic symmetry, although metastable with respect to the crystal, as our MC simulation study indicates. For large particle aspect ratios and in analogy with previous work [Phys. Chem. Chem. Phys. 5, 3700 (2003)], we have found a four--phase point where four spinodals, corresponding to phase transitions between phases with different symmetries, meet together. The location of this point is quite sensitive to particle cross section, which suggests that optimizing the particle geometry could be a useful criterion in the design of colloidal particles that can exhibit an increased stability of the biaxial nematic phase with respect to other competing phases with spatial order.Comment: 13 pages, 12 figures, submitted to PR

Free growth of a thermotropic columnar mesophase : supersaturation effects

The free growth (at constant temperature) of a columnar hexagonal liquid crystal is described. The chosen material is the hexaoctyloxytriphenylene. Three growth regimes are observed depending upon the value of the supersaturation Δ: a petal-shape regime for Δ 0.6. The first morphological transition is explained in the framework of the model of Brener et al. [6] by an effect of confinement and finite Peclet number. The second morphological transition remains unexplained for the moment.Nous décrivons la croissance libre (à température constante) d'un cristal liquide colonnaire hexagonal. Le matériau choisi est l'hexaoctyloxytriphénylène. Nous observons trois régimes de croissance suivant la valeur de la sursaturation Δ: un régime « pétale » pour Δ 0,6. La première transition est expliquée dans le cadre du modèle de Brener et al. [6] par un effet de confinement et de nombre Péclet fini. La seconde transition morphologique reste pour l'instant inexpliquée

X-ray structural study of the mesophases of some cone-shaped molecules

We have established by X-ray diffraction that cone-shaped molecules can form relatively well ordered columnar mesophases. Four mesophases have been studied. Three of them have the classical two-dimensional organization in parallel columns while the fourth one shows a quasi-crystalline structure : 3D-ordering takes place at the level of the long paraffinic chains which surround the conical core despite the fact that the chains are in a « molten state ».Grâce à la diffraction des rayons X, nous avons montré que des molécules coniques peuvent former des mésophases assez ordonnées. Parmi les quatre mésophases étudiées, trois sont classiques puisque les colonnes forment un réseau régulier plan, tandis que la quatrième est quasi cristalline : seules les chaînes paraffiniques participent à l'ordre tridimensionnel bien qu'elles soient à l'état fondu

Defects in a TGBA phase: A theoretical approach

Following our experimental observations of disclination lines in freely suspended droplets and free-standing films (Yu.A. Nastishin et al., Eur. Phys. J. E 5 353 (2001)), topological defects of the twist grain boundary (TGBA) phase are considered according to two aspects: topological and energetical. There are two classes of line defects, $disclinations$ (as in the cholesteric (N*) phase and the liquid vortices phase (NL*), relating to the directors tripod symmetries) and $dispirations$ (relating to the translation-rotation symmetries); there are no topological point defects. Differences between N*, NL* and TGBA disclinations are $physical$, not $topological$. The absence of focal conic domains in the TGBA phase is an immediate consequence of the materialization of the helical axis (along the ${\chi}$-director); the same feature, coupled to the trend to parallelism of the smectic layers, accounts for the predominance of $\lambda$-lines. Finally, the presence of defects akin to developable domains is explained in the frame of the $least~curvature~model$, that requires the introduction of a third type of defects: the $densities~of~edge~dislocations$ of the smectic layers

Segundo de Chomón, de l'ombre à la lumière ?

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Phasmids : a new class of liquid crystals

Two hexa-alkoxy derivatives (7a and 7b) of terephthal-bis-[4-benzoyloxyaniline] are described. They exhibit two new types of mesophase for which the term phasmidic is proposed : one has an hexagonal 2D lattice and the second an oblique 2D lattice.On décrit deux dérivés hexa alcoxy de la téréphtal-bis-[benzoyloxy-4-aniline] 7a et 7b. Ils présentent deux nouveaux types de mésophases pour lesquelles le qualificatif (phases) phasmidiques est proposé. L'une de ces mésophases possède un réseau bidimensionnel hexagonal, l'autre un réseau bidimensionnel oblique

Structure des deux phases colonnaires d'un ester de triphénylène chiral

By X ray and optical observations we have studied the structure of the two mesophases of a chiral disc-like molecule. The comparison of the two methods allows the determination of some structural parameters. The transition between the two mesophases appears to be a ferroelectric-antiferroelectric transition. The structure of each column is unchanged at the transition and presents special features related to the chirality of molecule.Nous avons étudié la structure des deux mésophases d'une molécule discoïde chirale par diffraction des rayons X et par observation optique. La comparaison des deux techniques nous a permis de déterminer quelques-uns des paramètres de la structure. La transition entre les deux mésophases semble être du type ferroélectrique-antiferroélectrique, tandis que la structure de chaque colonne reste la même et montre certaines caractéristiques liées aux propriétés de symétrie de la molécule

Reentrant nematic and columnar phases in disc-like liquid crystals at atmospheric pressure

We describe the first example of reentrant ND nematic and D r columnar mesophases at atmospheric pressure in a pure disc-like compound.Un composé discoïde présente, pour la première fois, deux mésophases « rentrantes » à la pression atmosphérique : une phase nématique ND et une phase en colonne Dr

Surface Tension and Elasticity of Hexagonal Columnar Mesophases

Using the grain boundary method and from the shape at equilibrium of liquid inclusions, we measured the surface tension and the Wulff plot of the hexagonal-isotropic interface of the lyotropic mixture C$_{12}$EO$_6+$ water and of the discotic liquid crystal C8HET. We also obtained the energy curve of the symmetric walls of the two systems as a function of the misfit angle of the columns. From their analysis we derive the product $KB$ of the elastic constants of curvature of an isolated column and of compressibility of the hexagonal array. Having measured $B$ mechanically, we then deduce a static estimate of $K$. In both systems, we find values of $K$ that are compatible with X-ray data.A l'aide de la méthode du joint de grain et en étudiant la forme d'équilibre d'inclusions liquides, nous avons mesuré la tension de surface et le diagramme de Wulff de l'interface hexagonal-isotrope du mélange lyotrope C$_{12}$EO$_6+$ eau et du cristal liquide discotique C8HET. Nous avons également obtenu la courbe d'énergie des parois symétriques dans les deux systèmes en fonction de l'angle de désorientation des colonnes. De l'analyse de cette courbe nous tirons le produit $KB$ des modules élastiques de courbure d'une colonne isolée et de compressibilité du réseau hexagonal. Ayant mesuré $B$ mécaniquement, nous en déduisons ensuite une estimation statique du module de courbure $K$ d'une colonne isolée. Nous trouvons dans les deux systèmes des valeurs de $K$ qui sont compatibles avec les données de rayons X