Dynamics of cholesteric structures in an electric field

Motivated by Lehmann-like rotation phenomena in cholesteric drops we study the transverse drift of two types of cholesteric fingers, which form rotating spirals in thin layers of cholesteric liquid crystal in an ac or dc electric field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge separation or flexoelectric charge generation can describe the drift of cholesteric fingers. We argue that the observed Lehmann-like phenomena can be understood on the same basis.Comment: 4 pages, 4 figures, submitted to PR

Investigations of the Mars Upper Atmosphere with ExoMars Trace Gas Orbiter

The Martian mesosphere and thermosphere, the region above about 60 km, is not the primary target of the ExoMars 2016 mission but its Trace Gas Orbiter (TGO) can explore it and address many interesting issues, either in-situ during the aerobraking period or remotely during the regular mission. In the aerobraking phase TGO peeks into thermospheric densities and temperatures, in a broad range of latitudes and during a long continuous period. TGO carries two instruments designed for the detection of trace species, NOMAD and ACS, which will use the solar occultation technique. Their regular sounding at the terminator up to very high altitudes in many different molecular bands will represent the first time that an extensive and precise dataset of densities and hopefully temperatures are obtained at those altitudes and local times on Mars. But there are additional capabilities in TGO for studying the upper atmosphere of Mars, and we review them briefly. Our simulations suggest that airglow emissions from the UV to the IR might be observed outside the terminator. If eventually confirmed from orbit, they would supply new information about atmospheric dynamics and variability. However, their optimal exploitation requires a special spacecraft pointing, currently not considered in the regular operations but feasible in our opinion. We discuss the synergy between the TGO instruments, specially the wide spectral range achieved by combining them. We also encourage coordinated operations with other Mars-observing missions capable of supplying simultaneous measurements of its upper atmosphere

Excitability and Defect-Mediated Turbulence in Nematic Liquid Crustal

We report the observation of one-arm spiral waves and of turbulence states in a nematic slab subjected to an external forcing. These one-arm spiral waves constitute a close analog to the spiral waves observed in chemical and biological excitable media. Those results were obtained by adding a small vertical magnetic field on the experimental apparatus described in [1], in which double-arm spiral waves were observed in a sample with homeotropic alignment. We also describe a Ginzburg-Landau dynamical model accounting for the main kinds of experimental behavior and patterns obtained. In particular, the experiments and the simulations of this model reveal the appearance of a new instability of the 2$\pi$ walls which is associated with the spontaneous formation of $+1$, $-1$ vortex, with the undoubling of the wavefronts, and which also leads to 2D turbulence

Ising-Bloch transition in a nematic liquid crystal

We consider a nematic slab with homeotropic anchoring and investigate the effect of a magnetic field, $H$, parallel to the plates and an electric field, $E$, perpendicular to them $(\epsilon_{\rm a} 0)$. The magnetic field is either fixed, or rotating at a frequency $\omega$ around an axis perpendicular to the plate. The main effect consists in an Ising-Bloch transition of the walls observed either in the ($H^2,~E^2$) or ($H^2,~\omega$) parameter space. The latter case corresponds to the experimental observation recently made at Brandeis University [1]. Some of these results are understood within the framework of a Ginzburg-Landau model, directly derived from the Oseen-Zocher-Frank expression for nematic elasticity, which accounts for the main static and dynamic experimental phenomena observed. The spiral shaped textures obtained in the dynamic case probably represent one of the first simple physical analogs of the chemical and biological phenomena observed in excitable media.Nous avons étudié, sur une lamelle nématique, avec un ancrage homéotrope, l'effet d'un champ magnétique, $H$, parallèle aux lames, associé à un champ électrique, $E$, parallèle à ces dernières $(\epsilon_{\rm a} 0)$. Le champ magnétique peut être fixe ou tournant à une vitesse $\omega$ autour d'un axe perpendiculaire aux lames. Le principal effet étudié ici concerne une transition Ising-Bloch des parois, observée dans les espaces de paramètres ($H^2,~E^2$) ou ($H^2,~\omega$). Ce dernier cas correspond à l'observation expérimentale récemment réalisée à l'Université de Brandeis [1]. Certains de ces résultats sont interprétés à l'aide d'un modèle à la Ginzburg-Landau directement déduit de l'expression de Oseen-Zocher-Frank de l'élasticité des nématiques. Les textures constituées de spirales obtenues dans le cas dynamique sont probablement l'un des premiers analogues physiques simples des spirales chimiques et biologiques étudiées dans le domaine des milieux excitables

Flow Alignment in Smectic Liquid Crystal

The experimental results of Cladis et al. [1] about the behaviour of smectic-C liquid crystal free-standing films subjected to a two-dimensional shear flow are reproduced via a Ginzburg-Landau model which is simulated numerically. The model takes into account effects that were first neglected in the theoretical analysis made by Cladis et al.; particular emphasis is put on the influence of the elastic anisotropy and the nonrotational part of the flow

Phases bleues "figées" dans un polysiloxane mésomorphe

For the first time, blue phases are observed with a low molecular weight side chain copolymer. Their characteristics are studied, as a function of the copolymerisation ratio of chiral groups. Properties, as the progressive disappearance of different blue phases with the cholesteric pitch values, orientation and symmetry of the observed platelets, seem to be comparable to the smallmolecule case. On the other hand, an original behaviour is obtained near the BP I - cholesteric transition, with an abrupt decrease of the reflected wavelength under normal incidence, associated with the birefringence reappearance. The main chain relating the mesogenic and chiral units is at the origin of a considerable supercooling of BP I allowing, for the first time, to quench blue phase platelets at room temperature, under the glass transition point of these compounds.Des phases bleues sont observées pour la première fois dans un copolymère à chaînes latérales de faible masse moléculaire. Leurs caractéristiques sont étudiées, en fonction du taux de copolymérisation des groupements chiraux. Des propriétés comme la disparition progressive des différentes phases bleues avec l'accroissement du pas cholestérique, l'orientation et la symétrie des "cristaux" observés, semblent comparables à celles qui sont décrites pour des petites molécules chirales. Par contre, un comportement original est obtenu au voisinage de la transition BP I - cholestérique, avec une décroissance brutale des longueurs d'ondes réfléchies sous incidence normale, associée à la réapparition de la biréfringence. La présence de la chaîne principale reliant les unités mésogènes et chirales est de plus à l'origine d'une surfusion considérable de BP I qui nous a permis, pour la première fois, de "geler" aisément des "cristallites" de phase bleue à température ambiante en dessous de la transition vitreuse de ces composés

Magnetic birefringence in mixtures of a nematic aromatic polyester and PAA

The magnetic birefringence was measured in a mixture of a small molecule liquid crystal (PAA) and a nematic polymer. The Cotton-Mouton constant CMfollows a law(CM)-1 = α(T - T *) in the entire concentration range. Both the slope α and the super cooling temperature T* increase with increasing polymer concentration. Comparison is made with a meanfield theory of a mixture of a worm-like polymer and a rigid rod liquid crystal.On a examiné la biréfringence magnétique dans un mélange d'un cristal liquide de petites molécules (PAA) et d'un polymère nématique. La constante de Cotton-Mouton (CM) suit une loi (CM)-1 = α(T - T*) pour toute la gamme de concentration. α ainsi que T* croissent avec la concentration de polymère nématique. On compare avec une théorie de champ moyen d'un mélange de polymère à longueur de persistance et d'un cristal liquide rigide