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

Lehmann rotation of cholesteric droplets subjected to a temperature gradient: role of the concentration of chiral molecules

International audienceWe present a systematic study of the Lehmann rotation of cholesteric droplets subjected to a temperature gradient when the concentration of chiral molecules is changed. The liquid crystal chosen is an eutectic mixture of 8CB and 8OCB doped with a small amount of the chiral molecule R811. The angular velocity of the droplets strongly depend on their size and on the concentration of chiral molecules. The Lehmann coefficient is estimated by using three different methods. Our results are consistent with a Lehmann coefficient proportional to the concentration of chiral molecules. We additionally show the existence of a critical size of the droplets below which they change texture and stop rotating

A chiral axial next nearest neighbour xyxy-model for antiferroelectric liquid crystals

We describe a chiral axial next nearest neighbour $xy$-model to account for the various subphases exhibited by antiferroelectric liquid crystals made of chiral rod-like molecules. The assumed form of the interlayer interactions is based on physical processes which are discussed. Using a discrete model, the predicted sequence of transitions is SmA-SmC$^*_\alpha$-SmC$^*_\beta$-FI$_H$-FI$_I$-FI$_L$-SmC$^*_A$, where FI stands for a ferriphase, as seen in many compounds. The ferri and SmC$^*_\alpha$ phases are characterized by relatively large angles between the $c$-vectors of successive layers and occur only when the compounds have high optical purity. The calculated field induced structures exhibit a plateau of the apparent tilt angle at $\sim\theta/3$, where $\theta$ is the tilt angle of the molecules in the ferriphase. The conoscopic figures in the presence of a field and ellipsometric parameters in the absence of a field have also been generated, which agree extremely well with the experimental results. Recent anomalous X-ray scattering studies prove the $xy$-character of the configurations, though the commensurate structures that are found in the ferriphase require an extension of the model to include lock-in terms

On a theoretical analysis on the influence of non-uniformity of the order parameter on the ssurface energy in nematics

Recently Pikin and Terent'ev have analysed the influence of the non-uniformity of the order parameter on the measured value of the surface cohesion energy in nematic liquid crystals (1988, Sov. Phys. Crystallogr., 33, 641). We argue that since the mathematical problem is ill-posed, the analysis has to be revised. In particular, quadratic terms in the second derivatives of the deformation angle and the order parameter should be taken into account

Linear flexo-electro-optic effect in a hybrid aligned nematic liquid crystal cell

A hybrid aligned nematic liquid crystal sample presents a bulk flexo-electric polarization P. The coupling of P with an external electric field normal to the bounding plates allows, for weak anchoring, a texture distortion. We report a new linear flexo-electro-optic effect in such a cell with a weak anchoring, which presents an electrically controlled birefringence. The results also indicate that the sin2 θs form of the surface anchoring energy is inadequate when the surface angle θ s is large. Low voltage screening by counterions is also discussed.Une cellule nématique en texture hybride présente une polarisation flexo-électrique de volume P. Le couplage de P avec un champ extérieur normal aux plaques produit une distorsion de texture, pour un ancrage faible sur les plaques. Nous observons la variation linéaire de biréfringence électriquement contrôlée qui en résulte, dans le MBBA. L'énergie de surface ne varie pas en sinus carré de l'angle, pour les grands angles de surface. L'écrantage de l'effet aux faibles valeurs de potentiel à cause d'électrodes imparfaites est aussi discuté

Effect of a strong electric field on a nematogen: evidence for polar short range order

PACS. 61.30.GdOrientational order of liquid crystals; electric and magnetic field effects on order - 64.70.MdTransitions in liquid crystals,

Anchoring energy for nematic liquid crystals: an analysis of the proposed forms

Restricted Access.We analyze the proposed functional forms describing the surface anchoring energy of MBBA nematic liquid crystal. Measurements of the surface torque versus tilt angle suggest that the simple cosinus-square form is not adequate for MBBA. The generalized form in an infinite series in cosinus-square is not useful since the set of functions is not orthogonal. Analyses using the orthogonal Legendre polynomials and Fourier expansion are given. Finally we analyse the data in terms of small tilt angles compared to the easy axis, and very far from it, which are the two limits of practical interest

Possibility of a deformed ground state in free standing nematic films

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