Measurement of the continuous Lehmann rotation of cholesteric droplets subjected to a temperature gradient

International audienceIn 1900, Otto Lehmann observed the continuous rotation of cholesteric drops when subjected to a temperature gradient. This thermomechanical phenomenon was predicted 68 years later by Leslie from symmetry arguments but was never reobserved to our knowledge. In this letter, we present an experiment allowing quantitative analysis of the Lehmann effect at the cholesteric-isotropic transition temperature. More precisely, we measure the angular velocity of cholesteric drops as a function of their size and the temperature gradient and we show that applying an electric field can stop the drop rotation. From these observations and a theoretical model we estimate the Lehmann coefficient

Clinical study of cerebral small vessel disease

Ageing and hypertension are the main causes of cerebral small vessel disease (SVD) known until today. Endothelial dysfunction, rupture of blood-brain barrier (BBB) and protein elimination failure angiopathy have become important in its pathophysiology. The main objectives of the study are identify the clinical risk profile and biomarkers of cognitive impairment (A 1-40), endothelial dysfunction (sTWEAK) and extracellular matrix dysfunction (MMPs) associated with initial phases or with the development of SVD. A prospective study was designed including long time hypertensive and diabetic patients with age between 60-75 years. Cognitive and neuroradiological evaluation were done; and ELISA tests were performed to determinate the serum concentration of sTWEAK, AB1-40, TIMP1, MMP-1, MMP-10, MMP-7, MMP-9, MMP-12, MMP-13 and MMP-3. Main results were bad clinical control of hypertension is the main factor associated with progression of SVD; sTWEAK, MMP7, MMP9 and AB 1-40 could be potential biomarkers related with progression of SVD

Zigzag instability of a chi disclination line in a cholesteric liquid crystal

We studied the formation of chi disclination lines in planar cholesteric samples placed in a temperature gradient near the cholesteric to smectic A phase transition. We observed that the first simple line which forms close to the smectic-cholesteric front zigzags when it is perpendicular to the direction of planar anchoring and is straight for other orientations. This instability is similar to Herring instability for crystalline surfaces. We show numerically that it originates from a strong increase of the elastic anisotropy close to the transition. In addition, we propose a new method to measure the pitch divergence at the smectic to cholesteric phase transition

Direct measurement of the thermomechanical Lehmann coefficient in a compensated cholesteric liquid crystal

International audienceThe thermomechanical Lehmann coefficient \nu is directly measured as a function of temperature in a compensated cholesteric liquid crystal. The method consists of observing the continuous rotation of the director in samples treated for planar sliding anchoring when a temperature gradient is applied perpendicularly to the director. The main result is that there is no relationship between the Lehmann coefficient and the equilibrium twist $q$. In particular, we confirm that $\nu$ does not vanish at the compensation temperature at which $q=0$, in agreement with previous static measurements of Éber and Jánossy (Mol. Cryst. Liq. Cryst. {\bf 72}, (1982) 233) and of ourselves (EuroPhysics Lett. {\bf 80}, (2007) 26001). In addition, the sign of the Lehmann coefficient is determined by observing between crossed polarizers the sense of rotation of the extinction branches of the disclination lines

Thermomechanically driven spirals in a cholesteric liquid crystal

International audienceWe show that the continuous cholesteric fingers, which form in homeotropic samples at the unwinding temperature of the cholesteric phase, drift and spiral when they are subjected to a temperature gradient. This phenomenon is attributed to the appearance of a Lehmann thermomechanical torque. Measurements of the finger drift velocity on both sides of the compensation temperature of a cholesteric mixture show that the Lehmann coefficient does not change sign (and so does not vanish) at this temperature contrary to the equilibrium twist. There is thus no direct relationship between the thermomechanical Lehmann coefficient and the equilibrium twist. The nonvanishing of the Lehmann coefficient at the compensation temperature is due to the absence of inversion symmetry in a compensated cholesteric in spite of its nematiclike structure. This comes from the chirality of the molecules. The ratio of the Lehmann coefficient over the rotational viscosity is also measured as a function of temperature

Sliding planar anchoring and viscous surface torque in a cholesteric liquid crystal

International audienceWe propose a new surface treatment allowing to obtain a sliding planar anchoring of nematic (or cholesteric) liquid crystals. It consists of depositing a thin layer of the polymercaptan hardener of an epoxy resin on an isotropic substrate (bare or ITO-coated glass plates). Microscopic observations of defect annihilations and capacitance measurements show that the molecules align parallel to the surface and slide viscously on it when they change orientation, which implies a zero (or extremely small) azimuthal anchoring energy. By contrast, the zenithal anchoring energy W_\theta is found to be larger than 3\times10^{-5} J/m{^2}. We also measured the liquid crystal rotational surface viscosity \gamma_S by a new thermo-optical method using the large temperature variation of the pitch of a compensated cholesteric mixture. We found that the sliding length \gamma_S/\gamma_1 (where \gamma_1 is the bulk rotational viscosity) is very large in comparison with the length of a liquid crystal molecule. This result is explained by a simple model which takes into account the diffusion of the liquid crystal within the polymer layer

A Sketch-Based Interface for Annotation of 3D Brain Vascular Reconstructions

Within the medical imaging community, 3D models of anatomical structures are now widely used in order to establish more accurate diagnoses than those based on 2D images. Many research works focus on an automatic process to build such 3D models. However automatic reconstruction induces many artifacts if the anatomical structure exhibits tortuous and thin parts (such as vascular networks) and the correction of these artifacts involves 3D-modeling skills and times that radiologists do not have. This article presents a semi-automatic approach to build a correct topology of vascular networks from 3D medical images. The user interface is based on sketching; user strokes both defines a command and the part of geometry where the command is applied to. Moreover the user-gesture speed is taken into account to adjust the command: a slow and precise gesture will correct a local part of the topology while a fast gesture will correct a larger part of the topology. Our system relies on an automatic segmentation that provides a initial guess that the user can interactively modify using the proposed set of commands. This allows to correct the anatomical aberrations or ambiguities that appear on the segmented model in a few strokes.Dans le domaine de l'imagerie médicale, la modélisation 3D de structures anatomiques est maintenant largement utilisée dans l'optique d'é}tablir des diagnostics plus précis qu'avec des données basées sur des images 2D. Aujourd'hui, de nombreux travaux mettent l'accent sur les méthodes automatique de reconstruction de modèles 3D mais ces méthodes induisent de nombreuses erreurs. Avec une structure anatomique (réseau cérébral) présente des parties assez fines et tortueuses, des erreurs sont introduites, cela nécessitent de la correction du modèle 3D, mais aussi des compétences et des heures que les radiologistes ne possèdent pas. Cet article présente une approche semi-automatique de reconstruction d'une topologie correcte de réseaux vasculaires issus d'images médicales en 3D. Notre système repose sur une segmentation automatique qui fournit une estimation initiale dont l'utilisateur peut modifier interactivement en utilisant un jeu proposé de commandes basées sur le croquis. Cela permet de corriger les aberrations anatomiques ou les ambiguïtés qui apparaissent sur le modèle segmenté en quelques traits

Lehmann effect in a compensated cholesteric liquid crystal: Experimental evidence with fixed and gliding boundary conditions

International audienceIn a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of Eber and Janossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). These results prove that one must take into account the chirality of the molecules and the absence of inversion symmetry at the macroscopic scale when deriving the constitutive equations of the phase at the compensation temperature. In this paper, we present the details of our experimental work and a new experiment performed in a sample treated for planar gliding anchoring. The latter experiment, coupled with a numerical simulation, supports the existence of a thermomechanical coupling in a compensated cholesteric