Infrared dichroism of gold nanorods controlled using a magnetically addressable mesophase

Gold nanorods have unique optical properties, which make them promising candidates for building nano-structured materials using a "bottom-up" strategy. We formulate stable bulk materials with anisotropic optical properties by inserting gold and iron oxide nanorods within a lamellar mesophase. Quantitative measurements of the order parameter by modelling the absorbance spectra show that the medium is macroscopically aligned in a direction defined by an external magnetic field. Under field, the system exhibits significant absorption dichroism in the infrared range, at the position of the longitudinal plasmon peak of the gold nanorods (about 1200 nm), indicating strong confinement of these particles within the water layers of the lamellar phase. This approach can yield soft and addressable optical elements

Etude de la dynamique et du désordre de surface des cristaux liquides nématiques

L'objectif de cette thèse est d'étudier la dynamique et le désordre spatial de l'orientation de l'axe facile, d'un cristal liquide nématique. La première partie du manuscrit est consacrée à l'étude de la dynamique de l'axe facile. Cette dynamique est généralement décrite au niveau moléculaire par deux procédés différents impliquant un mécanisme d'adsorption / désorption (AD) à la surface et la relaxation conformationelle d'un polymère. Nous avons élaboré une expérience et un modèle originaux capables de séparer l'influence de ces deux mécanismes microscopiques. Dans la deuxième partie, nous avons caractérisé statistiquement le désordre orientationnel de l'axe de facile pour différentes couches d'ancrage. Le désordre a des propriétés universelles, c'est à dire qu'il est le même sur toutes les surface analysées. Nous avons ensuite construit un modèle basé sur le « quenching » en surface des fluctuations nématique orientationnelles causé par le processus d'AD. Dans la dernière partie, nous avons mis en place une méthode avec une résolution spatiale et temporelle capable de capturer l'orientation moyenne de l'axe facile et sa distribution spatiale en fonction du temps. Nous montrons que le glissement de l'axe facile moyen se produit avec un changement relativement faible de son désordre spatial.The aim of this PhD is to investigate the dynamics and the spatial disorder of the easy axis orientation, in the case of a nematic liquid crystal. The first part of the manuscript is devoted to the study of the easy axis dynamics. This dynamics is usually described at molecular level by two different processes involving adsorption/desorption (AD) at the surface and polymer conformal relaxations. We devise an original experiment and we build a general model able to separate the influence of these two microscopic mechanisms. In the second part, we statistically characterize the orientational disorder of the easy axis in a whole set of different anchoring layers. This disorder is found to have universal properties, i.e. it is the same on all the analyzed layers. We then build up a model based on the surface quenching of the nematic orientational fluctuations due to the AD-process. In the final part, we set up a method with spatial and temporal resolution capturing the average easy axis orientation and its spatial distribution as function of time. We show that the drift of the average easy axis occurs with a relatively small change of its spatial disorder

Etude de la dynamique et du désordre de surface des cristaux liquides nématiques

L'objectif de cette thèse est d'étudier la dynamique et le désordre spatial de l'orientation de l'axe facile, d'un cristal liquide nématique. La première partie du manuscrit est consacrée à l'étude de la dynamique de l'axe facile. Cette dynamique est généralement décrite au niveau moléculaire par deux procédés différents impliquant un mécanisme d'adsorption / désorption (AD) à la surface et la relaxation conformationelle d'un polymère. Nous avons élaboré une expérience et un modèle originaux capables de séparer l'influence de ces deux mécanismes microscopiques. Dans la deuxième partie, nous avons caractérisé statistiquement le désordre orientationnel de l'axe de facile pour différentes couches d'ancrage. Le désordre a des propriétés universelles, c'est à dire qu'il est le même sur toutes les surface analysées. Nous avons ensuite construit un modèle basé sur le quenching en surface des fluctuations nématique orientationnelles causé par le processus d'AD. Dans la dernière partie, nous avons mis en place une méthode avec une résolution spatiale et temporelle capable de capturer l'orientation moyenne de l'axe facile et sa distribution spatiale en fonction du temps. Nous montrons que le glissement de l'axe facile moyen se produit avec un changement relativement faible de son désordre spatial.The aim of this PhD is to investigate the dynamics and the spatial disorder of the easy axis orientation, in the case of a nematic liquid crystal. The first part of the manuscript is devoted to the study of the easy axis dynamics. This dynamics is usually described at molecular level by two different processes involving adsorption/desorption (AD) at the surface and polymer conformal relaxations. We devise an original experiment and we build a general model able to separate the influence of these two microscopic mechanisms. In the second part, we statistically characterize the orientational disorder of the easy axis in a whole set of different anchoring layers. This disorder is found to have universal properties, i.e. it is the same on all the analyzed layers. We then build up a model based on the surface quenching of the nematic orientational fluctuations due to the AD-process. In the final part, we set up a method with spatial and temporal resolution capturing the average easy axis orientation and its spatial distribution as function of time. We show that the drift of the average easy axis occurs with a relatively small change of its spatial disorder.MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Dynamics of Molecular Exchange between Aligning Adsorbed Film of Liquid Crystal and the Bulk

The liquid crystal(LC) originally filled in thin cells is replaced by a different LC having a drastically different pretilt angle. The mechanism of desorption/adsorption of LC molecules from/on the adsorbed LCfilm at the substrate changes the composition of the adsorbed layer and thus the pretilt angle. The time dependence of molecular pretilt angle at solid surfaces of homogeneously aligned LC cells is measured. The characteristic molecular exchange time depends on the coefficients of desorption of the first LC. The results presented here provide an important insight into the molecular exchange between solid surface and bulk fluid.</p

Orientational quenched disorder in nematics: model and experiments

International audienceQuenched disorder refers to a disorder which is frozen; i.e., it does not change in time. In most experimental situations, this noise has a peculiar statistical signature with correlations decaying on some finite length as opposite to uncorrelated and scale invariant white noise [1]. In liquid crystals the orientational quenched disorder (OQD) profoundly affects the phase order close to a phase transition [2,3]. Unfortunately a direct characterization of these surface random fields necessary for an overall understanding of the measured phenomena is still in its early stages [4]. In this paper we show a complete set of measurement embracing different anchoring layers and liquid crystals. The obtained results support the universality of the measured OQD. We propose a possible physical mechanism which accounts for the measured OQD. In experiments the easy axis distribution was measured via the transmitted intensity trough a hybrid cell made by a hometropic counter-plate and filled with the nematic liquid crystal. Using a polarizing microscope equipped with a high resolution CCD camera we are able to measure the easy axis orientation at the pixel size (0.22 µm). The OQD on solid substrates as rubbed and photo-aligned polymers and SiO, presents the same statistical signature consisting in a compressed spatial auto-correlation function C(R) with unusually large (µm) correlation length (points in the Figure). In the model we consider the adsorption of the director fluctuation modes on the surface. The model main idea is that all fast modes fluctuating with characteristic times lower than a typical adsorption time T are averaged to zero. The model predictions agree with the experimental results by adjusting T ~= 1msec (lines in the Figure). The microscopic origin of the adsorption time will be discussed. References [1] M. A. Rubio, C. A. Edwards, A. Dougherty, and J. P. Gollub, Phys. Rev. Lett. 63, 1685 (1989). [2] T. Bellini, M. Buscaglia, C. Chiccoli, F. Mantegazza, P. Pasini, and C. Zannoni., Phys. Rev. Lett. 85, 1008 (2000). [3] M. Marinelli, F. Mercuri, S. Paoloni, and U. Zammit, Phys. Rev. Lett. 95, 237801 (2005). [4] M. Nespoulous, C. Blanc, and M. Nobili, Phys. Rev. Lett. 104, 9 (2010)

Quenched disorder of a nematic liquid crystal under a magnetic field

International audienceWe report measurements and theoretical predictions on the effect of an aligning magnetic field on the orientational disorder of a nematic liquid crystal in contact with isotropic solid substrates. Different types of substrates present a similar disorder and a similar dependency on the magnetic field amplitude, i.e. a decreasing of the angular distribution widths and spatial correlation lengths. Measurements are in qualitative agreement with a theory where the orientational disorder emerges from the competition between the aligning magnetic torque and the disorienting thermal fluctuations during the adsorption of the liquid crystal molecules on the substrate