No description available.

No description available.

L'essentiel de ce travail est l'étude de l'étalement de nano-gouttes smectiques sur un substrat de silicium rendu hydrophile par oxydation sous UV-ozone. De nombreuses expériences ont été menées à l'aide la récente technique SEEC (surface-enhanced ellipsometric contrast) qui permet la visualisation directe et en temps réel de couches moléculaires sur les surfaces. Cette technique offre couramment une résolution verticale supérieure à 0,1 nm. Un des cristaux liquides les plus utilisés dans ce travail est le 4’-n-Octyl-4-cyanobiphényle (8CB). Les expérimentations ont été conduites dans une gamme de température comprise entre 21,5°C et 33,5°C, dans cet intervalle de température le 8CB est en mésophase smectique.La dernière étape de l'étalement d'une gouttelette stratifiée dans le cas d'un mouillage impair est l'évolution à partir d'une tricouche vers une monocouche, par la disparition de la dernière bicouche dans la pile. Nous parlons de mouillage impair lorsque les milieux extérieurs sont hydrophile-hydrophobes (comme c'est le cas de nos mesures avec un substrathydrophile et un milieu extérieur (l'air) hydrophobe) et de mouillage pair lorsque les milieux extérieurs sont soit tous les deux hydrophiles soit tous les deux hydrophobes. Nous avons étudié le mouillage impair dans le cas des cristaux liquides smectiques 8CB sur une surface hydrophile. Le recul de la dernière bicouche est accompagné, lorsque la température est plus élevée de plusieurs degrés de celle de la transition Solide/Smectique A, par la formation de nucléations, qui apparaissent préférentiellement dans la partie extérieure de la bicouche. Sinon (lorsque l'on est près de la température de transition) on observe une rétraction de la bicouche sans nucléation. Les données expérimentales ne sont pas conformes à la seule théorie disponible (modèle de P.G. de Gennes et A.M. Cazabat), qui couvre les liquides faiblement stratifiés. Unnouveau modèle est proposé, en accord remarquable avec les expériences. Dans ce modèle le mécanisme de propagation semble être un processus quasi-statique gouverné par les interactions liquide/solide, la pression de Laplace à deux dimensions, et la distinction entre les coefficients de perméabilité de bord et de surface. Ce modèle permet de mieux expliquer nos résultat expérimentaux. De l'analyse des observations expérimentales en temps réel dans le cas de la formation de pores (nucléation), nous démontrons que les lignes de dislocation des boucles qui forment les frontières des pores ne sont pas situés à la même hauteur dans l'empilement des trois couches que les lignes de dislocation qui bordent la bicouche. En outre, une analyse minutieuse de nos résultats en utilisant une approche théorique récemment développée pour expliquer l'étalement de nano-gouttes de cristal liquide smectique suggère fortement que la nucléation des pores est déclenchée par la différence des potentiels chimiques entre les couches adjacentes, ce qui contraste avec le schéma classique où la nucléation est attribuée à la tension latérale le long des couches. Nous avons aussi imaginé un modèle de formation de pré-pores pour expliquer la perméation dans les strates adjacentes d'un liquide smectique. Enfin, une étude toujours en cours porte sur le nucléation hétérogène. Nous forçons une nucléation centrale et étudions son évolution. La tricouche est alors un anneau avec un rayon intérieur croissant et un rayons extérieur décroissant. Le système évolue jusqu'à la disparition totale de la tricouche. Notre modèle propose une solution qui est encore en cours de test. Letravail est ouvert et de nombreuses questions attendent leurs réponses... / The aim of this work is the study of the spread of nanodrops smectic on a silicon substrate made hydrophilic by oxidation under UV-ozone. Many experiments have been conducted using the recent SEEC technique (Surface-enhanced ellipsometric contrast) that allows direct visualization and real-time of molecular layers on surfaces. This technique provides a common vertical resolution greater than 0.1 nm. One of the most used liquid crystals in this work is the 4'-n-octyl-4-cyanobiphenyl (8CB). The experiments were conducted in a temperature range between 21.5 ° C and 33.5 ° C, in this temperature range the 8CB is in smectic mesophase. The last stage of the spreading of a droplet stratified in the case of an odd wetting is changing from a three-layer to a monolayer, that is, vanishing of the last bilayer in the stack. We talk about odd wetting when external environments are hydrophilic-hydrophobic (as is the case of our measurements with a hydrophilic substrate and an external medium (air) hydrophobie) and wetting even when external environments are either all two hydrophilic or bath hydrophobie. We studied the spreading in the case of smectic liquid crystals 8CB on a hydrophilic surface (odd wetting). Receding of the last bilayer is accompanied by formation of pores in it, which appear in the outer part of it, when the temperature is several degrees higher than that of the transition Solid / Smectic A. Otherwise (when it is near the transition temperature) there is a retraction of the bilayer without nucleation. The experimental data are consistent with the only available theory (PG de Gennes model and AM Cazabat) covering weakly stratified fluids. A new model is proposed in remarkable agreement with the experiments. In this model the propagation mechanism appear�� to be a quasi-static process governed by the interactions liquid / solid, the Laplace pressure in two dimensions, and the distinction between the coefficients of permeability of edge and surface. This model helps to explain our experimental results. Analysis of experimental observations in real time in the case of pore formation (nucleation), we demonstrate that the dislocation lines of the loops which form the pore boundaries are not located at the same height, in the stack of three layers, that dislocation lines bordering the bilayer. In addition, a careful analysis of our results using a recently developed theoretical approach to explain the spread of nana-drops of smectic liquid crystal strongly suggests that the pore nucleation is activated by the difference in chemical potential between adjacent layers, which contrasts with the conventional scheme in which nucleation is assigned to the lateral tension along the layers. We also imagined a pre-pore formation model to explain permeation in adjacent layers of a smectic liquid. Finally, still current study focuses on the heterogeneous nucleation. We force a central nucleation and study its evolution. The trilayer is then a ring with an increasing inner radius and an outer radius decreasing. The system evolvesuntil the complete disappearance of the three-layer. Our model offers a solution that is still being tested. The work is open and many questions awaiting answers ...

Polymer electrolytes are widely used as ion transport media in vital applications such as energy storage devices and electrochemical displays. To further develop these materials, it is important to understand their ionic conductivity mechanisms. It has long been thought that ionic conduction in a polymer electrolyte occurs in the amorphous phase, while the crystalline phase is insulating. However, this picture has recently been challenged by the discovery of the crystalline system LiXF6∙PEO6 (X=P, As or Sb) which exhibits higher conductivity than its amorphous counterpart. Their structures comprise interlocking hemi-helical PEO-chain pairs containing Li+ ions and separating them from the XF6- anions. The first Molecular Dynamics (MD) simulation study of the LiPF6∙PEO6 system is presented in this thesis. Although its conductivity is too low for most applications at ambient temperature, it can be enhanced by iso- and aliovalent anion doping. It is shown that the diffraction-determined structure is well reproduced on simulating the system using an infinite PEO-chain model. The Li-Oet coordination number here becomes 6 instead of 5; minor changes also occur in the polymer backbone configuration. The crystallographic asymmetric unit and diffraction profiles are also reproduced. On simulating a shorter-chain system (n=22), more resembling the real material, the structure retains its double hemi-helices, but the polymer adopts a more relaxed conformation, facilitating the formation of Li+-PF6- pairs. Infinite-chain simulation shows the ionic conduction to be dominated by anion motion, in contrast to earlier NMR results. The effects of doping are also reproduced. Shortening the polymer chain-length has the effect of raising the transport number for lithium, thereby bring it into better agreement with experiment. It can be concluded that it is critical to take polymer chain-length and chain-termination into account when modelling ionic conductivity mechanisms in crystalline polymer electrolytes.

Polymer electrolytes are the most obvious candidates for safe "all-solid" Li-ion batteries and other electrochemical devices. However, they still have relatively poor ionic conductivities, which limits their wider adoption in commercial applications. It has earlier been the conventional wisdom that only amorphous phases of polymer electrolytes show usefully high ionic conduction, while crystalline forms are insulators. However, this has been challenged in the last decade by the discovery of highly organized, low-dimensional ion-conducting materials. Specifically, the crystalline phases of LiXF6.PEO6 exhibit higher ionic conductivities than their amorphous counterparts, with the Li-ion conduction taking place along the PEO channels. Polymer chain-length and chain-end registry has emerged as potentially significant in determining ionic conduction in these materials. Molecular Dynamics simulations have therefore been made of short-chain, monodisperse (Mw~1000), methoxy end-capped LiPF6.PEO6 to examine relationships between ion conduction and mode of chain-ordering. Studies of smectic and nematic arrangements of PEO chains have revealed that ion-transport mechanisms within the smectic planes formed by cooperative chain-end registry appear to be more suppressed by ion-pairing than in-channel conduction. Disorder phenomena in the chain-end regions emerge as a critical factor in promoting Li-ion migration across chain-gaps, as does the structural continuity of the PEO channels. Simulations incorporating ~1% aliovalent SiF62- dopants further suggest an increase in Li-ion conduction when the extra Li-ions reside within the PEO channels, with the anion influencing charge-carrier concentration through enhanced ion-pair formation. XRD techniques alone are shown to be inadequate in ascertaining the significance of the various short-chain models proposed; atomistic modelling is clearly a helpful complement in distinguishing more or less favourable situations for ion conduction. Though providing valuable insights, it must be concluded that this work has hardly brought us significantly closer to breakthroughs in polymer electrolyte design; the critical factors which will make this possible remain as yet obscure.

In the present work, we study the effects of external fields and surfaces on liquid-crystalline systems. Initially, we investigate the effects caused by the addition of ferroelectric nanoparticles in a nematic liquid crystal. Using the technique of molecular dynamics simulation, we estimate the density of liquid crystal molecules, the orientational order parameter, and the polar and azimuthal angle profiles as a function of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the presence of ferroelectric nanoparticles enhance the nematic order of the liquid-crystalline medium changing many properties above the transition temperature Tn — Iso. In the second moment, we determine the interaction mediated by elastic deformations between colloidal nanoparticles adsorbed on the surface of free-standing smectic films. In these films, the elastic-mediated force between the adsorbed particles have a long-range character, which contrasts with the character of short-range in films on a solid substrate. The study of phase transitions and critical phenomena in free-standing smectic films has been a subject of many theoretical and experimental works. The McMillan-Miranstsev mean-field approach was used to show that the additional orientational order imposed by the surface anchoring stabilizes the surface-induced smectic siSmA and nematic N phases, leading to he collapse of the tricritical point and to the emergence of a critical end point. Usually strongly anchored free-standing smectic exhibit a stepwise reduction in the thickness when the temperature is raised above the smectic-isotropic bulk transition temperature. In this study, we demonstrate that a layer trinning transition, induced by and external field, can occur in smectic films with negative dielectric anisotropy even below the bulk transition temperature. Using an extended McMillian's model, we show that when the field increases above the bulk transition field, the film thickness reduction is well described by a power law. / Conselho Nacional de Desenvolvimento Científico e Tecnológico / No presente trabalho, estudamos os efeitos de superfícies e campos externos em sistemas líquido-cristalinos. Inicialmente, investigamos os feitos causados pela adição de nanopartículas ferroeléticas em um cristal líquido nemático. Utilizando a técnica de simulação por dinâmica molecular, estimamos a densidade de moléculas do cristal líquido, o parâmetro de ordem orientacional e os perfis dos ângulos polar e azimutal em função da distância ao centro da nanopartícula imersa. Foram consideradas diferentes temperaturas do sistema. Pudemos observar que a presença das nanopartículas ferroelétricas favorece a ordem nemática do meio líquido-cristalino, alterando muitas propriedades do meio acima da temperatura de transição Tn-Iso. Num segundo momento, determinamos a interação, mediada por deformações elásticas, entre nanopartículas coloidais adsorvidas em filmes esméticos livremente suspensos. Nesses filmes, a força de interação entre as partículas adsorvidas tem um caráter de longo alcance, o que contrasta com o comportamento de curto alcance em filmes sobre um substrato sólido. O estudo das transições de fase e fenômenos críticos em filmes esméticos livremente suspensos têm sido objeto de estudo de vários trabalhos teóricos e experimentais. Dentro desse contexto, nós estudamos a transição de fases esmética-nemática (SmA — N) que ocorre no centro de um filme livremente suspenso sujeito a um forte ancoramento superficial, promovido pelo gás ao seu redor. A aproximação de campo médio de McMillan-Mirantsev foi utilizada para mostrar que a ordem orientacional, imposta pelo ancoramento superficial, estabiliza as fases esmética induzida pela superfície (siSmA) e nemática (N), levando ao colapso do ponto tricrítico e ao aparecimento de um ponto crítico terminal. Usualmente, filmes esméticos livremente suspensos e fortemente ancorados apresentam uma redução camada por camada na espessura quando a temperatura aumenta acima da temperatura da transição esmético-isotrópico. Nesse trabalho, nós demostramos que uma transição por redução de camadas, induzida por um campo externo, pode ocorrer em filmes esméticos com anisotropia negativa, mesmo em temperaturas abaixo da temperatura de transição do bulk. Utilizando o mesmo modelo extendido de McMillan, nós mostramos que quando o campo cresce acima do valor do campo crítico, a redução da espessura do filme é bem descrita por uma lei de potência.

Orientadores: Ricardo Antonio Mosna, Guillermo Gerardo Cabrera Oyarzun / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-26T12:57:50Z (GMT). No. of bitstreams: 1 Souza_IbereOliveiraKuntzde_M.pdf: 20917156 bytes, checksum: bb95eeb451fb153542f18a4c8e165528 (MD5) Previous issue date: 2015 / Resumo: Nesse trabalho estudamos configurações geométricas de um cristal líquido bidimensional sobre substratos curvos. Em particular, estamos interessados na fase esmética-A desses materiais, em que as suas moléculas são organizadas em camadas. Isso é interessante pois grande parte das propriedades de um cristal líquido, como as propriedades ópticas e elásticas, é afetada pela curvatura do seu substrato. Diferentemente dos esméticos no plano euclidiano, em superfícies curvas a presença de curvatura gaussiana dá origem a defeitos topológicos e grain boundaries na estrutura dos esméticos. Mostrarei essa interação entre curvatura e defeitos topológicos em algumas superfícies no limite em que a contribuição à energia devido a compressão das camadas é muito maior do que as contribuições provenientes de outros tipos de deformação. Nesse regime, o estado de menor energia é obtido quando as camadas esméticas são igualmente espaçadas. Isso faz com que o vetor diretor siga as geodésicas da superfície, o que leva a uma interessante analogia entre esméticos e óptica geométrica. Além disso, é bem conhecido na comunidade de óptica que lentes planas de índice de refração não-uniformes podem ser tratadas como superfícies curvas, cujas geodésicas se propagam da mesma forma que a luz se propaga na lente. Com isso, pode-se fabricar, em princípio, superfícies com propriedades ópticas específicas e, dessa forma, construir texturas esméticas com diferentes defeitos e singularidades a partir da extensa literatura conhecida de lentes / Abstract: We study geometrical configurations of liquid crystals defined on curved bidimensional substrates. We are particularly interested in the smectics-A phase, whose molecules are organized in layers. This is an interesting problem since many of the liquid crystal characteristics, such as its optical and elastic properties, are affected by the curvature of its substrate. Differently from the planar case, in curved surfaces the presence of Gaussian curvature induces topological defects and grain boundaries in the smectic structure. We will illustrate this interplay between curvature and topological defects for different surfaces in the limit where the energy contribution due to the compression of the layers is much larger than the contributions from other types of deformations. At this regime, the ground state is obtained when the smectic layers are uniformly spaced. In this case the normals to the layers follows geodesics of the surface. This leads to an interesting analogy between smectics and geometric optics. Moreover, it is well known in the optics community that flat lenses with nonuniform refractive index can be treated as curved surfaces, where their geodesics propagate in the same way that light propagates in the lens. Therefore, one can manufacture, in principle, surfaces with specific optical properties and construct smectic textures with different topological defects and singularities by using the extensive literature of known lenses / Mestrado / Física / Mestre em Física

No description available.

No description available.

No description available.