Neste trabalho foi feito um estudo do comportamento magnético de cristais líquidos liotrópicos de laurato de potássio dopados com nanopartículas de magnetita. Medidas zero-field cooling (ZFC) e field cooling (FC) com campos de 50 Oe mostram que as nanopartículas magnéticas permanecem bloqueadas a temperatura ambiente devido à presença das micelas do cristal, e apresentam um comportamento superparamagnético quando dispersas somente em água. Simulações Monte Carlo indicam que o aumento da temperatura de bloqueio das nanopartículas imersas em cristais líquidos liotrópicos pode ser obtido considerando uma forte interação, similar à de troca, entre os momentos superficiais e uma camada extra, simulando a presença das micelas. Medidas ZFC-FC e de suscetibilidade magnética permitiram determinar as temperaturas de transição entre as fases micelar isotrópica e nemática discótica. A curva de magnetização na região da fase discótica parece seguir o mesmo comportamento do parâmetro de ordem desta fase. Foram obtidos nanofios de níquel pelo processo de eletrodeposição AC em lâminas de alumina porosas para dopar cristais líquidos liotrópicos na fase isotrópica. As medidas ZFC-FC desses sistemas foram mais delicadas que as das nanopartículas devido ao baixo sinal das amostras e baixa solubilidade dos nanofios no cristal líquido. A técnica para a dissolução dos fios e dopagem dos cristais foi aprimorada, no entanto são necessários mais estudos para melhorar a estabilidade dessas amostras. / In this work we studied the magnetic behavior of potassium laurate lyotropic liquid crystals doped with magnetite nanoparticles. Zero-field cooling (ZFC) and field-cooling (FC) measurements with 50 Oe applied field showed a blocking behavior of magnetic nanoparticles at room temperature due to the micelle presence, while presenting superparamagnetic behavior when dispersed in water. Monte Carlo simulations suggest that this increase in the nanoparticles\' blocking temperature when immersed in lyotropic liquid crystals can be acquired taking into account an exchange-like strong interaction between the nanoparticles\' superficial magnetic moments and an extra layer playing the role of the micelles. ZFC-FC and magnetic susceptibility measures allowed us to find the transition temperatures between the isotropic micellar and the discotic nematic phases. The magnetization curve in the discotic phase resembles the behavior of the order parameter in that phase. We obtained nickel nanowires by AC electrodeposition in anodic porous alumina to dope lyotropic liquid crystals in the isotropic phase. The ZFC-FC measurements in those systems were more intricate given the weak magnetization of the samples and low solubility of the nanowires in the liquid crystal. We refined the technique for dissolution and doping of the crystals, but more studies are needed in order to improve the samples\' stability.

Orientador: Harry Westfahl Junior / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-11T09:45:51Z (GMT). No. of bitstreams: 1 HernandezJimenez_Marcela_D.pdf: 3260686 bytes, checksum: f15b800ff735b84b434b7b6317322209 (MD5) Previous issue date: 2007 / Resumo: Os polímeros líquido-cristalinos são estruturas macromoleculares compostas por blocos moleculares flexíveis e rígidos, que combinam propriedades típicas de polímeros e de cristais líquidos. Como ocorre nos copolímeros de blocos flexíveis, a macro-separação de fases originada pela incompatibilidade química entre os diferentes tipos de blocos é frustrada pela conectividade do polímero. Isto resulta na microsegregação de fases, ou formação de microdomínios, característicos de cada um dos blocos e dispostos de maneira auto-organizada. Os polímeros líquido-cristalinos de cadeia lateral (PLCCL) são copolímeros graftizados ou enxertados1, que caracterizam-se por possuir uma cadeia flexível contendo moléculas rígidas (mesógenos) liigadas à mesma em intervalos regulares, através de unidades metilénicas denominadas espaçadores. O acoplamento entre os mesógenos (moléculas rígidas responsâveis pela formação de fases líquido-cristalinas) e a cadeia principal nestes polímeros, origina-se de uma competição entre o ordenamento dos mesógenos dos grupos laterais e a entropia conformacional da cadeia principal. Neste trabalho propomos um modelo microscópico para PLCCL, que leva em conta a competição entre as interações isotrópicas de volume excluído e as interações anisotrópicas de Maier-Saupe. Os blocos flexíveis do PLCCL são tratados como cadeias Gaussianas, enquanto os mesógenos são tratados como bastões rígidos. Usando um desenvolvimento em séries de potências dos parâmetros de ordem, conhecido como aproximação Random Phase Approximation (RPA) calculamos o funcional de energia livre para os PLCCL em função de dois parâmetros de ordem, um relacionado às flutuações de densidade e outro relacionado às flutuações de orientação. Mostramos que a estabilidade da fase isotrópica, em relação a essas flutuações, depende da razão entre os potenciais das interações Flory-Huggins (isotrópicas) e Maier-Saupe (anisotrópicas). Neste caso, três fases termodinâmicas são evidenciadas. A primeira delas, corresponde a uma fase nemâtica similar à fase nemâtica dos cristais líquidos monoméricos. A segunda é uma fase na qual hâ uma modulação da densidade, mas não há orientação dos mesógenos, ou seja, uma fase paranemática com modulação de densidade. A terceira é uma superposição dos dois tipos anteriores, e portanto, sugere a existência de uma fase esmética. Numa anâlise mais detalhada da transição nemática-esmética, estudamos a instabilidade da fase nemática em relação à formação de fases esmética A e C. Essas análises foram efetuadas variando-se os parâmetros de geométricos do PLCCL, tais como grau de polimerização, tamanho dos mesógenos, tamanho do espaçador e espaçamento entre os grupos laterais. Os resultados obtidos com este modelo apresentam uma concordância qualitativa com as observações experimentais relatadas na literatura. 1 A palavra "graftizado" é uma tradução da palavra inglesa "grafted", aceita pelo Comitê Brasileiro para Assuntos de Química junto à IUPAC, vide referências [1,2] / Abstract: Polymeric liquid crystals are materials that combine properties of both polymers and liquid crystals. These polymers can be thought as block copolymers made of ftexible and rigid molecular blocks. As in flexible block copolymers, the macrophase separation originated from the repulsion between different blocks is frustrated by the connectivity constraint imposed by the architecture of the polymer. This frustration results in the formation of alternated microdomains, rich in the fiexible or rigid component (microphase separation). In Side-Chain Liquid-Crystalline Polymers (SCLCP) the mesogenic units are periodically attached to a ftexible polymer (backbone) through a polymeric chain called spacer. Because of the coupling effect of the spacer, there is a competition between the ordering of the mesogens in the side groups and the conformational entropy of the backbone. In this work we propose a microscopíc model for SCLCP that takes into account the competition between the isotropic excluded volume interactions and the anisotropic Maier-Saupe interactions. The flexible blocks are treated as ideal Gaussian chains while the mesogens are considered as rigid mesogenic rods. Using an series expansion on the arder parameters, known as Random Phase Approximation (RPA), we calculate the free energy functional for the SCLCP as a function of two order parameters, one related to density ftuctuations and another related to orientational fluctuations. We show that the stability of the isotropic pha..,e against these fluctuations depends on the relative strength between the Flory-Huggins and the Maier-Saupe interactions. Three different thermodynamic phases are found within this model. The first one is a nematic phase similar to the nematic phase of low molar mass liquid crystals. The second one is a phase with modulated density without mesogen orientation, being this a paranematic phase. The third phase is characterized by both density modulation and orientational order, suggesting the formation of a smectic phase. In a more detailed analysis of the nematic-smectic transition, we studied the stability of the nematic phase against the formation of smectic A and smectic C phases. This analysis was performed for different values of the geometrical parameters of the molecule, such as degree of polymerization, length of the spacer, length of the meso.gen and spacing between side groups. The results obtained are in qualitative agreement with experimental data found in literature / Doutorado / Física da Matéria Condensada / Doutor em Ciências

Neste trabalho apresentamos uma revisão de métodos de simulação de energia eletrostática de sistemas de cargas e uma proposta de adaptação de algoritmo ultilizado na literatura de sistemas gravitacionais para estudo das propriedades estatísticas de sistemas coulombianos. Na primeira parte do estudo, revisamos os fundamentos teóricos do método de Ewald e suas condições de aplicabilidade, procurando esclarecer as referências mais importantes no assunto, que são de difícil compreensão, gerando equívocos na utilização do termo de dipolo. Detalhamos o estudo sobre a análise da convergência da série em que a técnica se baseia, bem como sua interpretação física mostrando a equivalência entre as duas abordagens . Na segunda parte do trabalho analisamos os fundamentos do Fast Multipole Method desenvolvido para interação gravitacional, para o qual construímos programas em linguagem C para uma versão na rede. Criamos um algoritmo que denominamos Fast Multipole Monte Carlo (FMMC) e desenvolvemos um programa para cálculo das propriedades termodinâmicas de sistemas coulombianos. Os programas são testados comparando resultados para a energia e propriedades térmicas do modelo LRPM com resultados de simulação através de cálculo direto. / In this work we present a review of methods of simulation for the electrostatic energy of charged systems and an adaptation of an algorithm from the literature on gravitational systems for the study of the statistical properties of Coulomb systems. In the first part of the work, we review the fundamentals for the theoretical method of Ewald and its conditions of applicability, seeking to clarify the most important references on the subject, which because of the involved mathematics, have led to misuse of the so-called dipole correction. We detail the study on the convergence of the series for the electrostatic potential on which the Ewald technique is based, as well as the physical interpretation given elsewhere, showing the equivalence between the two approaches. In the second part of this work, we analyse the foundations of the Fast Multipole Method developed for gravitational interactions, and present programs in C language for a network version of neutral charged systems. Finally, an algorithm, which we name Fast Multipole Monte Carlo, and the corresponding code for calculating the thermodynamic properties of Coulomb systems are presented. The programs are tested by comparing results for the energy and thermal properties of the Lattice Restricted Primitive model with results of simulations based on direct calculations for the Coulomb energies.

Este trabalho investiga a termodifusão em coloides magnéticos através da técnica de varredura Z. O ponto de partida é a generalização do modelo de lente térmica, supondo o surgimento de um gradiente de concentração dos grãos magnéticos devido ao gradiente de temperatura causado pelo feixe de laser sobre a amostra. A partir do uso da técnica de varredura Z foi possível o estudo do coeficiente Soret (S IND.T) em ferrofluidos iônicos, surfactados e citrados, em amostras com baixa concentração de grãos (fração volumétrica de Fe, ø, menor que 1%). Na generalização do modelo de lente térmica que efetuamos, consideramos que a variação no índice de refração da amostra, em uma experiência de varredura Z, depende da variação da intensidade do feixe laser (I), da variação da temperatura (T) e da variação da concentração de grãos magnéticos (ø), onde C IND.N, C IND.T e C IND.S são seus respectivos parâmetros adimensionais no modelo. Uma vez que o tempo característico da termodifusão é da ordem de segundos, uma varredura Z com pulsos da ordem de 20ms é utilizada para a determinação de C IND.N. C IND.T é obtido independentemente por meio de métodos de óptica linear.Após a determinação de C IND.N e C IND.T, uma varredura Z com duração de pulso da ordem de 1 segundo é feita para determinar C IND.S e, posteriormente, o coeficiente Soret. A partir do comportamento da curva de evolução temporal da tranmitância com pulsos de 1 segundo pode-se determinar o sinal do coeficiente Soret. O sinal está relacionado com a tendência dos grãos de migrarem para a região mais fria (termofóbico, S IND.T>0) ou mais quente (termofílico, S IND.T<0) da amostra, dependendo de suas características físico-químicas. Mostramos que o módulo de S IND.T é proporcional a ø, em concordância com resultados obtidos para soluções mais concentradas (ø1) através da técnica de Espalhamento Rayleigh Forçado. Uma possível origem física para os comportamentos termofóbico e termofílico dos fluidos magnéticos poderia estar relacionada a mudanças na intensidade das forças que mantêm o equilíbrio coloidal, por ação da temperatura. / This work explores the thermodiffusion in magnetic colloids through the Z-Scan technique. The starting point is the generalization of the thermal lens model based on the assumption that the concentration gradient of the magnetic grains emerges due to the temperature gradient caused by the laser beam on the sample. By using the Z-Scan technique it was possible to study the Soret coefficient (ST) for ionic, surfacted and citrated ferrofluids in samples with low concentration of grains (Fe volumetric percentage, ø, less than 1%). In this thermal lens model generalization, we have considered that the refraction índex variation in a Z-Scan experiment depends on the laser beam intensity (I), the temperature variation (T) and the variation of the magnetic grains concentration (ø), where CN, CT and CS are their respective dimensionless parameters in the model. As characteristic time of thermodiffusion is of the order of seconds, a Z-Scan with pulses around 20 ms is used in order to determine CN. CT is obtained independently by using lenear optics methods. After the determination of CN and CT, a Z-Scan with pulses around 1 second is made in order to determine CS and, Consequently, the Soret coefficient. Through the behavior of the time dependent transmittance with 1-second pulses we were able to determine the sign of the Soret coefficient. The sign is related to the tendency of the grains to migrate to the colder region (thermophobic, ST>0) or to the warmer region (thermophilic, ST<0) of the sample, depending on its physical-chemical characteristics. We have showed that the ST module is proportional to ø, in agreement with the results for higher concentration solutions (ø1%) obtained through Forced Rayleigh Scattering. A possible physical originfor the thermophobic and thermophilic behavior of magnetic fluids could berelated to changes in the intensity of the forces that keep the colloidal balance, by means of temperature.

Na presente dissertação investigou-se sistemas colestéricos liotrópicos induzidos e intrínsecos. Colestéricos induzidos por adição de pequenas quantidades de colesterol, em liomesofases à base de Laurato de Potássio (LK), foram estudados por RMN de 2H e microscopia óptica sob luz polarizada. A comparação dos perfis de ordem obtidos para liomesofases colestéricas induzidas, através de RMN de 2H de cadeias perdeuteradas de LK, com aqueles correspondentes a mesofases normais, mostrou a ausência de grandes distorções micelares, o que indica que o principal responsável pela colestericidade do sistema deve ser uma distribuição assimétrica de carga na superficie micelar. As amostras colestéricas induzidas tipo II apresentaram o padrão \"chevron\" esperado ao microscópio polarizado, após serem submetidas à ação de H<sub<o paralelo à superfície do capilar de paredes paralelas. Quando a estas liomesofases aplicou-se um campo Ho normal ao plano da cela, observou-se uma textura focal-cônica. Os padrões obtidos ao microscópio polarizado para as liomesofases colestéricas induzidas tipo I, submetidas à campo magnético, mostraram a existência de dois tempos de orientação. O primeiro devido ao alinhamento da hélice perpendicular a Ho e o segundo devido ao desenrolamento da superestrutura helicoidal. As liomesofases colestéricas intrínsecas se basearam no anfifílico N-Lauroil-Serinato de Potássio (NLSK) na forma levo. Este foi obtido pela acilação do amino-ácido correspondente, havendo retenção de configuração pelo produto. Todas mesofases obtidas foram caracterizadas por RMN de 2H da HDO e Microscopia Óptica. Estudos de microscopia sob luz polarizada, desenvolvidos em fases colestéricas intrínsecas do tipo I, expostas a campos magnéticos, corroboraram a hipótese de dois tempos de orientação estabelecido para as fases colestéricas induzidas, caracterizando definitivamente o sistema como colestérico. A caracterização de liomesofases colestéricas intrínsecas do tipo II foi feita por técnicas de RMN de 2H. Estas mesofases, após orientadas em campo magnético, apresentaram uma nova textura colestérica ao microscópico polarizado, diferente do padrão chevron esperado. Esta nova textura pode ser explicada pelas teorias de disclinações desenvolvidas para cristais líquidos termotrópicos, em especial para os colestéricos. Adicionalmente, verificou-se que a forma racêmica do anfifílico (d,1-NLSK) conduziu a mesofases \"nemáticas\". / Induced and intrinsic cholesteric lyotropic mesophases were studied in the present dissertation. Induced cholesteric lyomesophases based on potassium laurate (KL) system, with small amounts of cholesterol added, were studied by deuterium NMR and by polarizing microscopy. Order profiles obtained from deuterium NMR of KL perdeuterated chains in both induced cholesteric and normal mesophases were compared. The similarity of the profiles indicates that great distortions in micelles should not be present and an interaction between micelles with assyrnetric distribution of charges in their surfaces be responsible for the cholestericity of the system. The induced cholesteric type II samples presented the usual \"chevron\" pattern, when observed at the polarizing microscope after exposition to a magnetic field parallel to the capillar cell surface. When a field normal to the cell was applied a focal-conic texture was observed. For type I induced cholesteric mesophases, the patterns observed at the polarizing microscope indicated the existence of two orientational times in the magnetic field. The first is due to the alignment of the helix perpendicularly to the field and the second to the untwisting of the helicoidal structure. The intrinsic cholesteric lyotropic mesophases were based on the amphiphile potassium N-lauroyl-serinate (KNLS) in the resolved levo form. This compound was synthesized by acilation of the correspondent resolved amino-acid, with retention of configuration. All of obtained mesophases were characterized by deuterium NMR of the HDO and by optical microscopy under polarized light. The study of the type I intrinsic cholesteric mesophase at the polarizing microscope corroborated the hypothesis of two orientational times in magnetic field above mentioned for the induced cholesteric phases, and characterized the cholestericity of this system. Type II intrinsic cholesteric lyomesophase was characterized by deuterium NMR. Oriented samples of this mesophase when observed at the polarizing microscope showed a new cholesteric texture, different from the usual chevron pattern. The new texture was explained by the use of the theory of disclinations developed for thermotropic liquid crystals, specially for cholesteric type. Additionaly, it was verified that the racemic form of the amphiphile (d,l-KNLS) yielded \"nematic\" mesophases.

Cette thèse porte sur l’effet Lehmann, un effet hors d’équilibre couplant un gradient de température avec la rotation de la texture de gouttes cholestériques ou nématiques coexistant avec la phase isotrope. Nous avons d’abord caractérisé les couplages thermomécaniques de Leslie, Akopyan et Zel’dovich en mesurant en phase cholestérique la vitesse de rotation des molécules dans deux configurations invariantes par translation avec des orientations différentes.Nous avons ensuite caractérisé la texture des gouttes observées dans l’expérience de Lehmann en nous basant sur des observations optiques et des simulations numériques. Plus important, nous avons montré pour la première fois qu’il est possible d’observer l’effet Lehmann dans des gouttes nématiques achirales, à condition que la texture interne soit torsadée. Nous avons aussi utilisé un montage de photoblanchiment afin de montrer qu’il n’y a pas d’écoulements visibles au voisinage des gouttes. Ceci montre que la rotation observée est due à une rotation locale des molécules – pas à une rotation solide des gouttes.Enfin, nous avons proposé un modèle théorique « à la Leslie » de l’effet Lehmann incluant les termes de couplage thermomécanique d’Akopyan et Zel’dovich. En appliquant ce modèle généralisé aux textures calculées numériquement, nous avons ajusté les vitesses de rotation mesurées expérimentalement et avons trouvé des valeurs pour les constantes de couplage thermomécanique bien plus grandes que celles mesurées en dessous de la transition cholestérique/isotrope. Cela montre que ce modèle est faux et que le paradigme de Leslie doit être définitivement abandonné. / This thesis is focused on the Lehmann effect, an out-of-equilibrium effect which couples a temperature gradient with the rotation of the internal texture of liquid crystal droplets in coexistence with the isotropic phase. First, we characterized the thermomechanical couplings of Leslie, Akopyan and Zel'dovich by measuring the rotation velocity of the molecules in two translationally invariant configurations withdifferent orientations, below the cholesteric/isotropic transition.Then, we characterized the texture of the droplets observed in the Lehmann experiment, both usingoptical observations and numerical simulations. More important, we showed for the first time that it is possible to observe the Lehmann effect in achiral nematic droplets, providing that the internal texture ischiral. We also used a photobleaching experiment to show that there is no visible flow in the vicinity ofthe droplet, which implies that the texture rotation is due to a local rotation of the molecules -- not to asolid rotation of the droplet. Finally, we proposed a theoretical model of the Lehmann effect based on the thermomechanical coupling of Leslie, Akopyan and Zel’dovich. By applying this model to the numerically computed textures, we fitted the measured rotation velocities and found values for the thermomechanical coupling constants much bigger than those measured below the cholesteric/isotropic transition. This shows that this model is incorrect and that the Leslie paradigm must be abandoned.

L’émulsifiant est une matière première largement utilisée dans les systèmes formulés de type émulsion. Grâce à sa structure chimique amphiphile, cette molécule diminue la tension interfaciale entre les deux liquides non miscibles qui composent une émulsion, ce qui augmente la stabilité du système dans le temps. En plus, à cause de sa structure, il peut former des cristaux liquides dans les systèmes formulés, ce qui impacte considérablement les propriétés physico-chimiques et applicatives des émulsions. Ce projet vise l’étude d’un tensioactif mixte naturel alkyl polyglucoside/alcool gras. Les propriétés de celui-ci ont été d’abord investiguées via la construction du diagramme de phase. Ainsi, il a été démontré que la variation de la concentration et du ratio alkyl polyglucoside/alcool gras dans l’émulsifiant mixte favorise le passage de solutions micellaires vers les cristaux liquides de types lamellaire ou hexagonale dans les systèmes étudiés. La maitrise des propriétés des systèmes binaires a permis, par la suite, de comprendre les métamorphoses subies par le système lorsqu’une phase grasse est incorporée. La structure chimique des émollients (présence des hétéroatomes) affecte l’organisation des cristaux liquides formés par les molécules amphiphiles. Par conséquent, en fonction de l’émollient utilisé, les propriétés microscopiques et macroscopiques des systèmes changent (taille et distribution des gouttelettes, profil rhéologique, thermique et de texture). Finalement, les propriétés applicatives des émulsions sont aussi affectées, comme cela a été montré via l’analyse sensorielle et les mesures biométriques in-vivo. / Emulsifier is a raw material widely used in formulated systems of emulsion type. Due to its amphiphilic chemical structure, this molecule decreases the interfacial tension between two immiscible liquids that form an emulsion, which increases the stability of the system over time. In addition, it can form liquid crystals in the formulated systems because of its structure and this has a considerable impact on the physicochemical and applicative properties of emulsions. The objective of this project is to study a natural alkyl polyglucoside/fatty alcohol mixed surfactant. Its properties were first investigated via the construction of the phase diagram. Thus, it has been shown that the variation of the concentration and of the ratio of alkyl polyglucoside/fatty alcohol in the mixed emulsifier favors the transition between the micellar solutions towards liquid crystals of lamellar or hexagonal type in the studied systems. Next, the mastering of the binary systems properties allowed understanding the metamorphoses undergone by the system when an oil phase is incorporated into it. The chemical structure of emollients (presence of heteroatoms) affects the organization of liquid crystals formed by the amphiphilic molecules. Therefore, depending on the emollient type, the microscopic and macroscopic properties of the systems change (droplet sizes and distribution, rheological, thermal and texture profile). Finally, the applicative properties of emulsions are also affected, as shown by sensory analysis and in-vivo biometric measurements.

Nous présentons des résultats expérimentaux et théoriques sur les cristaux liquides en géométrie courbe. Nous étudions des coques de cristal liquide cholestérique dont la géométrie sphérique impose la présence de défauts topologiques. Ceci en fait un terrain de jeu idéal pour étudier la nature de ces singularités et leurs interactions. Nous observons un total de cinq configurations de défauts différentes, un atout remarquable dans le contexte d’auto-assemblage dans lequel ce projet s’inscrit. Les efforts combinés d’expériences et de simulations numériques nous permettent de décrire avec précision la structure des défauts. La complexité qui caractérise ces nouvelles structures est inhérente à la nature cholestérique de ces mésophases frustrées. Nous montrons qu’il est possible d’induire des transitions entre les différentes configurations, et examinons la dynamique qui y est associée. Nous établissons un modèle théorique qui rend compte de la position des défauts dans les différentes configurations. Nous discutons de l’équilibre subtil entre les interactions élastiques répulsives et le gradient d’épaisseur attractif qui résulte de la nature non-concentrique des coques. En outre, la confrontation du modèle aux expériences nous permet d’estimer les énergies associées aux nouvelles structures de défauts observées. Enfin, nous abordons les géométries toroïdales, et montrons que des transformations de formes peuvent nous permettre d’étudier la genèse et l’annihilation de défauts topologiques. / We present experimental and theoretical results on liquid crystals confined to curved geometries. We study cholesteric liquid crystal shells, the geometry of which imposes the presence of topological defects. This system constitutes an ideal playground to study the nature of these singularities and their interactions. We report a total of five different defect configurations, a remarkable feature in the context of self-assembly in which this work is set. Combining experiments and numerical simulations, we are able to accurately describe the inner structure of all observed defects. The complexity of these new structures is related to the cholesteric nature of the liquid crystal. We show that it is possible to induce transitions between the different configurations, and investigate the associated dynamics. We establish a theoretical model that successfully predicts the equilibrium defect positions in all configurations, and discuss the subtle balance between repulsive elastic interactions and attractive thickness gradients, arising from the eccentricity of the shells. Confronting the model to the experimental data, we are able to estimate the energies of nontrivial defect structures. Finally, we investigate toroidal geometries, and show how shape transformations can be interesting to study the genesis and annihilation of topological defects.

Cette thèse se concentre sur les cristaux liquides (CLs), leurs propriétés chirales et leurs interactions avec des surfaces et des nanostructures. Des cellules nématiques torsadées ont été remplies d'un CL dopé avec la molécule chirale CB15, qui compense la torsion imposée. Les résultats indiquent qu'un effet électroclinique (ECE) existe près de la surface en présence de molécules déracémisables conformes. Des mesures ECE ont aussi été réalisées afin de déterminer la réponse en ECE dans un CL dopé avec des organosilicates mésoporeux périodiques (OMP) chiraux. Les données montrent que la source principale du signal émane de l'extérieur des OMP plutôt que de leurs c¿urs. Un ECE est mesuré pour des CLs chiraux au-delà de leur température de transition de phase nématique/isotrope de bulk, et est observable dans les couches paranématiques induites par les substrats causant un alignement planaire. Nous avons dopé des stries huileuses de smectique A avec du CB15. Lors de ce dopage chiral, l'orientation des stries tourne d'un angle dépendant de la température, augmentant avec la concentration et maximum juste en-dessous de la température de transition smectique A/nématique. Ceci est expliqué comme une manifestation de l'ECE de surface sous la structure de strie huileuse permettant une rotation de cette dernière. Enfin, une structure innovante dans un système hybride aligné a été observée en-dessous de la température de transition Smectique A/Smectique C. Cette structure correspond à des stries périodiques sombres et claires perpendiculaires aux stries huileuses, éliminant les traces optiques des stries huileuses. Une origine possible de ces stries est présentée. / This dissertation focuses on liquid crystals (LCs), specifically their chiral properties and interactions with surfaces and nanostructures. Nematic twist cells were filled with a LC doped with the chiral molecule CB15, which compensates for the imposed twist. Using the electroclinic effect (ECE), results indicate that an ECE always exists near the surface in twist cells containing conformationally deracemizable molecules. ECE measurements were also performed to determine the source of the ECE response in a LC doped with chiral periodic mesoporous organosilica (PMO). The data show that the main source of the signal emanates from outside the PMO, and not inside the PMO pores. An ECE also is reported for chiral LCs above their bulk chiral isotropic–nematic phase transition, and is observable in the paranematic layers induced by the planar-aligning substrates. Optical microscopy measurements were performed on smectic-A oily streaks doped with CB15. When chirally doped, the stripe orientation rotated by a temperature dependent angle: This angle increased with concentration, was largest just below the nematic – smectic-A transition, and stabilized to near zero within ~1C below TNA. This is explained as a manifestation of a surface ECE. Finally, a novel structure in a hybrid aligned system was observed below the Smectic-A – Smectic-C transition. The structure appeared as periodic dark and light streaks running perpendicular to the oily streaks, and formed by extending discretely from one oily streak to the next, eliminating optical evidence of the oily streaks. At lower temperatures the streaks undulated in a 2D chiral pattern. A possible origin of these streaks is presented.

Cette thèse porte sur la dynamique des différentes mésophases liquides cristallines et des transitions de phase d'un modèle colloïdal de particules en forme de bâtonnent: les virus fd. L'étude de l'auto-organisation des cristaux liquides colloïdaux traite des phénomènes les plus simples de formation de systèmes structurés. Dans un système où les particules anisotropes ont des interactions de noyau rigide, l'auto-organisation est purement entropique en raison de la maximisation du volume libre du système. Ainsi, il y a une modification de la dynamique qui peut être mesurée, et fournisse des informations sur le volume libre disponible et la structure de la mésophase.La dynamique des bâtonnets fd peut être mesurée avec précision à l'aide de techniques de microscopie à fluorescence. Nous quantifions la dynamique autour des transitions de phase et l'effet de la flexibilité et de la longueur en tant que mécanisme pour relâcher la contrainte de leurs voisins. En outre, dans une structure lamellaire guest-host, nous avons prouvé la pérmeation favorisée des bâtonnets longs guest à travers les couches de la matrice Smectique hast formée de plus petites particules. Dans ces conditions, la super-diffusion de la particule invitée est également observée lorsqu'elle se diffuse dans une limite de grain. Il s'agit d'un pas en avant pour comprendre la dynamique des systèmes structurés colloïdaux et aussi dans le développement des nouveaux matériaux basés sur des diffuseurs rapides avec des applications potentielles dans la biologie médicale. Les résultats expérimentaux sont très prometteurs et stimulantes. / This thesis treats the dynamics of the different liquid crystalline mesophases and phase transitions of a model colloïdal of rod-like particles: the fd viruses. The study of the self-organization of colloïdal liquid crystals treats the simplest phenomena of forming structured systems. In a system where anisotropie particles have hard core interactions, the self-organization is purely entropy driven due to the maximization of the free volume of the system. Thus, there is a change on the dynamics at single particle level that can be measured, providing information on the available free volume and the structure of the mesophase.The fd rods are a versatile colloïdal system and their self-dynamics can be accurately measured using fluorescence microscopy techniques. We quantify the relaxation of the dynamics around the phase transitions and the effect of flexibility and length as a mechanism to release the constraint of their neighbors. Moreover, in a guest-host lamellar structure we demonstrate that a big guest particle is faster than the small host, if the guest particle is not commensurate in the host energy landscape. In these conditions, also the super-diffusion of the guest particle is observed when it diffuses into a grain boundary. This is a step forward to understand the dynamics of colloïdal structured systems and also in the development the new materials based on fast diffusers with potential applications in drug delivery. The extensive experimental results are completed by a whole analysis and interpretation, being very promising and challenging.