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Doctoral Thesis
DOI
https://doi.org/10.11606/T.43.2018.tde-04042018-173836
Document
Author
Full name
Eduardo dos Santos Nascimento
E-mail
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 2018
Supervisor
Committee
Vieira, Andre de Pinho (President)
Evangelista, Luiz Roberto
Oliveira, Cristiano Luis Pinto de
Oliveira, Ítalo Marcos Nunes de
Oliveira, Mario Jose de
Title in Portuguese
Modelos microscópicos para cristais líquidos nemáticos
Keywords in Portuguese
cristais líquidos
modelo de Maier-Saupe
modelo de Onsager
modelos microscópicos
transições nemáticas
Abstract in Portuguese
Neste trabalho estudamos, no contexto de campo médio, modelos microscópicos que possam descrever o comportamento termodinâmico das fases nemáticas em sistemas líquido-cristalinos. Considerando apenas interações atrativas, investigamos modelos de interações quadrupolares para objetos intrinsecamente biaxiais. Esses modelos apresentam mesofases nemáticas uniaxiais e biaxiais, pontos triplos e multicríticos (tricríticos, pontos de Landau, etc.). Ainda no contexto de forças atrativas, introduzimos um modelo de mistura binária de objetos intrinsecamente uniaxiais e objetos intrinsecamente biaxiais, numa formulação annealed. Essa mistura apresenta diagramas de fases bastante ricos, com topologias diversas, onde identificamos estruturas uniaxiais e biaxiais, fases reentrantes e uma grande variedade de pontos multicríticos (tricríticos, pontos críticos terminais, etc.). No caso de interações estéricas, estudamos uma teoria do funcional densidade para sistemas anisotrópicos densos, construída a partir de uma aproximação de van der Waals. Para esferoides prolatos, o modelo prevê um espaço de orientações com regiões não-acessíveis para as partículas. Além disso, o sistema apresenta uma região de coexistência entre as fases nemática e isotrópica.
Title in English
Microscopic models for nematic liquid crystals
Keywords in English
liquid crystals
Maier-Saupe theory
microscopic models
nematic transitions
Onsager theory
Abstract in English
We study, in a mean-field approximation, microscopic models which can lead to nematic liquid-crystalline phases. Considering attractive forces, we investigate models with quadrupolar interactions for intrinsically biaxial objects. These models present uniaxial and biaxial nematic mesophases, triple and multicritical points (tricritical point, Landau point, etc.). We also introduce a model for a binary mixture of intrinsically uniaxiail and biaxial objects, in an annealed treatment. The mixture exhibits phase diagrams with very rich topologies, where we find uniaxial and biaxial structures, reentrant phases and many different multicritical behaviors (tricritical point, critical endpoint, etc.). Moreover, assuming steric interactions, we investigate a density functional theory for hard anisotropic bodies at high densities, based on a van der Waals approximation. For hard spheroids, the model leads to an orientation space with forbidden regions for the particles. Also, the system phase separates in a nematic and an isotropic phases.
 
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Publishing Date
2018-04-26
 
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