Efeitos de campos externos na reorientação molecular de cristais líquidos nemáticos twist-bend

• Ano de defesa: 2016
• Autor(a) principal: Dalcol, Denise da Silva
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Estadual de Maringá; Brasil; Programa de Pós-Graduação em Física; UEM; Maringá, PR; Centro de Ciências Exatas
• Palavras-chave em Português: Nemático twist-bend; Modelo teórico; Tempos de relaxação; Fase nemática twist-bend; Passo da hélice; Cristais liquidos; Campos magnéticos; Brasil.; Twist-bend nematic phase; Magnetic fields; Theoretical model; Relaxation times; Helix pitch; Brazil.; Ciências Exatas e da Terra; Física
• Link de acesso: http://repositorio.uem.br:8080/jspui/handle/1/2680

This work presents a study of the effects of external magnetic fields (H ⃗) on the molecular structure of the twist-bend nematic phase (N_tb). In order to accomplish this, was used a recently proposed theoretical model to describe the said phase. The results for the theoretical model demonstrate the stability of the phase N_tb from an elastic point of view with a natural extension of the Frank elastic energy, where two vectors intervene as elements of symmetry [1]. The molecules in such a phase form an oblique helicoidal structure, in which the director n ⃗ rotates toward the axis t ⃗ keeping a constant angle θ, with a helix pitch in a nanometer scale. Therefore, in this work, the influence of external magnetic fields was analytically studied on a liquid crystal N_tb sample, using the elastic theory of continuum proposed by Barbero et al. [1], with positive magnetic susceptibility anisotropy. The results suggest that when H ⃗ is applied perpendicular to the helical axis, t ⃗, by increasing its intensity, the senoidal component of the director n ⃗ is distorted by increasing the helix pitch. As the intensity of H ⃗ increases, the helix unwinds forms a uniform nematic phase. Dynamic processes were investigated in the reorientation and relaxation of the director for the phase N_tb induced by a magnetic field H ⃗ parallel to t ⃗, assuming a case where θ=θ(z). In the situation of strong anchoring on the surfaces, an expression for the critical magnetic field was found analytically, and from this result, the relaxation times were obtained.