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Doctoral Thesis
DOI
10.11606/T.43.1998.tde-03122013-164807
Document
Author
Full name
Sandro Fontanini
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1998
Supervisor
Committee
Figueiredo Neto, Antonio Martins (President)
Becerra, Carlos Castilla
Evangelista, Luiz Roberto
Figueiredo, Wagner
Mesquita, Oscar Nassif de
Title in Portuguese
Propriedades Elásticas de um Cristal Líquido Liotrópico Nemático nas Vizinhanças da Interface Sólido-Cristal Líquido
Keywords in Portuguese
Cristais Líquidos
Matéria condensada
Abstract in Portuguese
Efetuamos o estudo da constante elástica efetiva de splay-bend K IND.13 por técnicas ópticas para um cristal líquido liotrópico. Neste experimento encontramos a razão entre K IND.13 e a constante elástica usual de Frank como sendo positiva e da ordem de 1. A análise teórica mostra que próximo à transição nemática-isotrópica o modelo termodinâmico para a temperatura de transição na superfície funciona razoavelmente bem. Na região de baixas temperaturas próxima à transição lamelarnemática são observados desvios importantes do comportamento teórico. No sentido interpretar os dados experimentais, propomos uma extensão fenomenológica simples do modelo termodinâmico que leva em conta as diferentes contribuições para a energia de superficie e a ordem lamelar residual próxima à superficie na fase nemática. Mostramos além disso que a constante clássica efetiva de splay-bend depende da concentração de ferrofluido na amostra. Analisamos também a influência do tratamento de superfície sobre a parte anisotrópica da tensão superficial de um cristal líquido liotrópico dopado com ferrofluido. Nosso resultado mostra que a superfície plana do vidro, independentemente da presença ou ausência de qualquer tratamento, estabiliza sobre si própria uma camada ou bicamada lamelar formada pelas moléculas anfifílicas do cristal líquido. Esta camada blinda o efeito dos filmes de Langmuir-Blodgett, ou o substrato puro sem nenhum tratamento, sobre o alinhamento de mesofases nemáticas liotrópicas.
Title in English
Elastic Properties of a Nematic Liquid Crystal in lyotropic Neighbourhoods Interface Solid-Liquid Crystal
Keywords in English
Condensed matter
Liquid crystals
Abstract in English
The effective splay-bend elastic constant K13 is studied by means of an optical technique for a lyotropic nematic liquid crystal. The ratio between K13 and the usual Frank elastic constant found in this experiment is positive and of the order of 1. The theoretical analysis shows that near the nematic-isotropic transition, the termodynamical model for the temperature surface transition works reasonably well. In the low temperature region near the lamellar-nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the termodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration. The influence of the surface treatment on the ansiotropic part of the surface tension of a discotic micellar lyotropic nematic crystal doped with ferrofluid analysed. Our results show that the flat glass surface, independently of the presence or of the absence of any treatment, stabilizes onto itself a lamellar layer - bilayer - formed by the amphiphilic molecules of the liquid crystal. This layer screens the effect of the Langmuir-Blodgett film - or of the pure substrate without any treatment - on the alignment of the lyotropic nematic mesofase.
 
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Publishing Date
2014-02-18
 
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  • ALEXE-IONESCU, A. L., et al. Surface fields and the splay-bend elastic constant [doi:10.1103/PhysRevE.53.R4299]. PHYSICAL REVIEW E (STATISTICAL PHYSICS, PLASMAS, FLUIDS AND RELATED INTERDISCIPLINARY TOPICS) [online], 1996, vol. 53, p. 4299-4301.
  • FONTANINI, S., et al. Measurements of the splay-bend elastic constant in lyotropic ferronematic liquid crystals: The influence of the bounding surfaces [doi:10.1063/1.473241]. Journal of Chemical Physics [online], 1997, vol. 106, p. 6187-6193.
  • FONTANINI, S., et al. Self-screening of langmuir-blodgett films by a discotic micellar lyotropic liquid crystal [doi:10.1080/026782998206597]. Liquid Crystals [online], 1998, vol. 24, nº 6, p. 793-798.
  • FONTANINI, S., BARBERO, G., and Neto, A.M.F. Determination of the effective splay-bend elastic constant of a lyotropic nematic liquid crystal [doi:10.1103/PhysRevE.53.2454]. Physical Review E. (Cessou em 2000. Cont. 1539-3755 Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics) [online], 1996, vol. 53, p. 2454-2460.
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