Yazar "Figueiredo Neto, A. M." seçeneğine göre listele

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    Lyotropic mixture made of potassium laurate/1-undecanol/K2SO4/water presenting high birefringences and large biaxial nematic phase domain: a laser conoscopy study
    (Springer, 2012) Akpınar, Erol; Reis, D.; Figueiredo Neto, A. M.
    The lyotropic liquid crystalline quaternary mixture made of potassium laurate (KL), potassium sulphate, 1-undecanol and water was investigated by experimental optical methods (optical microscopy and laser conoscopy). In a particular temperature and relative concentrations range, the three nematic phases (two uniaxial and one biaxial) were identified. The biaxial domain in the temperature/KL concentration surface is larger when compared to other lyotropic mixtures. Moreover, this new mixture gives nematic phases with higher birefringence than similar systems. The behavior of the symmetric tensor order parameter invariants sigma(3) and sigma(2) calculated from the measured optical birefringences supports that the uniaxial-to-biaxial transitions are of second order, described by a mean-field theory.
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    Shear viscosity and rheology of ternary and quaternary lyotropic liquid crystals in discotic and calamitic nematic phases
    (Springer, 2015) Risi, Celso L. S.; Figueiredo Neto, A. M.; Fernandes, Paulo R. G.; Sampaio, Anderson R.; Akpınar, Erol
    We report rheological studies of micellar nematic phases of potassium laurate/ decanol/ water mixtures with and without addition of salt (K2SO4) at a fixed temperature of 20 C-a similar to. Using a cone-plate rheometer, we verified that the ternary mixture (i.e., without salt) exhibiting the discotic (N (D) ) nematic phase showed only a slight pseudo-plastic (or shear-thinning) behavior. In contrast, a pronounced pseudo-plastic effect was observed in the N (D) phase of the quaternary samples. On the other hand, all the N (C) samples (salted or unsalted) showed a quasi-Newtonian behavior. In addition, we have made viscosity measurements using a (variation of a) rotating Couette flow viscometer. Finally, we also performed density measurements, using a density cell attached to the viscometer. Our results showed a contrasting behavior between the two types of nematic phases in two aspects. Firstly, regarding the ternary samples only, the apparent viscosity eta at moderate shear rates was lower in the N (C) than in the N (D) phase. The second aspect, regarding both ternary and quaternary samples, is that eta in the N (D) phase dramatically increased as the salt concentration increased, while in the N (C) phase eta was quite insensitive to the addition of salt. In order to analyse the results, we have used two different theoretical frameworks, namely the nematodynamics in the first aspect and the classical theory of electrolyte solutions in the second one.
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    A survey on the phase diagram of the system sodium dodecyl sulphate/1-decanol/D2O concerning an eventual coexistence of uniaxial nematic phases
    (Taylor & Francis Ltd, 2018) Oliveira-Filho, Arnaldo G.; Akpınar, Erol; Reis, Dennys; Figueiredo Neto, A. M.
    We examine an eventual coexistence of discotic () and calamitic () nematic uniaxial phases in the sodium dodecyl sulphate (SDS)/1-decanol (DeOH)/D2O mixture phase diagram. This coexistence is claimed in some previous works. Four different molar ratios [D2O]/[SDS] were investigated, where the square brackets represents the molar concentration of the compound, and 33, with temperatures ranging from 20 degrees C up to about 52 degrees C. The experimental techniques employed for phase identification were laser conoscopy, polarised optical microscopy and small-angle X-ray scattering. In the investigated surface of the phase diagram, no such nematic phases coexistence region was found. The discotic () and biaxial () nematic phases were identified in some regions of the phase diagram. Coexistence domains of the nematic ND/lamellar, and lamellar/micellar isotropic were identified. Our results agree with the theoretical predictions and symmetry arguments that consider the nematic/nematic phase transitions of second order, well described by a mean-field Landau-type model, preventing the nematic/nematic coexistence domains. Moreover, these results are consistent with all the experimental results we encountered in more than 30 different lyotropic mixtures investigated by using complementary experimental techniques to univocally identify the mesophases and phase-coexistence regions. [Graphics] .