W.S. Braga

Biaxial phase and coexistence of the two uniaxial nematic phases in the system sodium dodecyl sulphate–decanol–D2O

The system sodium dodecyl sulphate (SDS)/decanol (DeOH)/water presents, with temperature increase, an unusual phase transition between a stable biaxial phase and a coexistence region of the two uniaxial nematic phases (discotic ND + cylindrical NC). This has been detected previously by several methods for a sample with water/SDS molar ratio Mw = 36. Here, this system is investigated changing the ratio Mw to 32, where previously the coexistence region was reported after a discotic ND phase, without the biaxial phase. We report now the existence of a biaxial NB in the temperature range of ~2°C, defining for both values of Mw and the phase sequence as NB – (ND + NC) – NC. The change in temperature is followed by conoscopic and orthoscopic optical techniques and also analysed through the curve obtained by the technique of digital image processing of the textures, which reveals a continuous transition ND – NB – (ND + NC). While the biaxial phase is stable for at least 10 hours, in a reproducible way, the coexistence region evolves with time, and drops of the discotic phase grow immersed in a NC matrix. Results are explained in terms of recent theories dealing with stabilisation of mixtures of cylinders and discs.

An image processing study of a reentrant discotic cholesteric – biaxial cholesteric phase transition

ABSTRACT In this work, we study and characterize the cholesteric sequence of phases (ChDr − ChB − ChD), where the first ChDr is the reentrant cholesteric discotic phase, ChB is the cholesteric biaxial phase and the second ChD is the cholesteric discotic phase. This sequence of phases is studied through polarized light microscopy and image processing technique, where, for the first time, the domains and borders of these transitions are established and characterized. They are also investigated and optically characterized throughout their textures.

Uniaxial and Biaxial Nematic Phases in Sodium Dodecyl Sulphate - Decanol - D2O Mixtures. An Optical Conoscopy Study

The system SDS/decanol/water is investigated by polarized optical microscopy. A reported biaxial island is here confirmed by conoscopy, in the temperature interval 20°C to 26°C. Coexistence regions of the two uniaxial nematic phases (discotic ND + cylindrical NC) are also confirmed, in different temperature intervals. While the biaxial phase is stable for at least 10 hours, in a reproducible way, along the same time the coexistence region evolves with time, and drops of the discotic phase grow immersed in a NC matrix. Results are explained in terms of recent theories dealing with stabilization of mixtures of cylinders and discs.

Refractive index measurements in a reentrant discotic nematic–biaxial nematic phase transition

ABSTRACT The optical characterization of uniaxial (reentrant discotic – and discotic – ) and biaxial nematic ( phases requires the measurements of two and three refractive indices, respectively. These optical parameters were determined near the reentrant isotropic -isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and . Biaxial positive ( nematic phase is optically characterized between two uniaxial discotic nematic phases from refractive index data and confirmed via conoscopic images. The micellar configuration of uniaxial and biaxial nematic phases is discussed as a consequence of our experimental data.

Conoscopic image of a biaxial nematic phase in a sodium decyl sulfate – decanol – D2O mixture

ABSTRACT The nematic phases of a lyotropic system NadS/ decanol/ heavy water are investigated using optical conoscopy and image processing. The phase diagram obtained from these lyotropic materials predicts the occurrence of a direct phase transition, which does not present the biaxial nematic phase, between the discotic (ND) and calamitic (NC) nematic phases. A biaxial nematic (NB) phase is optically characterized and confirmed through conoscopic image, inside the biaxial range, between the two uniaxial nematic phases. Also, their respective transition points are determined by means of image processing. The NB phase observed here is discussed as part of the nature of the micellar configuration of lyotropic materials which exhibit uniaxial nematic phases.