Temperature dependence of refractive indices near uniaxial–biaxial nematic phase transition

An optical microscopy study of the biaxial ‐ calamitic nematic lyotropic phase transition

“…As one see from figure 1, the temperature range of the biaxial nematic phase is around 2.6 °C. This result is in accordance with the phase diagrams reported in the literature [1,4] and it is also consistent with optical birefringence measurements performed on the similar lyotropic mixtures [7,8,17,18]. Figure 2 shows the nematic textures obtained upon heating from the reentrant isotropic phase near the I RE -N D --N B -N C + -I phase transitions.…”

“…This result also has been confirmed via digital image processing method of the nematic textures [16,22]. ) Note that, the N B -N C + transition point is identified as well as the domain of the N B phase by optical microscopy observations, in accordance with the optical birefringence data [3,7,8] presented in figure 1. Aligned nematic sample in a planar glass cells 0.2 mm of light path between crossed polarizers, at 45° of the light polarizing direction, is observed through optical microscope connected to the CCD camera.…”

“…In this sense, they are connected by l = m × n [7,15,19]. The sketch at upper right corner shows the sample holder and the laboratory frame axes adopted [3,8]. Furthermore, we believe that the alignment of the N B phase along the 3 axis is induced by the boundary conditions parallel to the director l. This experimental procedure employed here is justified by Yu and Saupe [1] where the principal diamagnetic susceptibilities satisfies the relations χ 1 > χ 2 > χ 3 .…”

“…Biaxial nematic lyotropic phase has been the subject of extensive studies by several authors [1][2][3][4][5][6][7][8][9][10] from the onset of the 1980's. The uniaxial nematic phases have been shown to be prolate (calamitic -N C + ) and oblate (discotic -N D -) micellar aggregates dispersed in water [11].…”

“…From order parameter point of view, the N B phase is characterized by a second-rank tensor and two macroscopic order parameters are required to describe the amount of order of this biaxial phase [15]. optical birefringence [3] and refractive index measurements [8] were utilized to establish the existence of a biaxial nematic phase observed between the discotic and calamitic nematic phases for the potassium laurate, 1-decanol and D 2 O lyotropic mixtures [1,3]. optical birefringence [3] and refractive index measurements [8] were utilized to establish the existence of a biaxial nematic phase observed between the discotic and calamitic nematic phases for the potassium laurate, 1-decanol and D 2 O lyotropic mixtures [1,3].…”