Biomedical subjects

A J Palangana

Publications and source records attributed to A J Palangana.

11 recordsLinked to original sources

PubMed2005

Bend and splay elastic constants at a reentrant isotropic-calamitic-nematic phase transition.

By measuring the ratio between the major and minor axes of closed elliptical loops created by a magnetic field, the ratio between the bend (K33) and splay (K11) elastic constants of a lyotropic mixture of potassium laurate, decanol, and water in the calamitic nematic phase is measured as a function of temperature. Since these systems present two nematic-isotropic phase transitions--the usual one, at high temperatures, and a reentrant one, at low temperatures--such measurements are used to compare the behavior of these elastic constants at the neighborhoods of these two distinct regions, which have in common the vanishing of the order parameter. The experimental data have revealed the existence of two elastic constant branches, characterizing a distinct low- and high-temperature behavior. To explain such asymmetry we support that, because of the lyotropic nature of the compound, the micellar variation, which is mainly responsible for the reentrant phase, plays a distinct role at these two phase transitions.

Journal Article↗

PubMed2005

Viscosity peaks at the cholesteric-isotropic phase transitions.

In this work, the effective viscosity of the cholesteryl myristate and cholesteryl nonanoate liquid crystals is studied as a function of temperature at the region of their cholesteric-to-isotropic phase transition, where blue phases are found. Using a change of scale it is shown that the viscosity peaks that characterize these phase transitions are shape invariant, which suggests that large-scale fluctuations on the two-point correlation function give an important contribution to the observed viscosity. The consequences of this fact are investigated and, from the experimental data, the critical exponents associated with the diverging two-point correlation function are calculated. The results found for both compounds are essentially the same, being also in good agreement with the known values of the corresponding critical exponents of nematic-isotropic phase transition.

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PubMed2003

Periodic structures in a biaxial nematic phase.

In this work experimental and theoretical results will be used to show that under convenient conditions; appropriated geometry, well chosen boundary conditions, and suitable external fields, the elastic and dynamic behavior of a sample of a biaxial nematic liquid crystal can become similar to the one observed on usual uniaxial nematic liquid crystals. It is shown that this result can be used to determine the values of some combinations of the rheologic parameters of the biaxial phase.

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PubMed2002

Refractive index measurements in a reentrant isotropic-calamitic nematic phase transition.

In this work the temperature dependence of the extraordinary/ordinary refractive indices and optical birefringence are determined near the reentrant isotropic (I(RE))-calamitic nematic (N(C)) and N(C)-isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol, and water. This reentrant isotropic phase is verified by our experimental data and discussed in terms of thermal variation of the micellar shape anisotropy. The results showed also, with basis in the Vuks's equation, that near the maximum optical birefringence in the range of N(C) phase the extraordinary coefficient of electronic polarizability (phi(parallel)) is equal to the ordinary coefficient (phi( perpendicular)).

Journal Article↗

PubMed2001

Temperature dependence of the coefficient of electronic polarizability in calamitic nematic liquid crystals.

In this report the temperature dependence of the coefficient of electronic polarizability (straight phi(i)) is determined by means of the thermal expansion coefficient (beta) and ordinary/extraordinary refractive indexes measurements near the calamitic nematic-isotropic phase transition in a lyotropic mixture of sodium decylsulphonate, decanol, and water. These parameters (straight phi(i) and beta) were related to the extraordinary and ordinary refractive indexes via the temperature derivative of the Vuks's equation. The results showed that near the nematic-isotropic phase transition, the measured value of straight phi(i) was found to be about two orders of magnitude smaller than that obtained for thermotropic, showing a remarkable difference in the molecular electronic polarizability strength between lyotropic and thermotropic liquid crystals.

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PubMed2001

Induction of orientational order in the isotropic phase of a nematic liquid crystal.

The orientational properties of an isotropic dense liquid composed by anisotropic molecules, such as a liquid crystal in an isotropic phase, is studied. Using a Langevin-like equation it will be shown that the rotational motion of each molecule can be divided in two elements describing two kinds of physical motion. The first describes the Brownian rotational motion and another the coherent rotation induced by the external fields. It will be shown that, even at the isotropic phase, an order parameter describing the mean degree of alignment of the molecules around a given point can be defined. This order parameter also separates the order coming from the coherent motion from the order generated by the anisotropy in the thermal fluctuations. At the end the proposed model is compared with an experiment and it is shown that the coherent motion is enough to explain the experimental results.

Journal Article↗

PubMed2001

Temperature dependence of the refractive index near the reentrant-isotropic--calamitic-nematic phase transition.

The laser-induced nonlinear optical response of a lyotropic liquid crystal system in the reentrant-isotropic and calamitic-nematic phases is investigated by the use of the thermal lens technique. The occurrence of an inversion in the temperature coefficient of the ordinary refractive index, dn(radially)/dT, near the reentrant-isotropic-calamitic-nematic phase transition, is discussed. This effect is attributed to the behavior of the electronic polarizability due to the change in micelle shape near the isotropic-nematic transition, and correlated with the results obtained near the nematic-isotropic transition, previously reported.

Journal Article↗

PubMed2001

Arising of magnetic walls in the vicinities of the Frèedericksz transition.

In this paper, the dynamical formation of twist-bend periodic walls at the neighborhoods of the Frèedericksz critical point of a nematic sample of liquid crystals is studied. The theory that describes the arising of these periodic structures affirms that the mode with the fastest initial growth will fix the observed properties of these patterns. But, just above the Frèedericksz threshold there is a region where this leading mode becomes null and, therefore, a homogeneous bending of the director may be detected. This prediction was not confirmed by the experiment, and walls with very well defined wavelength were found. We will show here that the fastest growing mode cannot be defined around the Frèedericksz threshold and, therefore, a new way to compute the observed periodicity must be formulated. We assume that the observed wall results from a sum of a continuum and nonsharp distribution of modes in which the null mode is at the center.

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PubMed2000

Inversion in the change of the refractive index and memory effect near the nematic-isotropic phase transition in a lyotropic liquid crystal.

This work demonstrates the occurrence of dn/dT inversion from negative to positive near the nematic-isotropic phase transition in a lyotropic liquid crystal. It is suggested that this effect can be attributed to a sudden increase of the electronic polarizability due to a change in the micelle shape near this phase transition. Formation of a long lasting lenslike element within the sample when it is irradiated at moderately high laser powers is also reported. This permanent lens is erasable by increasing the temperature above the nematic-isotropic transition temperature.

Crystallization↗

PubMed1999

Forbidden walls.

In this paper the presence of periodic twist-bend walls in the upper vicinity of the Frèedericksz critical point of a nematic calamitic sample is reported. According to the existing theory, the formation of these structures could not happen in this region of the magnetic field. The length of the periodicity of these walls as well as the time spent with their formation have been measured, and it was demonstrated that the coherent motion of the nematic material cannot drive their construction. We assume that the mechanism responsible for their appearance results from the magnification of some selected random fluctuations occurring at the neighborhood of the Frèedericksz critical point.

Journal Article↗

PubMed1999

Micellar shape anisotropy and elastic constants in discotic lyotropic liquid crystals.

The elastic constants of a discotic lyotropic nematic liquid crystal are calculated by means of a pseudomolecular approach as functions of the micellar shape anisotropy. By assuming that the temperature dependence of the ratio of the elastic constants comes from the temperature dependence of the micellar shape anisotropy, the theoretical predictions are connected with experimental measurements for the ratio K(33)/K(11). This procedure permits one to determine, in a phenomenological way, the temperature dependence for the ratio of elastic constants and for the micellar shape anisotropy near the nematic-isotropic transition in agreement with the experimental data.

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