Biomedical subjects

G Barbero

Publications and source records attributed to G Barbero.

At least 37 records · Page 2Linked to original sources

PubMed2002

Periodic deformations in nematic liquid crystals.

We reconsider the possibility of periodic deformations in nematic liquid crystal samples, and present a simple method to analyze their stability near the threshold. Our method consists in finding the matrix characterizing the total energy in terms of the integration constants of the linearized solutions of the variational problem. In the undeformed state all the integration constants are identically zero. Hence the analysis of the stability of the undeformed state reduces to the analysis of the sign of the determinants of the principal minors of the matrix of the quadratic form representing the total energy of the nematic sample. We discuss the role of the saddle-splay elastic constant and of the anchoring energy strength in the stability of the modulated structure. The role of the thickness of the sample, as well as of the polar and azimuthal anchoring energies, in the phenomenon is also considered.

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PubMed2002

Intermediate periodic "saddle-splay" nematic phase in the vicinity of a nematic-smectic-A transition.

We consider possible spontaneous modulations of the nematic director induced by the elastic saddle-splay K24 term when the value of the elastic constant K24 does not satisfy the Ericksen stability condition for the homogeneous ground state. According to the standard formula expressing K24 in terms of the twist elastic constant K22, this can be expected close to the nematic-smectic-A transition where K22 becomes very large. It is predicted that in a planar nematic layer (or, more generally, if the surface director alignment is sufficiently close to a planar one), a modulated phase with observable long wavelength period can occur in samples considerably thicker than the anchoring extrapolation length. The modulated nematic phase is expected to persist into the smectic phase so that its temperature of the transition to smectic phase has to be lower than that for the homogeneous nematic liquid crystal. Low amplitude short wavelength modulations are predicted for any thickness if the surface director is sufficiently far from a pure homeotropic alignment. At the expense of this mode the temperature of a nematic-smectic-A transition in a planar cell with isotropic surfaces has to be lower than that for a homeotropic cell even if the periodic structure is not accessible for the direct observation.

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PubMed2002

Photomanipulation of the anchoring strength of a photochromic nematic liquid crystal.

The behavior of the Fréedericksz threshold voltage of a planarly oriented sample filled with liquid crystalline azobenzene material possessing a nematic phase is investigated under UV illumination. It is found that the anchoring strength strongly decreases with the UV-exposure time due to the photoisomerization process that takes place in the sample. The changes in the anchoring strength are due to a selective adsorption of cis isomers at the surface. We show that the experimental data can be explained by a model that takes into account the changes in the molecular shape and in the net molecular dipole moment due to the cis-trans isomerization. We obtain a good agreement between the predictions of the model and the experimental data.

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PubMed2002

External electric-field effect on nematic anchoring energy.

The influence of an external field on the effective anchoring energy of a nematic liquid crystal in contact with a substrate is theoretically analyzed. Our analysis is performed on the hypothesis that the electrodes are perfectly blocking and that there is no selective ion adsorption. The proposed theory predicts an effective anchoring energy dependent on the applied dc voltage. According to the sign of the dielectric anisotropy and of the flexoelectric coefficient the dependence of the anchoring energy strength with the bias can be monotonic or not. For large bias voltage the effective anchoring energy strength tends to a constant value. Our theory is in qualitative agreement with published data investigating the influence of the bias on the saturation voltage.

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PubMed2002

Local self-consistent approach to the phase transition at the nematic liquid-crystal-wall interface.

The nematic-isotropic transition for a semi-infinite sample is analyzed by means of the Maier-Saupe theory. The effect of a delocalized surface field acting on the nematic molecules, and of the incomplete interaction between the nematic molecules, are taken into account in the van der Waals approximation. We show that the existence of a surface transition is governed by the strength of the surface potential. The spatial profile of the order parameter and the degree of extra order near the walls are determined for different temperatures. The surface tension of nematic origin is calculated near the bulk transition temperature. It is estimated to be one order of magnitude smaller than the total surface tension experimentally detected with standard techniques.

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PubMed2002

Ions and nematic surface energy: beyond the exponential approximation for the electric field of ionic origin.

We present a general model to describe the influence of the ionic adsorption on the anisotropic part of the surface energy of a nematic liquid crystal in contact with a substrate. We show that in the limit of small adsorption energy, the exponential approximation for the electric field of ionic origin works well. In this limit, the dielectric and flexoelectric contributions to the surface energy are quadratic and linear on the density of adsorbed ions, respectively. In the opposite limit of large adsorption energy, the exponential approximation for the electric field does not work, and the two contributions to the surface energy are both found to depend linearly on the surface density of adsorbed charges. Approximated formulas reported in literature are derived from our general equations as particular cases, and their limits discussed. An expression for the surface polarization in nematic liquid crystal due to the ionic adsorption is also deduced. Our analysis is performed in the framework of the Poisson-Boltzmann theory, where dimensionless ions are treated within a mean field approach. Possible extensions of our model are indicated.

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PubMed2001

Temperature-induced surface transition in nematic liquid crystals oriented by evaporated SiOx.

Temperature induced surface transitions observed in nematic liquid crystals oriented by obliquely evaporated SiOx are analyzed. It is shown that the experimental data can be interpreted with a theory for the thermal renormalization of the anisotropic part of the surface energy based on the mean field approximation. By assuming that the nematic scalar order parameter is given by the Maier-Saupe theory, the fit is obtained with only one free parameter. The temperature behaviors of the polar and azimuthal angles are in agreement with the proposed model over the whole temperature range.

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PubMed2001

Adsorption phenomenon and external field effect on an isotropic liquid containing impurities.

The steady-state distribution of ionic charges in a liquid, in the presence of surface adsorption, is determined. The effect of an electric field applied by means of blocking electrodes is considered. The analysis shows that the surface adsorption of ions dissolved in the liquid is responsible for an asymmetry in the electric-field distribution. In the model, the liquid is assumed to be dielectric but to contain impurities. These impurities, by means of a chemical reaction, can bring about ions. The theory takes into account the activation energy for the ionization chemical reaction and the adsorption energy of the ions at the surface.

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PubMed2001

Liquid-crystal-electrochromic-material interface: a p-n-like electro-optic junction.

A liquid crystal (LC) cell has been made by using a standard sandwich configuration with one of the indium tin oxide (ITO) electrodes covered by a thin layer of tungsten trioxide (WO3). In this kind of cell optical polarization switching (observed under a crossed polarizer microscope) occurs for only one of the two polarities of an ac external applied electric field, while in the usual liquid crystal cells the electro-optic response does not depend on the sign of the field. The inhibiting switching configuration corresponds to anodic polarization of the tungsten trioxide film in which the deintercalation of cations occurs. Here we present the time behavior of charge and discharge for both the anodic and cathodic currents. A model based on charge carrier exchange between the ITO-WO3 and WO3-LC interfaces and also electrochemical processes is reported. Our model is also capable of explaining the electric and electro-optic asymmetric responses of the cell. Numerical calculations confirm the model.

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PubMed2000

Model for the planar-homeotropic anchoring transition induced by trans-cis isomerization.

We present a model to explain the planar-homeotropic anchoring transition of azobenzene induced by UV illumination via trans-cis isomerization. We consider bulk and surface as two different phases (separated by an infinitely sharp interface) which are in equilibrium. We obtain a relation for the exposure time after which the transition takes place.

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PubMed2000

Thermal renormalization of the anchoring energy of nematic liquid crystals

The temperature dependence of the anchoring energy of a nematic liquid crystal on thermal fluctuations is studied. We consider the weak anchoring case, where the interaction of a nematic molecule on the surface with the substrate is small with respect to the mean field energy due to the other nematic molecules. The analysis is performed by means of a perturbation method in which the expansion parameter is the surface interaction. The presented model is valid for any value of the scalar order parameter. We show that the renormalization of the anchoring coefficients due to thermal fluctuations is proportional to the generalized scalar order parameters. We show also that, at the lowest order in the scalar order parameter, Landau-like theories agree with our mean field approach. An expression for the thermal renormalization of the anchoring coefficients valid in the low temperature region, where the fluctuations are small, is derived. The agreement between our theoretical predictions and the experimental data obtained by other groups is fairly good over a large temperature range.

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PubMed1999

Nematic ordering in a cell with modulated surface anchoring: effects of flexoelectricity.

We have analyzed molecular ordering in a nematic sample sandwiched between two parallel substrates, characterized by a periodically varying anchoring easy axis. If the periodicity lambda is smaller than the Debye screening length l(D) and the nematic material possesses flexoelectric properties, it is necessary to take into account also the electrostatic and flexoelectric contributions in the thermodynamical potential when the actual director field is determined. In this framework, for small deviations from the homeotropic alignment we have derived analytical expressions for the tilt angle (theta) and the electrical potential. To establish a connection with experimentally observable quantities, we have related the theta profile to the average and investigated its behavior for different values of lambda, the flexoelectric coefficient, and the anchoring strength w. Our results indicate that in a nematic with pronounced flexoelectric properties for small enough lambda, a kind of subsurface deformation appears, which substantially decreases . Therefore, effects of flexoelectricity cannot be neglected in treating nematic cells with modulated anchoring which allows bistable ordering.

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PubMed1996