Biomedical subjects

G Barbero

Publications and source records attributed to G Barbero.

At least 19 recordsLinked to original sources

PubMed2006

Ionic contribution to the electric current in an electrolytic cell submitted to an external voltage.

The ionic contribution to the electrical current in an electrolytic cell submitted to an external voltage linearly increasing with the time is evaluated. The investigation is performed in the limit of small and large electric field, in which the density of ions depends on the actual electric field in the sample. In the analysis, it is assumed that the ionic separation induced by an external field can be described by a surface density of charge. We show that the ions are responsible for a peak in the current, followed by a delay in the application of the external voltage. From the analysis of the peak and its delay, it is possible to obtain information on the density of ions in thermodynamical equilibrium, and on the mobility of the ions in the considered liquid.

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PubMed2006

Dependence of the anchoring energy on the applied voltage in a nematic cell.

A nematic liquid crystal cell in the shape of a slab of thickness d and containing ionic impurities is considered in the presence of a dc voltage. A complete theoretical model to determine the electric field distribution across the sample is used to explain the experimental dependence of the effective anchoring energy of the cell on the applied voltage, in the limit of high voltage.

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PubMed2006

Effective screening length of isotropic liquid samples submitted to an applied voltage.

A cell of isotropic liquid in the shape of a slab of thickness d and containing ionic impurities is considered. It is shown that the screening effect produced by the ionic charges on the external field is characterized by an effective surface length, lambda(S)(U), depending on the applied voltage U. The analysis indicates that lambda(S)(U)) << lambda(D) when the applied voltage is very large, and lambda(S)(U) --> lambda(D) for very small values of the applied voltage, where lambda(D) is the Debye screening length. The presence of the ions is responsible also for a counterpotential, v, that for small U is such to cancel the effective electric field in the sample, whereas in the opposite limit it is inversely proportional to the applied difference of potential.

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PubMed2006

Electrical impedance for an electrolytic cell.

We analyze in which experimental conditions the concept of electrical impedance is useful for an electrolytic cell. The analysis is performed by solving numerically the differential equations governing the phenomenon of the redistribution of the ions in the presence of an external electric field and comparing the results with the ones obtained by solving the linear approximation of these equations. The control parameter in our study is the amplitude of the applied voltage, assumed a simple harmonic function of the time. We show that the bulk distribution of ions close to the electrodes differs from the one obtained by means of the linear analysis already for small amplitudes of the applied voltage. Nevertheless, the concept of electrical impedance remains valid. For larger amplitudes, the current in the circuit is no longer harmonic at the same frequency of the applied voltage. Therefore the concept of electrical impedance is no longer meaningful.

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PubMed2006

Substrate induced 1D tilt-modulated state in nematic monolayers.

Symmetry considerations yield the general form, up to second order terms, for the deformation elastic energy of a nematic monolayer, composed by symmetric achiral molecules, on a rigid planar substrate. The deformation energy contains an elastic contribution linear in the deformation tensor, whose elements are the spatial derivatives of the average molecular orientation. This linear Lifshitz-invariant-like term can be responsible for a ground state of the nematic monolayer periodically deformed if the relevant elastic constant is stiffer than a critical value. The wave-length of the modulation diverges at the transition threshold. We show that only large variations of the tilt angle form stable states. The effect of a destabilizing electric or magnetic field on the layer is to induce (i) the transition towards the tilt-modulated phase, while (ii) for higher enough values of the field the modulation is destroyed.

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PubMed2005

Impedance spectroscopy of water solutions: the role of ions at the liquid-electrode interface.

We discuss the influence of the ions dissolved in a liquid on the impedance spectroscopy of a cell. Our analysis is performed in the small-voltage regime, where the actual bulk density of ions is only slightly perturbed by the external electric field. In this framework, we show that the presence of the ions can be taken into account by a surface density of charge. The agreement between the theoretical prediction, on the basis of the assumption that the ionic mobility is frequency independent, and the experimental data for the real and imaginary parts of the impedance is fairly good for frequencies larger than 100 Hz. In the low-frequency range, the agreement of the theory with the experiment is rather poor. In this region, the experimental data can be successfully fitted by introducing the impedance of the metal-electrolyte interface, which is accurately represented by Zi = w(i omega)(-nu), where w and nu are two constants, with 0 < nu < 1. From the analysis of the experimental data, we determine w and nu. The theoretical predictions of our model are in good agreement with the experimental data in the investigated frequency range.

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PubMed2005

Comment on "local elastic stability for nematic liquid crystals".

We discuss a theory recently proposed by Virga and co-workers for the analysis of the local elastic stability of nematic liquid crystals [R. Rosso, E. G. Virga, and S. Kralj, Phys. Rev. E. 70, 011710 (2004)]. The periodic instability in a nematic sample induced by the saddle-splay elastic constant or by the coupling of an external field with the flexoelectric polarization is reconsidered. We show that in the case in which the two surfaces limiting the sample are characterized by the same easy angle the analysis previously proposed by us coincides with the one proposed by Virga.

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PubMed2005

Influence of adsorption phenomenon on the impedance spectroscopy of a cell of liquid.

We investigate the influence of the adsorption phenomenon on the impedance spectroscopy measurements. The analysis is performed by assuming that the ions have the same mobility and the electrodes are perfectly blocking. We find that in the low frequency range the presence of the adsorption phenomenon is responsible for an increasing of the real part of the impedance of the cell, similar to the one usually described by means of the impedance of the metal-electrolyte interface. The frequency dependencies theoretically predicted by our model for the real and imaginary parts of the complex dielectric constant are in qualitative agreement with the experimental data published by other groups.

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PubMed2005

Modulated structures in nematic monolayers formed by symmetric molecules.

An analysis based on symmetry yields a general form for the deformation elastic energy of a nematic monolayer, formed by achiral symmetric molecules, deposited on a solid substrate. Lifshitz-invariant-like terms in the energy, which originate from the substrate field, can induce a modulated-tilt state if the anchoring energy is sufficiently low. A way to enhance the symmetry breaking is to apply a destabilizing magnetic or electric field that serves to lower the anchoring energy. In the case of an initial state with homeotropic alignment, the phase diagram displays a cusp-shaped tilt-modulated state intervening between two uniform tilt states.

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PubMed2004

Deformation free energy and elastic description of a self-assembled system.

A continuum model is proposed to describe orientational states of a self-assembled system formed by rodlike molecules, in contact with an isotropic solid substrate. The total free energy is determined by taking into account the interactions between the molecules forming the film and between the molecules and the substrate. A phase diagram is presented, demonstrating that a critical surface molecular density exists, depending on the the ratio between the surface and the bulk free energy, separating homeotropic from tilted phases. The behavior of the elastic constants is investigated as a function of the surface molecular density. The elastic description leads to the presence of a linear term in the free energy, which accounts for the existence of possible spontaneous elastic distortions induced in the system.

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PubMed2004

Adsorption phenomenon of neutral particles and a kinetic equation at the interface.

The adsorption phenomenon of neutral particles in a sample having the shape of a slab is theoretically investigated by using a particular form for the kinetic equation at the limiting surfaces. The time evolution of the bulk and surface densities is determined in a closed form by means of a simple expression. A discussion on the characteristic times entering in the problem is reported. Finally, a microscopic model giving rise to a kinetic equation similar to the one used in the analysis is proposed, and the phenomenological parameters determined. The analysis is suitable for the description of the adsorption phenomena of dyes in nematic liquid crystals.

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PubMed2004

Influence of the ions on the dynamical response of a nematic cell submitted to a dc voltage.

The influence of the ions present in a liquid crystal on the dynamical response of a nematic slab submitted to a dc voltage is studied. The evolution of the system toward the equilibrium state is investigated by solving the continuity equation for the electric charge, taking into account the current of drift and of diffusion. Our analysis shows that the formation of the double layers close to the electrodes strongly modifies the distribution of the electric field across the sample. We evaluate the surface polarization due to the ions movements and the contribution to the anisotropic part of the surface energy having a dielectric origin. We show also that, even if the optical response of the liquid crystal is a slow phenomenon, the distribution of the ionic charge is rather fast. Consequently, the presence of the ions cannot be neglected in the determination of the flexoelectric coefficients when the nematic sample is submitted to a square wave having a period of the order of 1 s.

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PubMed2004

Influence of the bias-voltage on the anchoring energy for nematic liquid crystals.

The influence of the bias-voltage on the anisotropic part of the nematic surface energy is analyzed. The experimental data show a strong dependence of the anchoring strength on the bias-voltage when the electrodes of the nematic cell are covered with WO3. The observed dependence can be interpreted taking into account the ions dissolved in the liquid crystal. We propose a model in which the effect of the bias-voltage is to collect the ions near the electrodes, in a surface layer whose thickness is of the order of the Debye's screening length. The surplus of electric field due to this ions confinement gives rise to an electrostatic contribution to the total energy that can be considered as a surface energy. The proposed model is in good agreement with the experimental data. The model is used to interpret the observed independence of the anchoring strength on the bias-voltage when the (indium-tin-oxide) electrode is covered with a film of polyimide, or it is without any covering. The influence of a charge emission from the electrodes under the bias voltage on the anchoring energy is also analyzed. Possible applications of the observed phenomenon are discussed.

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PubMed2004

Statistical interpretation of the kinetic equation in the adsorption problem.

A statistical interpretation for the adsorption phenomenon is presented in the framework of the Maxwell-Boltzmann statistics. A model for an adsorbing surface is proposed and the connection between the phenomenological parameters, entering in the kinetic equation at the boundary surfaces, with the microscopic model is derived.

Adsorption↗

PubMed2003

Comment on "Optical determination of flexoelectric coefficients and surface polarization in a hybrid aligned nematic cell".

We argue that the experimental determination of the flexoelectric coefficients and the surface polarization of the p-pentyl-cyanobiphenil liquid crystal, recently reported by Mazzulla et al. [Phys. Rev. E 64, 021708 (2001)] is questionable. This conclusion follows from the hypothesis performed by the authors to extract the flexoelectric coefficients and the surface polarization from the reflectivity curves taken under an applied voltage. In fact, they assume that the liquid crystal can be considered as a perfect insulating material and that the ionic charges do not contribute to the electric field profile inside the sample. Using our recent results concerning nematic liquid crystals submitted to an external electric field we show that this hypothesis is far from being correct. In the experimental conditions of Mazzulla et al., due to the low frequency of the applied alternative square pulsed signal, the ions play an important role in the molecular orientation of the liquid crystal induced by the external field.

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PubMed2003

Splay-bend periodic deformation in nematic liquid crystal slabs.

We predict a two-dimensional splay-bend periodic deformation in a nematic slab with homeotropic boundary conditions. The nematic director modulation is induced by surface contributions in the elastic energy, which are linear in the deformation tensor. The instability appears only if the surface anchoring energy strength is small enough, and if the two surfaces are different. The wave vector of the stripe modulation is proportional to the thickness of the nematic slab. The order of magnitude of the surface elastic constants relevant to the linear elastic terms in the deformation tensor, and the critical value of the anchoring energy, greater than or approximately 10(-6) J/m(2), to observe the instability are discussed.

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PubMed2003

Modulated structures of flexoelectric origin in nematic liquid crystals.

A structural instability of flexoelectric origin is predicted in a homeotropic cell, of insulating nematic liquid crystal, by the action of an electric field applied in the direction of the initially nonperturbed nematic director. The instability gives rise to a two-dimensional periodic structure. The critical field to observe the predicted modulated structure as well as the wavelength at the threshold are evaluated. Both vary as the inverse square root of the cell thickness. The role of the dielectric anisotropy on the phenomenon is investigated. Our analysis is performed in the limit of weak anchoring energy strength, where the extrapolation length is large with respect to the thickness of the nematic sample.

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PubMed2003

Spontaneous periodic distortions in nematic liquid crystals: dependence on the tilt angle.

The possibility of spontaneous periodic distortions, depending on the tilt angle in a nematic liquid crystal sample, is investigated by means of a general formulation of the stability problem. It is shown that due to the presence of a surfacelike term in the free-energy density, the uniform pattern can be destabilized, giving rise to a periodic distortion of the director. Our analysis establishes, in general terms, the conditions for the formation of stable periodic structures in nematic samples. In particular, we determine the wavelengths for which the periodic distortion exists by investigating its dependence on the tilt angle, characterizing the uniform pattern, and on the saddle-splay elastic constant. The effect considered in our paper is a finite size effect, related to the slab geometry of the nematic sample under consideration.

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