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I Lelidis

Publications and source records attributed to I Lelidis.

13 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

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

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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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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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

Nonspontaneous surface-induced nematic phase.

By means of a Landau-de Gennes mean field model, we predict the existence of a nonspontaneous surface nematic phase in a smectogenic compound in contact with a suitable solid substrate. In the bulk the system does not show any nematic phase, the latter being solely induced by the substrate-liquid crystal interaction. Depending on the strength of the surface potential, a prewetting line, terminating at a critical point, may appear. For strong enough coupling, a new surface smectic phase can be induced, accompanied by a reentrant behavior. Our analysis might explain some recent experimental results [T. Moses, Phys. Rev. E 64, 010702(R) (2001)]: to validate it we suggest possible further experimental investigations.

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PubMed2001

Experimental evidence of the Halperin-Lubensky-Ma effect in liquid crystals.

The orientational order parameter decrease, due to nematic director macroscopic fluctuations, is obtained by birefringence measurements around the nematic <--> smectic-A transition temperature of the liquid crystal 4'-n-octyl-4-cyanobiphenyl. The measured nematic order reduction shows the same amplitude as the secondary order parameter discontinuity at the transition providing direct evidence of the Halperin-Lubensky-Ma effect. The importance of nematic director thermal fluctuations on the character of the transition is revealed as their quenching by an electric field of approximately 20 V/micrometer restores the second order character of the transition.

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