1,721,290 research outputs found
Influence of adsorption phenomenon on the impedance spectroscopy of a cell of liquid
No full textWe 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
Some considerations on the elastic theory for nematic liquid crystals
No full textThe elastic theory for nematic liquid crystals is critically analysed. After a review on the variational calculus formalism, the range of applicability of the Lagrangian method for the solution of practical problems is discussed. It is underlined that only a limited number of problems can be solved by means of a variational approach. The role at the Jacobi equation is also discussed. The importance of the non linear character of the K13-problem is analyzed in the framework of a simple molecular model. Finally, the principle of virtual work is applied to the elastic theory of nematic liquid crystals. Our analysis shows that the K13 elastic problem is an ill-posed one, since this problem can only be solved by means of a variational, or virtual work, approach by modifying the bulk elastic free energy and taking into account new terms quadratic in the second order deviatives. However it is necessary to remember that, in the proximity of a surface, a spatial variation of the density and of the scalar order parameter of the liquid crystal are expected, and hence a true elastic description is no longer possible
One sign ion mobile approximation
No full textThe electrical response of an electrolytic cell to an external excitation is discussed in the simple case where only one group of positive and negative ions is present. The particular case where the diffusion coefficients of the negative ions, D-m, is very small with respect to that of the positive ions, D-p, is considered. In this framework, it is discussed under what conditions the one mobile approximation, in which the negative ions are assumed fixed, works well. The analysis is performed by assuming that the external excitation is sinusoidal with circular frequency omega, as that used in the impedance spectroscopy technique. In this framework, we show that there exists a circular frequency, omega*, such that for omega > omega*, the one mobile ion approximation works well. We also show that for D-m << D-p, omega* is independent of D-m. (C) 2011 American Institute of Physic
Second order elasticity in nematics: a new anchoring source
No full textBy considering, in the expression of the nematic free energy density, an additional term in the square of the director second derivatives, an unexpected anchoring source results, due only to surface and bulk elastic constants. As an example, the case of a planar homogeneous and of a homeotropic nematic cell, equally anchored on both walls, is discussed. In both situations the new anchoring source has a destabilizing effect
Nematic phases with spontaneous splay-bend deformation: standard elastic description
No full textRecently, possible deformed states in nematic liquid crystals have been discussed assuming that Frank's bend elastic constant is negative. The presence of a stable deformed state has been justified by introducing in the elastic energy density a term proportional to the second derivative square of the deformation parameter. This term, widely used in field theory, has been introduced in the elastic theory of liquid crystals mainly to investigate the effect of the splay-bend term in the nematic deformation in confined samples. Nevertheless, in the analysis of bulk properties this term is questionable. An alternative justification of possible spontaneous distortions in nematic liquid crystals is proposed. Our description of the elastic properties is based on an elastic energy density expanded to the fourth order in the deformation parameter. In this framework, we show that a stable nematic state characterised by a uniform deformation, related to a position-independent elastic energy density, is possible
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