Giovanni Barbero

Selective ions adsorption and nonlocal anchoring energy in nematic liquid crystals

Recent measurements have demonstrated the nonlocal character of the angular anchoring energy of a nematic liquid‐crystal orientation close to a solid substrate. Assuming a sign selective adsorption for ions on such a substrate, we calculate the surface charge and field. It exists a macroscopic cell thickness below which the surface can adsorb most of the volume charges. In this regime, because of the coupling of the surface field with the nematic director, the surface anchoring energy depends on the thickness, and can even become zero. This model explains the observed thickness dependence of the anchoring energy and the ‘‘spontaneous’’ Freederickcsz transition.

Impedance spectroscopy of an electrolytic cell limited by ohmic electrodes

We investigate the influence of the ohmic character of the electrodes on the impedance spectroscopy of a cell of a dielectric liquid containing ions. According to our calculations, the finite conductivity of the electrodes is responsible for an increasing of the real part of the electrical impedance in the low frequency range. It follows that the anomalous increasing of the resistance of the cell, experimentally observed by several groups, could be related also to the exchange of charge at the electrodes. Our analysis is based on a generalization of the Butler-Wolmer equation, for small values of the external voltage.

The structure of the distortion free-energy density in nematics: second-order elasticity and surface terms

SummaryBy means of a phenomenological approach, we demonstrate that the mixed splay-bend elastic constantK13 in the free energy density of nematic liquid crystals must be considered zero, unless the bulk contributions of the squares of the distortion second-order derivatives are taken into account, together with the squares of the first-order derivatives times the second-order derivatives, and with the fourth powers of the first-order derivatives. Such contributions just reduce to one in the presence of—and close to—a threshold. Furthermore, the saddle-splayK24-term instead is shown always to play an essential role, as the bulk first-order elasticity, in determining the distortion free energy of nematics with weak anchoring subjected to spatial deformations. Finally, the new surfacelike elastic constants are shown to have a nilpotent character: thus they behave as well asK24 from the point of view of the variational calculus.

Optical determination of large distortion surface anchoring torques in a nematic liquid crystal

We have measured the angular dependence of the surface torque associated with a relatively weak anchoring of a nematic liquid crystal, for large deviations θ (near π/2) from the orientation of minimum energy. Analysing the torque in a Fourier series, we find that in addition to the usual first term, viz. sin 2θ, an important contribution from a sin 6θ term is needed to adequately represent the experimental variation.

Order electricity and the nematic, smectic A, smetic C phase transitions

Abstract Smectic liquid crystal are quadrupolar oriented systems, with a spatial modulation across layers. This modulation creates an order electric antiferroelectric polarization. The associated dielectric depolarization energy, originating from long range forces, creates a new non analytic term, which must be added to the usual Landau-de Gennes expansion. This new term can physically explain the molecular tilt inside the layers and the smectic A-Nematic reentrance. The tilt appears as an intrinsic property of well ordered quadrupolar layered systems.

Impedance spectroscopy analysis of an electrolytic cell limited by Ohmic electrodes: The case of ions with two different diffusion coefficients dispersed in an aqueous solution

We analyze the influence of Ohmic electrodes on the impedance spectroscopy of an electrolytic cell in the shape of a slab. The electrolyte is assumed completely dissociated. The positive and negative ions have different diffusion coefficients. We show that in the very low frequency limit, the electrical impedance of the cell reduces to a pure resistance, whose value depends on the diffusion coefficients and on the conductivity of the electrodes. The ratio between the diffusion coefficients determines the numerical value of the plateaus of the resistance, and the position and amplitude of the local minimum of the reactance of the cell.

Experimental evidence for the adsorption-desorption phenomenon on the spectroscopy impedance measurements of an electrolytic cell

The frequency dependence of the real and imaginary parts of the electrical impedance of an electrolytic cell in the low frequency range is experimentally investigated. We show that a possible candidate able to explain the anomalous increasing of the real part of the electrical impedance of the cell is the adsorption-desorption phenomenon at the bounding surfaces. Measurements are performed on cells of water solution containing KCl, of different thickness and concentration of KCl, limited by the same type of gold electrodes. The experimental data can be fitted with the same adsorption and desorption coefficients, appearing in the kinetic equations, in the Langmuir approximation. This result confirm that the electrical impedance of the interface introduced to interpret the observed increasing of the resistance of the cell, in the low frequency region, could be connected with the adsorption-desorption phenomenon.

On a possible mechanism for the spontaneous Freedericksz effect

Abstract Kaznacheev and Sonin have presented a model to explain the so-called spontaneous Freedericksz transition in nematic liquid crystals (1983, Sov. Phys. solid Sr, 25, 528; 1984, Ibid, 26, 486). A surface polarization, coupled with the negative anisotropy of the nematic, turns the two homeotropic anchoring plates into planar anchoring plates. We show that this model, correctly solved, cannot explain the observed critical thickness. The spontaneous Freedericksz transition is in fact the surface instability of a hybrid cell with weak planar anchoring.

Electrical response of an electrolytic cell submitted to a small direct current electric field when the electrodes are ohmic

We theoretically investigate the influence of the ohmic character of the electrodes on the direct current (dc) response and on the ionic relaxation time of an electrolytic cell, in the shape of a slab. The analysis is performed by assuming that the fundamental equations of the problem can be linearized. This implies that, for monovalent ions, the applied difference of potential is smaller than 25 mV. We show that the finite ohmic conductivity of the electrodes is responsible for a reduction of the relaxation time. An explicit expression for the relaxation time, valid when the Debye length is very small with respect to the thickness of the sample is deduced. The analysis of the dc current allows us to determine the contribution of the electrodes to the effective electrical resistance. We show also that the equivalent circuit relevant to the present problem cannot be used to evaluate the relaxation time of the cell.

Thickness dependence of the anchoring energy of a nematic cell

The cell gap thickness dependence of the anchoring energy strength of a nematic cell is investigated. The relaxation time of the elastic deformation of a nematic liquid crystal layer imposed by an external electric field is analyzed as a function of the thickness of the nematic layer. The measurements have been performed with conventional sandwich cells with cell gap ranging from 1u2009μm to 10u2009μm. According to our data, the anchoring energy strength is not only a characteristic of the interface substrate-nematic liquid crystal but it also depends strongly on the cell thickness.