A.L. Alexe-Ionescu

Role of the diffuse layer of the ionic charge on the impedance spectroscopy of a cell of liquid

We investigate the role of the diffuse layer of the ionic cloud on impedance spectroscopy measurements. The analysis is performed assuming that the ions have equal mobility, the electrodes are perfectly blocking and adsorption phenomenon can be neglected. We find that the dielectric permittivity, in the limit of high frequency ω, tends to the dielectric permittivity of the pure liquid as ω−3/2. The relationship between the detected equivalent permittivity and conductivity of the cell with the real and imaginary part of the complex dielectric constant is discussed. We show also that the presence of the ions is responsible for a distribution of relaxation times. An application to nematic liquid crystals is presented.

Complex dielectric constant of a nematic liquid crystal containing two types of ions: limit of validity of the superposition principle.

We investigate the influence of two groups of ions on the complex dielectric constant of a nematic liquid crystal limited by perfectly blocking electrodes. The analysis is performed by solving the equations of continuity for the two groups of cations and anions, and the equation of Poisson relating the actual electric field to the net density of charge. We consider a typical experiment of impedance spectroscopy, and evaluate the equivalent resistance and reactance of the cell, in the series representation, versus the frequency of the applied voltage to the cell. We show that the presence of two groups of ions gives rise to two plateaux in the spectrum of the resistance, similar to those related to the ambipolar and free diffusion in the case where there is only one type of ions, but for which the cations and anions have different diffusion coefficients. The correspondence between the usual ambipolar and free diffusion coefficients and those related to the presence of two groups of ions is discussed. The spectra of the real and imaginary parts of the complex dielectric constant are obtained, and their dependence on the bulk densities of the two types of ions is investigated. The nonvalidity of the superposition principle is discussed.

Electric response of a cell of hydrogel: Role of the electrodes

We investigate the dielectric properties of hydrogel by means of the impedance spectroscopy technique. Our experimental data relevant to the frequency dependence of the real and imaginary parts of the electrical impedance of the cell indicate that, in the low frequency region (f 100u2002Hz), the electric response of the sample is mainly due to the bulk properties. The observed frequency dependence of the electrical impedance of the cell can be well interpreted by means of the Poisson–Nernst–Planck model taking into account the Ohmic character of the interface electrodes-hydrogel.

Measurement of the splay-bend elastic constant in lyotropic ferronematic liquid crystals: The influence of the bounding surfaces

We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined. The theoretical analysis shows that near the nematic–isotropic transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory.

Effect of the viscosity on the electrical response of a hydrosolution

The electric response of a hydrosolution is investigated. We show that our experimental data can be interpreted by means of the standard drift-diffusion model only if the frequency dependence of the viscosity of the hydrosolution is taken into account. From this result, it follows that measurements of impedance spectroscopy can give information on the non-newtonian character of complex fluids.

Electrical behavior of nematic cells oriented by polypyrrole surface treatment

We have studied the electrical response of nematic liquid crystal cells with uniform planar alignment imposed by ClO4− doped polypyrrole layers. From the measured electrical current on a resistance in series with the nematic sample when applying a triangular potential with different periods and amplitudes, the diffusion of the ions from the polypyrrole layer into the liquid crystal was evidenced. The sample is described as an insulating liquid containing a distribution of ions depending on the characteristics of the applied external field and also of the polymer layers. A simple model concerning the diffusion of the ions from the aligning doped polypyrrole film inside the liquid crystal is proposed. The agreement between experimental data and numerical results is good.

Anchoring energy for the nematic liquid crystal–Langmuir Blodgett film interface

Abstract We analyse the effect of a Langmuir-Blodgett (LB) multilayer on the surface properties of a nematic liquid crystal (NLC). We show that the easy axis of the LB-NLC interface coincides with that of the LB-solid surface interface. On the contrary, the effective anchoring energy of the LB-NLC interface is lower than that associated with the LB-solid substrate interface. We show in a first approximation that the anchoring energy characterizing the NLC may be separated into three contributions: one connected with the interaction between the LB film and the solid substrate, one due to the direct LB-NLC interaction and the other one having an elastic origin. Nevertheless, to be more precise, one has to consider also the term associated with the interaction energy between the NLC and the substrate, which is screened by the LB film. The elastic contribution is of the order of the elastic constant of the LB film over the thickness of the multilayer. This quantity is estimated to be of the order of 10−2–10−1...

Influence of the rheological properties on the electrical impedance of hydrogels

We show that the anomalous frequency dependence of the real part of the electrical impedance of hydrogels can be interpreted by taking into account their non-Newtonian character, according to which the effective viscosity, η, depends on the frequency of the imposed deformation. Our experimental results are in good agreement with the Poisson–Nernst–Planck model if the ionic viscosity is taken proportional to the macroscopic one, measured by means of a viscositymeter, and the diffusion coefficient is assumed frequency dependent according to the formula of Stokes Du2009=u2009KBT/(κη), where κ is a shape coefficient. From this result it follows that the non-Newtonian character of complex liquid can be investigated by means of the impedance spectroscopy technique.

Independence of the effective dielectric constant of an electrolytic solution on the ionic distribution in the linear Poisson-Nernst-Planck model

We consider the influence of the spatial dependence of the ions distribution on the effective dielectric constant of an electrolytic solution. We show that in the linear version of the Poisson-Nernst-Planck model, the effective dielectric constant of the solution has to be considered independent of any ionic distribution induced by the external field. This result follows from the fact that, in the linear approximation of the Poisson-Nernst-Planck model, the redistribution of the ions in the solvent due to the external field gives rise to a variation of the dielectric constant that is of the first order in the effective potential, and therefore it has to be neglected in the Poissons equation that relates the actual electric potential across the electrolytic cell to the bulk density of ions. The analysis is performed in the case where the electrodes are perfectly blocking and the adsorption at the electrodes is negligible, and in the absence of any ion dissociation-recombination effect.