F. C. M. Freire

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 100 Hz), 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.

Universal patterns in sound amplitudes of songs and music genres.

We report a statistical analysis of more than eight thousand songs. Specifically, we investigated the probability distribution of the normalized sound amplitudes. Our findings suggest a universal form of distribution that agrees well with a one-parameter stretched Gaussian. We also argue that this parameter can give information on music complexity, and consequently it helps classify songs as well as music genres. Additionally, we present statistical evidence that correlation aspects of the songs are directly related to the non-Gaussian nature of their sound amplitude distributions.

Transient effects in electrolytic cells submitted to an external electric field

We analyse transient effects in an electrolytic cell submitted to an external voltage and determine the relaxation time of the redistribution of the ions and of the potential. We consider the case in which adsorption effects at the interface with the electrodes are present and show that the typical relaxation time, for applied voltage V 0⩽25 mV, is of the order of tens of seconds for commercial nematic liquid crystals. When V 0>25 mV the linearized analysis is no longer valid. In this case, the relaxation time depends on the applied voltage. Increasing V 0, but still remaining in the low amplitude limit, the relaxation time starts increasing. This indicates that the reduction of the actual field in the sample, for moderate values of V 0, plays an important role. For large values of V 0, the relaxation time is a decreasing function of V 0. This result is interpreted in terms of a simple model, according to which the ionic charge is localized in a surface layer whose thickness depends on the amplitude of the applied voltage.

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.

Voltage decay time of a liquid crystal cell submitted to a large difference of potential

The voltage decay time of a nematic liquid crystal cell submitted to a large difference of electrical potential is theoretically investigated. The authors show that ions are responsible for a current transient in the electrical circuit containing the cell. The analysis is performed by solving the equations of continuity for the ions and the equation of Poisson for the electrical potential. The results of the authors’ numerical analysis are compared with the experimental data and a good agreement is found. From the best fit of the experimental data the authors obtain the ionic mobility and the bulk density of ions, in thermodynamical equilibrium.

Exact tilt angle profiles for splay-bend deformations in nematic liquid crystals

The exact tilt angle profiles for splay–bend deformations, in nematic liquid crystal samples limited by inhomogeneous surfaces, are determined in the one‐constant approximation. The boundary value problem concerning the situation of strong anchoring at the surfaces of a sample of slab shape of thickness d (Dirichlets problem) is analytically solved in the presence of an external uniform field. The boundary value problem concerning the weak anchoring situation (mixed problem) is also exactly solved in the absence of an external field. The results are used to obtain the thickness dependence of the optical path difference between the ordinary and extraordinary rays, from which the physical properties of the sample can be deduced.

Dielectric characterization of doped M5.

We analyze the dielectric properties of the liquid crystal M5 used to investigate the electrohydrodynamics instability in nematic liquid crystals. We show that the spectra of the electrical impedance of doped samples of M5 with several concentrations of salt and of acid can be interpreted by assuming the doped liquid crystal as a dispersion of ions in a dielectric liquid. The analysis has been performed by considering the electrodes as blocking. From the best fit of the experimental data relevant to the real and imaginary parts of the electrical impedance of the cells, we derive the diffusion coefficients for the positive and negative ions and their bulk density. According to our analysis, in the limit of small concentrations of doping substance, the effective dielectric constant of the resulting liquid is, practically, independent of the doping.