Ions and nematic surface energy: Beyond the exponential approximation for the electric field of ionic origin

Abstract

We present a general model to describe the influence of the ionic adsorption on the anisotropic part of the surface energy of a nematic liquid crystal in contact with a substrate. We show that in the limit of small adsorption energy, the exponential approximation for the electric field of ionic origin works well. In this limit, the dielectric and flexoelectric contributions to the surface energy are quadratic and linear on the density of adsorbed ions, respectively. In the opposite limit of large adsorption energy, the exponential approximation for the electric field does not work, and the two contributions to the surface energy are both found to depend linearly on the surface density of adsorbed charges. Approximated formulas reported in literature are derived from our general equations as particular cases, and their limits discussed. An expression for the surface polarization in nematic liquid crystal due to the ionic adsorption is also deduced. Our analysis is performed in the framework of the Poisson-Boltzmann theory, where dimensionless ions are treated within a mean field approach. Possible extensions of our model are indicated.

Liquid Crystal Dispersions of Carbon Nanotubes: Dielectric, Electro-Optical and Structural Peculiarities

“…We begin with the intermediate frequency range (B). In this range, large active resistances of the double electric layers (DEL) formed near the cell electrodes are shunted by their capacitance diminishing with a field frequency f (Koval'chuk, 1998;Koval'chuk, 2000;Barbero & Olivero, 2002;Koval'chuk, 2001b). Because of this, the range (B) characterizes volume properties of samples.…”

“…The dielectric spectra in the most low frequency range (A) (f<10 2 Hz) are mainly determined by the near-electrode processes and electron exchange between electrodes and ions (Koval'chuk, 1998;Koval'chuk, 2000;Barbero & Olivero, 2002;Koval'chuk, 2001c). In this range, the most important changes, provoked by presence of CNTs, were observed for the imaginary (ac conductance) component , while changes for real (capacitive) component  were smaller.…”

Dielectric properties of nematic liquid crystals with Fe3O4 nanoparticles in direct magnetic field

“…5. Frequency dependences for the ratio of dielectric permittivity value for 6СНВТ in the magnetic field  В to that without the magnetic field  0 for  (1,2),  (3,4) and different values of the magnetic field induction 0.45 T (1, 3) and 0.60 T (2, 4) at the temperature 293 K.…”

“…To create the electric field in a LC cell, it is necessary to deposite the electrodes transparent in the visible spectrum onto the boundary surfaces. Using the electro-optical effects, one needs to take into account the influence of near-electrode processes [1,2]. All these factors that may create substantial problems (one of the basic problems is increase of the electro-optical response time) would be avoided by the influence of magnetic field on LC.…”

Adsorption of ions and thickness dependence of conductivity in liquid crystals

“…It was shown in [1][2][3] that, after filling the cell with liquid crystal, some part of ions is adsorbed on the electrode surface. So, the total amount of charge carriers in LC will decrease, which in some way should affect the value of conductivity.…”