Role of Surface Order on the Total Electric Conduction in NLC Samples

Abstract

Current vs voltage measurements have revealed two different conduction zones in a NLC sample. A standard sandwich type sample was filled with K18 (6CB) which was aligned (approximately) homeotropically by the two ITO surfaces previously cleaned in orthochromic mixture. On two opposite sides of the cell, the NLC was limited by two Mylar stripes acting as spacers. The other two sides were left open, and the liquid crystal had two free surfaces (limited only by air) in the form of a cylindrical meniscus. The overall conductivity of the cell depended strongly on the length of the free surface. For the same area, samples with a larger free surface experienced a higher overall conductivity. In the absence of an electric field, we considered the order in the free surface zone to be planar with n being normal to LC−air interface. For small electric fields the conduction along director n in the bulk is less than the conduction perpendicular to n on the free zone. At higher fields, the Freedericksz transition which operates on the free zone reduces the overall conductivity. This reduced conductivity was practically equal to that obtained when ITO electrodes were completely immersed in liquid crystal; that is, the free surfaces do not contribute to the conduction. Considering the measured values, we have concluded that the order in the free surface zone was planar for a depth of about 50 μm.

Role of Delocalized Electrons in Polyaniline−Nematogen Cyanobiphenyls Interaction

“…van der Waals dispersion attraction between PANI phenyl rings and those of 5CB and 6CB strengthen this anchoring, due to the larger polarizability of π electrons. The same π electrons contribute to the electric conductivity anisotropy of these liquid crystals, as mentioned earlier in this paper. − …”

“…The same π electrons contribute to the electric conductivity anisotropy of these liquid crystals, as mentioned earlier in this paper. [1][2][3][4][5] Let us imagine, succinctly, a mechanism that could explain the above-mentioned experimental facts. With the ITO/PANI side negative, electrons tunnel the thin ITO/PANI interface, traverse the PANI layer by fast and reversible charging of it, and enter liquid crystal reducing the aromatic rings that contain electron-withdrawing cyano groups.…”

“…In a recent paper we have presented that the electric conductivity in ordered phases of pentylcyanobiphenyls (5CB) or hexylcyanobiphenyls (6CB) across phenyl rings σ ⊥ (i.e., perpendicular to the nematic director n ) is much higher than along n ( σ ∥ ). Typically, σ ∥ ∼ 6 × 10 -8 S / m, σ ⊥ ∼ 9 × 10 -7 S / m, leading to a relative anisotropy This large negative anisotropy has been explained by considering the contribution to the electric conduction of delocalized π electrons.…”

Molecular Simulation of the Free Surface Order in NLC Samples

“…Recently, a nonstandard behavior of electric conductivity within some samples with NLC has been observed, which can be associated with the local orientational order at the free surfaces of the liquid crystal. 1 Current vs voltage measurements have revealed two different conduction zones in a NLC sample. A standard sandwich type sample was filled with K 18 (6 CB) that was aligned (approximately) homeotropically by the two ITO surfaces previously cleaned in orthochromic mixture.…”

Role of the Adsorption Phenomenon on the Ionic Equilibrium Distribution and on the Transient Effects in Electrolytic Cells

“…Furthermore, experimental evidence of the importance of the ionic charge on the anchoring energy of the interface solid substrate−nematic liquid crystal has been recently reported, suggesting an interpretation of its dependence on the sample thickness and on the voltage applied to the cell . Finally, the presence of the ions can also play a role in the total electric conduction , or in the measurements and in the theory presented in ref . The aim of this contribution is to extend previous analyses to take into account the adsorption phenomenon at the limiting surfaces.…”