A surface forces platform for dielectric measurements

Abstract

Articles you may be interested in Dielectric relaxation in ionic liquids: Role of ion-ion and ion-dipole interactions, and effects of heterogeneity J. Chem. Phys. 140, 014504 (2014) Methods are described to implement dielectric spectroscopy ͑frequency range 10 Ϫ1 -10 6 Hz) within a surface forces apparatus by using as electrodes silver sheets on the backside of mica. These methods are applied to study the competitive effects of surface alignment, confinement, and shear field on 5CB ͑5-cyanobiphenyl͒, a nematic liquid crystal at the experimental temperature of 25°C. In the planar orientation, films could be squeezed to a minimum thickness of Ϸ5 Å, the molecule's thickness. In the perpendicular ͑homeotropic͒ orientation, films could be squeezed to Ϸ25 Å, the expected thickness of the head-to-tail 5CB dimer. It was difficult to discuss responses at f Ͼ10 5 Hz quantitatively because the peak was not visible in the experimental frequency window. Nonetheless, the onset of the relaxation mode for the planar oriented molecules appears at higher frequency than for the homeotropic orientation. A slower relaxation mode, peaked at f Ϸ10 Hz, was assigned to electrode polarization due to the mobility of trace ions within the 5CB samples although these samples were Ͼ99.7% pure. The peak frequency was a factor of 3 slower with homeotropic than planar alignment and, in both cases, independent of film thickness except when the film thickness exceeded 10 m. This was explained using a simple model based on the assumption that trace ions move to oppositely charged electrodes to form electric double layers. A small influence of shear on the dielectric response was observed but only when the dielectric response was measured at the same frequency as the large-amplitude shear deformation. Also described is the use of capacitance to measure force-distance profiles.

Investigation of the liquid crystal alignment layer: effect on electrical properties

“…The alignment of LC itself involves complex physicochemical interactions between the LC molecules and the alignment film. It was found that the homeotropic alignment of LC is caused by the weak anchoring [16] and is favoured when the surface is hydrophobic [12].…”

“…It is in the nematic phase between 14.5 and 29 • C. Its physical properties have been studied extensively. The dielectric anisotropy is positive because the dipole moment of the cyano group in the molecule is nearly parallel to the long molecular axis [12]. The LC material was syringed into a d = 20 µm thick symmetric cell with Mylar spacers between the two functionalized ITO substrates.…”

Perfluoropolymer Surface for Shock‐free Homeotropic Alignment of Smectic Liquid Crystals

“…Hence, vertical orientation of LC is much more favorable on an alignment layer with low surface energy. [6,7] Now we need to explain this phenomenon in more detail because the alignment of LC is induced by complex physicochemical interactions between the LC molecules and the aligned films. To explain the orientational recovery to VA, several types of interactions must be considered, including p-p electron coupling, geometric effect, dipole-dipole interaction, and van der Waals interaction.…”

“…Usually the homeotropic alignment of LCs is realized by the hydrophobic surface if there is no preferential orientation or surface roughness. [6,7] One way to achieve such alignment of LCs is to adsorb a surfactant monolayer on the surface to render it hydrophobic. [8,9] Geometrical factors can also be responsible; for example, LC molecules align parallel to the flexible long chains of a surfactant attached to substrate surfaces perpendicularly.…”

Analysis of Optical Performance Degradation in an Ion-Doped Liquid-Crystal Cell with Electrical Circuit Modeling

“…To analyze the degradation with time, we fabricated an ion-doped LC cell and measured the haze and impedance. Impedance and dielectric spectroscopic analyses were carried out to extract the impedance parameters, such as the resistance and capacitance of the LCs and physical parameters of the ionic materials in the bulk regions and near the alignment layers [29][30][31][32][33][34][35]. With the increase in time under the applied electric field, the LC cell exhibited nonuniformity with the decrease of the haze, capacitance of the LC cell, and thickness of the diffusion layer, along with the increase of the resistance of the LC cell and surface concentration of the ions.…”