Abstract
Capacitance measurements have been extensively used to measure the anchoring extrapolation length L at a nematic–substrate interface. These measurements are extremely delicate because the value found for L often critically depends on the sample thickness and the voltage range chosen to perform the measurements. Several reasons have been proposed to explain this observation, such as the presence of inhomogeneities in the director distribution on the bounding plates or the variation with the electric field of the dielectric constants. In this paper, I propose a new method to measure L that takes into account this second effect. This method is more general than the one proposed in Murauski et al. Phys. Rev. E 71, 061707 (2005) because it does not assume that the anchoring angle is small and that the anchoring energy is of the Rapini–Papoular form. This method is applied to a cell of 8CB that is treated for planar unidirectional anchoring by photoalignment with the azobenzene dye Brilliant Yellow. The role of flexoelectric effects and the shape of the anchoring potential are discussed.
Highlights
Measuring the anchoring energy W of the director at the bounding plates of a nematic cell is necessary to predict its electrooptical properties under electric field
In this paper, I focus on the measurement of the polar anchoring energy in planar cells filled with a liquid crystal (LC) of positive dielectric anisotropy
I notice that the behavior observed in my analysis, namely a value L that systematically decreases when the fits are performed at large voltages, is the same as the one reported by Nastishin et al [5] by using the YvS or the RV techniques
Summary
Measuring the anchoring energy W of the director at the bounding plates of a nematic cell is necessary to predict its electrooptical properties under electric field. Nastishin et al have shown that similar results can be obtained by just measuring the optical retardation in the same range of voltage (RV method) [5,17] This method is interesting because it allows a local measurement of L. I propose an alternative method to measure the polar (zenithal) anchoring energy W in a planar cell based on capacitance measurements at high electric field. The main difference with previous works is that I take into account the variation of the dielectric constant ε with the electric field and I use the full integral equations to fit the experimental data This way, the limitation to small tilt angles on the plates is waived and any form of the anchoring potential can be used. There is no pretilt angle at the electrodes, which simplifies the problem
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