Determination of azimuthal anchoring energy in grating-aligned twisted nematic liquid-crystal layers

Abstract

In this work sinusoidal diffraction gratings with a range of pitches and amplitudes are used to align nematic liquid-crystal layers in a twisted homogeneous configuration. The grating profiles are accurately characterized using optical surface plasmon polariton spectroscopy, which then allows a calculation of the anchoring energy as predicted by the simple Berreman expression. The experimental Rapini–Papoular anchoring energy is also obtained by a measurement of the director twist away from the alignment direction (easy axis) at room temperature. A linear relationship is found between the two anchoring energies, except when it falls below 4×10−7 J m−2. Noticeably, the correlation between the two theories is not unity, if room-temperature elastic constants are used in the calculation. This apparent inconsistency is explained if the effect of surface memory on the system is considered. Indeed, if elastic constants, corresponding to a higher temperature at which surface memory effects are absent, are used in the Berreman expression, good agreement between the predicted and experimentally measured energies is found.

Anchoring Energies of Liquid Crystals Measured on Surfaces Presenting Oligopeptides

“…This model uses an approximation for surface-anchoring torque proposed by Rapini and Papoular 40 that is not valid for φ > 45°. 29 The measured values of φ for liquid crystal in contact with χ peptide ) 0.05 and χ peptide ) 0.03 exceeded this limitation (when recorded at a film thickness of liquid crystal of 5.5 ( 0.5 µm, as determined by observation of interference colors a polarized light microscope 27 ). The large value of φ measured on surfaces with χ peptide ) 0.05 and χ peptide ) 0.03 suggests very weak anchoring at those interfaces.…”

“…Next, we calculated W az using the measurements summarized in Figure A and the torque-balance model given in eq 2. This model uses an approximation for surface-anchoring torque proposed by Rapini and Papoular that is not valid for φ > 45° . The measured values of φ for liquid crystal in contact with χ peptide = 0.05 and χ peptide = 0.03 exceeded this limitation (when recorded at a film thickness of liquid crystal of 5.5 ± 0.5 μm, as determined by observation of interference colors using a polarized light microscope 27 ).…”

“…The approach reported in this paper to measurement of the azimuthal anchoring energy of peptide-decorated interfaces is a variant of the elastic torque-balance method. ,− In brief, when using the torque-balance method, liquid crystals are confined between two surfaces such that η 0 at the top surface is rotated approximately 90° relative to the orientation of η 0 at the bottom surface (Figure A). This configuration induces a twist distortion across the film of liquid crystal.…”

Measurement of the Azimuthal Anchoring Energy of Liquid Crystals in Contact with Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers Supported on Obliquely Deposited Gold Films

“…9,10 Several groups have isolated the contributions that surface topography can make to azimuthal anchoring energies by studying LCs in contact with substrates having feature sizes that can be systematically controlled (e.g. surface gratings 11,12 or periodic microrelief structures prepared by the deformation of hard coatings supported on soft polymeric substrates 13 ). Distortion of the director of the LC over the topography of the surface can create an "elastic contribution" to the anchoring energy.…”

Effective anchoring and scaling in nematic liquid crystals

“…Controlled anchorings can be designed by means of grooved surfaces [12,[25][26][27], spatially mixed patterns of different aligning potential [28], or stochastic patterns [29]. The effective anchorings at larger scale are usually calculated by assuming that the director is uniform at infinity.…”