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Surface limitations to the electro-mechanical tuning range of negative dielectric anisotropy cholesteric liquid crystals
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Image of FIG. 1.
FIG. 1.

(Color online) (a) The typical cell configuration for electromechanical tuning of −Δɛ CLC including the following important parameters such as the cell gap thickness (g), the unglued region spacing (L), the rubbing direction, and the tuning region (dashed circle) where the maximum tuning occurs. The director configuration for a CLC confined between anti-parallel rubbed Elvamide with rubbing directions along the dashed arrows. Due to the finite surface energy the orientation of the director may differ from that of the rubbing direction by (± θ/2). (b) The electromechanical tuning mechanism shown near the tuning region where distortions of g occur under applied voltages resulting in distortions of the pitch.

Image of FIG. 2.
FIG. 2.

(Color online) (a) The experimental setup used to measure the twist angle, θ, in a long pitch cholesteric sample. (b) Numerical calculations for the transmitted intensity for a cholesteric with a twist angle θ = 30° at various retardation values Г = 2πΔnd/λ as a function of the analyzer angle α. (c) A graph of the minimum intensity (Imin) vs the analyzer angle at the minimum (αmin) for different values of Г along with a parabolic fit.

Image of FIG. 3.
FIG. 3.

(Color online) (a) Example transmission spectra for a CLC at 0 V with a rubbing number of 13 along with the locations of their band edges (vertical lines) as determined by the analysis software. (b) A plot of the band edges identified as the high energy (short wavelength) and low energy (long wavelength), and their average (notch position) as a function of applied voltage. The dashed vertical line in this graph identifies the tuning limit. (c) The voltage dependence on the notch position as the voltage is cycled from 0-190 V (blue), then 190-(−190 V) (red), and finally from (−190)-190 V (black).

Image of FIG. 4.
FIG. 4.

(Color online) Experimentally obtained tuning range limit vs notch position for multiple 5 μm cells. Using the tuning range limit model to fit the tuning range limit data (solid curve), an extrapolation length Le = 0.76 ± 0.05 μm was obtained resulting in an anchoring energy Wθ = 13 μJ/m2 assuming that K22 = 10 pN. Also shown are curves having different anchoring strengths with values of 50 μJ/m2 (dotted) and 330 μJ/m2 (dashed-dotted), along their predicted tuning range limits at 1500 nm that shows significant increases with higher anchoring energy.

Image of FIG. 5.
FIG. 5.

(Color online) The measured anchoring energy Wθ between E44 and Elvamide (circles–left axis) and the measured tuning range limit for ZLI-4788 with Elvamide (squares–right axis) as a function of the rubbing number. The solid curve is a guide to show the general trend of the data and is not theoretically determined. For the tuning experiments, the notch position λn ranged between 620 and 623 nm.

Image of FIG. 6.
FIG. 6.

(Color online) (a) A graphical interpretation of the free energy per area and possible solutions to the free energy for weak and strong anchoring. (b) The maximum strain (umax) as a function of the number of half pitches for a cholesteric using the analytic model for various values of πLe/g. (c) A comparison of the analytical model to numerical results obtained for the same equation for a 10 μm cell, K22 = 10 pN, and the anchoring energies described in the legend.


Generic image for table
Table I.

Experimental results from the anchoring strength measurements using Elvamide.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Surface limitations to the electro-mechanical tuning range of negative dielectric anisotropy cholesteric liquid crystals