Polar anchoring energy

FIGURE 7.26. Electrooptical switching of FLC cells for different polar anchoring energies Wp. Short pulses of the electrical field E with different signs are applied... 

The qualitative analaysis of bistability based on the catastrophe theory was carried out [159]. The bistability conditions were shown to depend crucially on polar anchoring energy Wp. For large Wp, the occurrence of a third stable state with a splayed director profile should be taken into account. If the polar energy is small, compared with the dispersion energy which is close to experimental data [76], the following phenomenological correlations between the bistability threshold and the FLC parameters were obtained [159] ... 

Figure 11. Polar anchoring energy of POSS/PI alignment layers as a function of POSS concentration doped in homogeneous Pis (Hwang et al. 2010).

The high electric field technique could also be used to measure the polar anchoring energy [14]. In this technique the extrapolation length d is estimated by using the relationship between the measured values of the electric capacitance (C) and the optical retardation (R)... 

D. Shenoy, L. Beresnev, D. Holt, and R. Shashidhar, Tuning polar anchoring energy through chemical modification of photodimerized surfaces. Applied Physics Letters 80, 1538 (2002). 

The estimated values of the azimuthal and polar anchoring energy of the photoaligned LC layer were 4 x 10 " J/w and 5 x 10 J/m respectively, which are almost comparable with that of the rubbed polyimide layer. Consequently, photoaligned LCOS panels have the same electro-optical characteristics as the usual ones prepared by rubbing (Figure 6.6). 

Here, the quantities JVo, Wa and Wp stand for the isotropic, azimuthal, and polar anchorings, respectively. The unit vectors % and point along the NP symmetry axis and perpendicular to it. W is integrated over the particle surface, neglecting its end parts. The resulting surface-anchoring Ifee-energy penalty is... 

When the liquid crystal director n is aligned along the direction specified by the polar angle G and azimuthal angle (p, the anisotropic part of the surface energy - referred to as the anchoring energy function - of the liquid crystal isfs=fs G,(p). When G = Gg and

[Pg.38]

It should be noted that there is currently some discrepancy between the values of the flexoelectric coefficients derived by various methods even for the same material, in particular, in MBBA. This may be caused by difficulties in calculating the real values of the anchoring energy of the molecules to the surface, or by not allowing for the sinrface polarization of the nematic liquid crystal caused by the polar nature of its molecules [189]. 

The flexoelectric distortion in Hybrid Aligned Nematic (HAN) configuration was studied [190, 191]. In HAN cells the director is homeotropic on one of the substrates and homogeneous on the other. The measme-ments of transmitted intensity in HAN samples allow us to evaluate for MBBA both a nematic anchoring energy W for the homeotropic orientation VF 6 X 10 erg/cm and the sum of the flexoelectric moduli 11 + 33 —4.5 X 10 dyn / [191]- In an electric field normal to the initial director plane, the flexoelectric effect results in the appearance of twist deformation, observed by the rotation of linearly polarized light [190]. 

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