Flexoelectric effect homogeneous fields

Fig. 11.29 Conversed flexoelectric effect in cells with homeotropic (a) and homogeneous (b) director alignment and electric field applied along the cell normal. Weak anchoring energy at the bottom plate allows the flexoelectric deflection of the director 3 at the surface propagating up in the vertical direction (e = 0)...

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]. 

This effect is observed in the geometry shown in Fig. 6.7(a) when the cholesteric axis h is homogeneously oriented in the plane of the cell (along x), and an electric field is applied to the electrodes of a sandwich cell along the 2 -axis [37, 81, 82]. In this case, the helical structure, even an ideal one, is incompatible with the planar boundary conditions and splayed and bended regions form near the boundaries. Thus, according to (3.19) the flexoelectric polarization arises in those regions which can interact with the electric field. 

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