Helfrich deformation cholesterics

In the Helfrich deformation shown in Figure 10.26, the cell thickness of the cell is h. The cholesteric liquid crystal has the pitch P and dielectric anisotropy A (> 0). For small undulation, calculate the field threshold Eueijnch and the wavelength A of the undulation. 

W. Helfrich, Deformation of Cholesteric Liquid Crystals with Low Threshold Voltage, Appl. Rhys. Lett., 17, p. 531 (1970). 

Fig. 4.6.3. Deformation of a planar structure due to a magnetic field acting along the helical axis of cholesteric liquid crystal composed of molecules of positive diamagnetic anisotropy. A similar deformation superposed in an orthogonal direction results in the square-grid pattern (see fig. 4.6.4). (Helfrich. )...

Helfrich, W., Deformation of cholesteric liquid crystals with low threshold voltage, Appl. Phy.s. Lett., 17, 531-532 (1970). 

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