Pitch electrically induced change

Figure 10.20. Electroclinic effect in TGB phases electric field-induced change of the X-ray diffraction intensity for observation of the compound IIF2BTFO1M7 perpendicular to the pitch axis ( m-scan ) (a) TGBa phase and (b) TGBc phase (from [138]).

Field untwisting of a helix for thin cells with planar boundary conditions occurs differently from the case of an infinite cholesteric medium. For a field perpendicular to the axis of a cholesteric helix in the planar texture, a stepwise change in the pitch with increase in the field is predicted [128]. The size of the step increases with a decrease in the ratio 2electric field perpendicular to the helix axis has been observed for the case of planar cholesteric texture with a>0 and with strong anchoring of the molecules to the limiting surfaces [129]. Under these conditions the relaxation of the field-induced (i.e. untwisted) state is accompanied by the formation of spatially modulated structures in the form of strips or grids. 

F. J. Kahn, Electric-Field-Induced Color Changes and Pitch Dilation in Cholesteric Liquid Crystals, Rhys. Rev. Lett., 24, p. 209 (1969). 

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