Field untwisting, cholesteric helix

FIGURE 6.14. Schematic representation of the untwisting of a cholesteric helix by a magnetic field. 

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 a planar texture, a stepwise change in the pitch with an increase in the field is predicted... 

FIGURE 6.15. Illustration of the field-induced untwisting of a cholesteric helix in a thin cell with a planar orientation by the walls. 

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. 

The optical properties of cholesteric liquid crystals are very specific and are determined by the pitch and arrangement of the axis of the helix and the polarization of the incident light. In an external field changes occur both in the direction of the axis of the helix (texture transitions) and in its pitch (untwisting of the helix). Before considering these field variations let us give a brief account of the optical properties of cholesteric liquid crystals in the absence of a field. Comprehensive reviews of the topics have been given recently in [1, 2]. 

Stepwise untwisting of the helix, by an electrical field perpendicular to its axis, has been observed for the case of a planar cholesteric texture with As > 0, and with strong anchoring of the molecules to the limiting surfaces... 

The linear-in-field fiexoelectric coupling, discussed in Section 4.5, for nematics can modity the threshold field for helix untwisting in cholesterics. The flexo-effect stabilizes the helical structure, thus, it results in a decrease of an apparent dielectric anisotropy [40]... 

If the cell walls are treated to produce a homeotropic orientation in a cholesteric liquid crystal with As < 0, for a particular relation between the thickness and the helical pitch (1 < Po/d < 2) a new texture appears under the influence of the electric fleld. These are the bubble domains [77] mentioned in Section 6.1.1. When there is no field present the liquid crystal has a homeotropic nematic structure, since the helix is untwisted by the walls. The application of a low-frequency field induces an electrohydrodynamic instability. After the removal of the field a new stable texture appears in the form of cholesteric bubbles dispersed through the homeotropic nematic phase. Thus a memory is created, while erasure of the information can be performed either by a high-frequency field or by the mechanical... 

Figure 4.2. Field induced untwisting of a cholesteric liquid crystal (a) ideal sinusoidal helix (b) increase of pitch and deviation from ideal helix (c) complete aligtunent along applied field and infinite pitch.

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