Electrically Switched Bistable Structures

The bistable states observed in nemato-cholesteric mixtures (Ae 0) could be divided into two parts [57, 58] transitions between two ordered states of minimum energy separated by an energy barrier [59] and order-disorder transitions with the switched-off state in the form of an almost transparent spiral structure or a scattering structure [60-64]. 

Berreman and Heffner [59] considered the cholesteric Grandjean texture with tilted director orientation on the boundaries, Fig. 6.16. In the absence of the tilt, the free energy g is minimum at the following thicknesses d of the Cano wedge  

Another type of bistability, called in [57] thermodynamical bistability, is observed, if one of the bistable states is disordered, resulting in strong light scattering [60-64]. Consider, for example, unwinding the cholesteric structure with a homeotropically oriented director at the boundaries. Fig. 

FIGURE 6.16. Bistable switching in long-pitch cholesterics with a tilt of the director o- (a) Tilted states with n/2 turns in zero field, = 55 . (b) Free energies g as functions of thickness to pitch ratio d/Po at zero field, dlPoY = 0.89 is the operating point, (c) g(d/PoY in an electric field versus reduced volteige U/Ufi Uf is the Frederiks transition threshold, Ae 0. 

FIGURE 6.17. Thermodynamic bistability in cholesteric liquid crystals with homeotropically oriented director at opposite boimdaries (a) scattering state (b) transparent homeotropic state and (c) intensity curve of the cell, placed between crossed polarizers, Ae 0. 

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The spontaneous molecular polarization of ferroelectric liquid crystals, arising from their structure when constrained in small cell gaps, results in unique features that can be exploited in display devices. A low electric field of only a few volts can switch the ferroelectric liquid crystal between two equally stable states with opposing polarization directions. This is commonly referred to as bistability. In contrast, nematic displays generally require the electric field to maintain the ON state. The power required to run ferroelectric liquid crystal displays is consequently much less than that required for a nematic display. Since active switching is used in both directions, ferroelectric liquid crystals can switch hundreds of times faster than a... 

All ferroelectric materials are also piezo- and pyroelectric, although there are no known examples of ferroelectric LB films. However, bulk phase phthalocyanine (2) is known to undergo a ferroelectric transition at low temperature. By modifying the structure to achieve reversible spontaneous polarization upon application of an external electric field, it may be possible to produce ferroelectric LB films of such materials at more accessible temperatures. Applications for materials of this type include bistable switching devices and memory storage systems. 

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