Switching flexoelectric

F. Castles, S.M. Morris, D.J. Gardiner, Q.M. Mahk and H.J. Coles, Ultrafast-switching flexoelectric liquid-crystal display with high contrast. Journal of the SID 18(2), 128-133, (2010). doi 10.1889/JSID18.2.128... 

As noted earlier, the incorporation of chiral groups in the liquid crystal moieties can have the effect of inducing non-linear properties, which include thermochromism, ferroelectricity, antiferroelectricity, electrostriction, and flexoelectricity. In a now classical study, Hult [82] demonstrated that it was possible for supermolecular material 34 to exhibit two-state ferroelectric switching. The remarkable material he investigated, shown in Fig. 30, was found to exhibit two hitherto unclassified mesophases between the smectic... 

Figure 6.26 The flexoelectric effect in BaTiO (a) the evolution of the potential energy curve under a homogeneous stress and in a strain gradient (b) domain switching via mechanical stress imposed by a probe (Original data from Lu et al. (2012))...

The simple expression for / means that the response time will be inversely proportional to the field, r oc 1/ PE) when there is flexoelectric coupling. This is the same dynamics as in ferroelectric liquid crystal (FLC) switching. In the cholesteric case the expression for / is somewhat more complicated and, as we will see below, it turns out that r is independent of E. 

R. Barberi, M. Giocondo and G. Durand, Flexoelectrically controlled surface bistable switching in nematic hquid crystals, Appl. Phys. Lett. 60(9), 1085-1086, (1992). doi 10.1063/1.106452... 

L.A. Parry-Jones, R.B. Meyer and S.J. Elston, Mechanisms of flexoelectric switching in a zenithally bistable nematic device, J. Appl. Phys. 106(1),... 

The liquid crystal in the Z-bistable ceU has two stable states at zero field. One is the high-tilt state shown in Figure 10.18(a). The other is the low-tilt state shown in Figure 10.18(b). The flexoelectric effect plays an important role in switching the liquid crystal between the two stable... 

FIGURE 3.14. (a) The flexoelectric cell and (b) the distortion of a homeotropi-cally oriented nematic. Curve 1 strong anchoring, = oo, electric field (E) is switched on curve 2 finite anchoring, 0 < VF < oo, E is on, H is off curve 3 finite anchoring E and H are switched on. 

We now turn to the changes that occur in the macroscopic structure of a liquid crystal due to a destabilization and reorientation of the director under direct action of an electric or magnetic field. The external field might be coupled either to the dielectric (diamagnetic) anisotropy (magnetically or electrically driven uniform Frederiks transition and periodic pattern formation) or to the macroscopic polarization (flexoelectric effect and ferroelectric switching) of the substance. The fluid is considered to be nonconductive. 

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

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