Flexoelectricity chiral nematics

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

Table A.9. Flexoelectric coefficients of chiral nematic liquid crystals using naming convention in Eq. (A.l). 

Figure 35. Representation of the director ( v, z) plane and the helix axis (x=h) for a chiral nematic undergoing flexoelectric distortion in negative ( <0) and positive ( >0) polarity electric fields ( ,). The z axis is out of the plane of the figure towards the observer.

The flexoelectro-optic effect in cholesterics (which is described in more detail in Chapter 7 by Rudquist and Lagerwall ) offers another way to measure 61 — 63. A tight-pitch helical structure can easily be induced in nematics by adding a few per cent of chiral dopant. Recently this technique has been adopted to test the flexoelectric response of another BC nematic, 4-cyano-l,3-phenylene bis [4-[4 -(hexyl) benzoyloxy] benzoate] (C6ban). ... 

S.P. Palto, N.J. Mottram and M.A. Osipov, Flexoelectric instability and spontaneous chiral-symmetry breaking in a nematic liquid crystal with asymmetric boundary conditions, Phys. Rev. E 75(6), 061707/1-8, (2007). 

Another bistable device is the Binem display, which does not involve flexoelectric effects but is sufficiently similar for a brief mention. This device uses a chirally doped nematic in a cell with parallel alignment directions... 

P.S. Salter, C. Kischka, S.J. Elston and E.P. Raynes, The influence of chirality on the difference in flexoelectric coefficients investigated in uniform lying helix, Grandjean and twisted nematic structures, Liq. Cryst. 36(12), 1355-1364, (2009). doi 10.1080/02678290903233991... 

Although the nematic phase is nonpolar, there are very interesting and important polar effects in this phase, in a sense analogous to piezoelectric effects in solid crystals. This was recognized by Meyer [18] in 1969. These so-called flexoelectric effects are discussed in Sec. 2.4 of this Chapter. Meyer also recognized [61] in 1974 that all chiral tilted smectics would be truly polar and the first example of this kind, the helielectric smectic C, was presented [62] in 1975. Out of Meyer s discovery grew the whole research area of ferroelectric and antiferroelectric liquid crystals, which is today a major part of liquid crystal physics and chemistry. 

The fact that the flexoelectric effect is not related to the symmetry of the medium, in particular means that it is not related to chirality. It means that the effect is common to all liquid crystals. Therefore expressions like, for instance, a flexoelectric nematic , which are encountered now and then, are nonsensical. 

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