Liquid crystal piezoelectric effects

Meyer, R.B. Piezoelectric effects in liquid crystals. Phys. Rev. Letters 22, 918 (1969). 

The third electro-optical effect using calamitic nematic liquid crystals makes use of a flexoelectric effect manifested by a curved asymmetrical nematic medium. This corresponds to piezoelectricity in crystals. The existence of flexoelectricity in a nematic phase under certain boundary conditions was predicted in the late 1960s and then confirmed experimentally several years later. However, LCDs using this effect, such as bistable nematic displays are only in the development stage and as such they will not be discussed in this monograph. 

The viscosity sensor uses a microacoustic sensor device [11], in which the piezoelectric effect in a crystal is used to excite (and detect) acoustic vibrations. For microacoustic liquid sensing, special wavetypes are used in order to avoid unwanted radiation losses due to the excitation of compressional waves in the liquid. This... 

Fig. 2 Microfluidic biosensor chip. Pictorial representation of a piezoelectric bioassay a piezoelectric acoustic wave device (such as a quartz crystal resonator) is coated with a selective, passivating layer to which an analyte-specific receptor can be immobilised. Liquid containing the analyte of interest is then passed across the surface using appropriate microfluidics, which results in selective capture of the analyte. This additional bound material in turn modulates resonance of the acoustic wave device, which is transformed into an electrical signal due to the piezoelectric effect...

For lyotropic systems, such as black lipid membranes (BLM), flexoelectric coefficients much larger than those estimated for thermotropic liquid crystals were observed as early as 1973, and were called a peculiar kind of piezoelectric effect . Later Petrov et studied this effect in detail... 

A. Jakli and N. Iber, Piezoelectric effects in liquid crystals. In A. Bukaed., Modem Topics in Liquid Crystals. World Scientific, Singapore, 1993. pp. 235-256. 

The term piezoelectric was borrowed from the physics of solids by analogy to the piezoelectric effect in crystals without center of symmetry. As a rule, the piezoelectric polarization manifests itself as a charge on the surfaces of a crystal due to a translational deformation, e.g. compression or extension. Piezo-effects are also characteristic of polar liquid crystalline phases, e.g., of the chiral smectic C phase. The polarization, we are interested now, is caused by the mechanical curvature (or flexion) of the director field, and, following De Gennes, we call it flexoelectric. 

Like solid ferroelectrics, the ferroelectric liquid crystals, particularly the FLCPs, show a pyroelectric effect and a piezoelectric effect and are capable of switching polarization direction (dielectric hysteresis). Moreover, they can switch propagating or reflected polarized light. Finally, the polar symmetry of the phase leads to nonlinear optical properties of the FLCPs such as second-harmonic generation, the Pockels effect, and the Kerr effect. These physical properties of the ferroelectric LC polymers are discussed in the following sections. 

It is well known that a nematic liquid crystal is nonpolar as a result of the free or hindered rotation of its constituent molecules around their axes. In the absence of an external field the distribution of the dipoles in an undistorted nematic liquid crystal has a nonpolar cylindrical symmetry. This is shown schematically in Fig. 4.29(a). However, as Meyer [183] has shown, a polar axis can arise in a liquid crystal made up of polar pear-shaped molecules when it is subjected to splay deformations, or in a liquid crystal made up of banana-shaped molecules subjected to bend deformations. In this case, the polar structure corresponds to closer packing of the molecules (Fig. 4.29(b)). Thus, the external mechanical deformation of the nematic liquid crystal results in the occurrence of a charge at electrodes perpendicular to the polar axis, i.e., there is a similarity to the piezoelectric effect in solid crystals. 

This effect has been called the piezoelectric effect in many publications on liquid crystals, but there is good reason for giving it a different name. The piezoelectric effect corresponds to the occurrence of a charge on the surface of the crystal when there is a translational deformation, e.g., with compression or extension. The crystal in this case must be non-centrosymmetric. An effect of this type is also characteristic of polar liquid crystalline phases, e.g., of the chiral smectic C (Chapter 7). The effect, however, in which we are interested here is caused by flexion, a purely orientation deformation in a nematic liquid crystal. Consistent with this argument [1], we will call... 

The storage effects could also be realized in polymer liquid crystals. On cooling, ferroelectric liquid crystal polymers with the electric field applied, the macroscopic polarization is frozen in the glassy state [74]. Thus, the polymer film becomes a pyroelectric and a piezoelectric. Unfortunately, the glassy state is too viscous to allow the field-induced reorientation of the polarization and the film cannot be considered to be a ferroelectric. 

The chirality of the columnar phase may manifest itself, as in other liquid crystal phases, in a pronounced circular dichroism, in an amplified specific rotation, in a piezoelectric response, and in electro-optical switching effects. The columnar organization of concentrated DNA is one of the most prominent examples of the occurrence of liquid crystal phases in chiral natural systems. 

The phase structure of the phase is at the origin of the piezoelectric effects. While low molar mass Sq liquid crystals flow under the influence of an external mechanical held, the network structure of the Sq elastomers prevents macro-Brownian motions of the mesogens and deformations with large amplitudes are feasible. On the other hand, compared to solid-state crystals, the modulus of the elastomers is smaller by orders of magnitude and, moreover, can be modified by the cross-linking density of the network. With these exceptional properties, S() elastomers offer a new class of electromechanical materials that stimulate theoretical and experimental activities. 

A liquid crystal (LC) in which the electric dipoles point in the same direction as the respective LC directors should exhibit not only a nonuniform strain but also a piezoelectric response when it undergoes one or more of the three nonuniform deformation modes that are identified as splay, bend, and twist. Accordingly, three different modes of piezoelectricity from nonuniform strain distributions were postulated for liquid crystals (Meyer 1969), but it was not clear whether the resulting piezoelectric effects were large enough to be observed in real experiments (Helfrich 1971). In the meantime, since the early concepts, a whole new field - flexoelectricity in liquid crystals (Buka and Eber 2013) - has developed from the pioneering work of Meyer and Helfrich on splay and bend deformation in liquid crystals. 

Mellinger A (2003) Dielectric resonance spectroscopy a versatile tool in the quest for better piezoelectric polymers. IEEE Trans Dielectr Electr Insul 10 842-861 Meyer RB (1969) Piezoelectric effects in liquid crystals. Phys Rev Lett 22 918-921 Newnham RE (2005) Properties of materials anisotropy, symmetry, stmeture. Oxford University Press, Oxford/New York... 

The ferroelectric effect was discovered in 1920 by Valasek, who obtained hysteresis curves for Rochelle salt analogous to the B-H curves of ferromagnetism [5.5], and studied the electric hysteresis and piezoelectric response of the crystal in some detail [5.6]. For about 15 years thereafter, ferroelectricity was considered as a very specific property of Rochelle salt, until Busch and Scherrer discovered ferroelectricity in KH2PO4 and its sister crystals in 1935. During World War II, the anomalous dielectric properties of BaTiOs were discovered in ceramic specimens independently by Wainer and Solomon in the USA in 1942, by Ogawa in Japan in 1944, and by Wul and Goldman in Russia in 1946. Since then, many ferroelectrics have been discovered and research activity has rapidly increased. In recent decades, active studies have been made on ferroelectric liquid crystals and high polymers, after ferroelectricity had been considered as a characteristic property of solids for more than 50 years. 

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. 

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