Effects of Electric Field on Liquid Crystals

One of the main reasons, if not the only reason, that liquid crystals are of great importance in display applications is their ready response to externally applied electric fields [1,2]. Their direction can be easily changed by electric fields produced by the application of a few volts across the liquid crystal cells. They are either dielectric or ferroelectric materials with high resistivities and thus consume little energy. When the liquid crystals reorient, their optical properties change dramatically because of their large birefringences. In this chapter, we will first discuss how liquid crystals interact with externally applied electric fields, and then consider their applications. 

---

Since the electro-optical effect of LCs was discovered and its extensive applications were recognized, the effects of electric field on the liquid crystal materials have been of considerable interest to both academy and industry. The direct influences of the electric field on LC include shift of the phase transition temperature, variation in order parameter, and change in symmetry [44,45]. 

Helfnch, W. Effect of electric fields on the temperature of phase transitions of liquid crystals. Phys. Rev. Lett. 24, 201-203 (1970)... 

Effect of Electric Field on Nematic Liquid Crystal (Electro-Optical Effect)... 

Polarized microscopy is particularly powerful when combined with electrical measurements on liquid crystal films (known as electro-optical measurements). From microscopic observations of the birefringence in a material as a function of different applied fields, changes in molecular orientation can be inferred, and it is possible to deduce the mechanisms by which the liquid crystal molecules respond to electric or magnetic fields. It is most common to look at the effects of the electric fields on liquid crystal materials. Although magnetic fields will also result in molecular reorientation, the effect is much weaker and therefore has not been commercially useful. 

Traditionally, the term electroconvection is used in at least four different physical contexts, of which three pertain to flows of liquid dielectrics. Thus, it is used to describe the electric field-induced flow of nematic liquid crystals, the flow of liquid dielectrics caused by the action of electric field on the space charge of ions of the appropriate sign injected in a low quantity into a fluid, or the effects of an electric field acting on the surface charge accumulated at the interface between two weakly conducting fluids. The latter process was studied by G. I. Taylor, who in the mid-1960s introduced the leaky dielectric model... 

For example, for a liquid crystal with h = 1.6 x 10 J/m, c = 3.9 x 10 J/m and a = 10 (the dielectric anisotropy in the nematic phase at zero field is about 10), the critical field is = 15 V/pm. The induced order parameter at the critical point is about 0.15. This has been experimentally confirmed [5]. At such a high field, attention must be paid to avoid the heating effect of the field on the liquid crystal cell and electrical breakdown of the material. 

Comparing the effect of an electric field on low-molecular and polymeric liquid crystals it is necessary to stress the following ... 

Independently, a highly interesting problem recently revived is that of the alignment of molecules in liquid crystals due to externally applied static and alternating electric fidds. The problem was first approached by Jezewski and Kast, and was developed by Carr, Helfrich, Wysocki et al. and many others, studying various aspects, such as dielectric loss in electric fields and anomalous alignment in the smectic phase and domains, the effect of an electric field on the temperature of mesomorphic-isomorphic phase tranritions of liquid crystals, electric... 

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

T he effect of an externally applied electric field on the alignment of molecules in liquid crystals has been studied by many investigators, and the results have frequently been conflicting. The literature regarding the effect owing to electric fields has been reviewed recently by Gray (8). 

Optical properties of organic compounds are determined by the polarization of electrons along the induced electric field. On the other hand, photochemical reactions change the chemical structures and electronic states of organic materials. Many researchers are interested in photoisomerization of azo dyes because it changes orientations of dye molecules and can be controlled with light irradiation. This effect is utilized for optically-induced alignment of liquid crystals(i, 2) and... 

---