Nematic liquid crystals electric field, effect

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

Lopatina and Selinger recently presented a theory for the statistical mechanics of ferroelectric nanoparticles in liquid crystals, which explicitly shows that the presence of such nanoparticles not only increases the sensitivity to applied electric fields in the isotropic liquid phase (maybe also a possible explanation for lower values for in the nematic phase) but also 7 N/Iso [327]. Another computational study also supported many of the experimentally observed effects. Using molecular dynamics simulations, Pereira et al. concluded that interactions between permanent dipoles of the ferroelectric nanoparticles and liquid crystals are not sufficient to produce the experimentally found shift in 7 N/ so and that additional long-range interactions between field-induced dipoles of nematic liquid crystal molecules are required for such stabilization of the nematic phase [328]. 

Electric field effects have been studied in cholesteric liquid crystals by Muller (17) and Harper (9). Since the structure for this phase is different from the nematic phase, their work and the work discussed in this article cannot be compared at this time. 

Liquid crystals have interesting electro-optical properties. When subjected to small electric fields, reorientation and alignment of the liquid crystal molecules takes place, which produces striking optical effects because light travels more slowly along the axes of the molecules than across them. This has led to their use in optical display devices for electronic instruments such as digital voltmeters, desk calculators, clocks, and watches. Nematic liquid crystals are most commonly used in these applications. Cholesteric materials are added to provide memory effects. 

In addition to the pear-shaped molecules, bent-shaped molecules were used to illustrate the dipolar origin of the flexoelectric effects in nematic liquid crystals. It was assumed that the constituent molecules of the nematic liquid crystals are free to rotate around their axes, and in the absence of electric fields, their dipole moments average out so the net polarization of the material is zero. However, when liquid crystals made from polar pear- or banana-shaped molecules are subjected to splay or bend deformations, respectively, they can become macroscopically polar, because the polar structures correspond to a more efficient packing of the molecules. It follows from symmetry considerations that the deformation-induced fiexo-electric polarization Pa can be written as ... 

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... 

By measuring velocity of a spherical particle sinking in a liquid under gravity force the viscosity of the liquid can be found (the buoyancy effect should be taken into account). Note that in Section 7.3.3, using an electric field as an action force, the same Stokes law has been applied (with some precautions) to evaluation of velocity and mobility of spherical ions in isotropic liquids or nematic liquid crystals For large Reynolds numbers, Re = pv//ri>l the flow in no longer laminar and even becomes turbulent. Then, the convective term (vV)v should be added to the left part of the Navier-Stokes equation... 

Optics and Electric Field Effects in Nematic and Smectic A Liquid Crystals... 

If a nematic liquid crystal has negligible conductivity the results of Sections 11.2.1-11.2.5 for the Frederiks transition induced by a magnetic field may be directly applied to the electric field case. To this effect, it suffices to substitute H by E and all components of magnetic susceptibility tensor Xij hy correspondent components of dielectric permittivity tensor s,y. From the practical point of view the electrooptical effects are much more important and further on we discuss the optical response of nematics to the electric field. 

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