Liquid crystals dielectric measuring cell

In this chapter, we will be describing different electro-optic techniques that have been developed specifically for measurement of the flexocoeffi-cients. In these techniques, an electric field is applied to a nematic liquid crystal in a cell with well-defined boundary treatments. The net torque on the medium, which involves dielectric, elastic and flexoelectric components, is set to zero both in the bulk and at the two surfaces in order to And the equilibrium director configuration if a DC field is applied. In AC techniques, the above torques are balanced with the viscous torque. The optical properties of the medium are in turn calculated for the distorted profiles and compared with experimental measurements. The flexoelectric, and in some... 

The reorientation of the molecules has only been inferred by detailed investigation of the integral of the dielectric function Recent studies of surface layers with surface plasmons have shown that now a very narrow region next to a surface can be measured specifically, the optical constants and thicknesses of thin overlayers on a metal.We have applied this technique of surface plasmons to study the electrode surfaces of liquid crystal cells to observe the molecular properties as implied from the changes in the refractive indices First the method will be... 

In an extensive work on polysiloxane polymer liquid crystals, Finkelmann et al. examined their performance in a twisted nematic electro-optical cell. In parallel with the polymer measurements, analogous studies were carried out on low molar mass nematic compounds of equivalent dielectric anisotropy (Ae). Both the low molar mass nematic materials and the polymer side-group moieties were mixtures of methoxy- (or alkyloxy-) and chlorine-terminated benzoate esters. Cells were prepared using rubbed polyimides and in the polymer case the samples were annealed for several hours to obtain defect-free aligned textures. Measurement of the threshold voltages, for both the low molar mass compounds and the polymer, showed ... 

In order to avoid the effect of double electric layers and the electrochemical injection from electrodes, a flat or cylindrical stainless steel cell (4) is used having a suflSciently large thickness, around 1 mm. A generator (2) of pseudoconstant voltage U = 50 V, / 1 Hz) prevents fast electrolytic purification and dielectric dispersion phenomena in a liquid crystal (5). The current passed through the cell is measured with high accuracy by means of integrating electrometric amplifier (1). 

Extensive measurements of the Prederiks transition in various smectic C liquid crystals have been carried out by the Halle group [117, 118]. In experiments, the conventional sandwich cells with optically transparent electrodes were used. The director was oriented uniaxially by rubbing the electrodes and the smectic layers were tilted with respect to the cell plane (a bookshelf geometry). An electric field applied along the cell normal ( -direction), due to positive dielectric anisotropy, induces the director rotation around the normal to the smectic layers. Fig. 6.32. Two optical effects are observed ... 

Yasuda et al. performed dielectric relaxation measurements on metastable solid MBBA, using a parallel-plate capacitor. Glass plates with strips of transparent tin-oxide-conducting coating served as electrodes. The cell was mounted in a copper beryllium pressure vessel, pressurized with liquid isopentane. A similar setup was used for the study of ferroelectric liquid crystals. Ferroelectric liquid crystals have also been studied by Chandrasekhar and co-workers who used a sapphire cell setup in a high-pressure apparatus. The sample was sandwiched... 

The Helmholtz free energy represented in this equation is extremal in configurations adopted by the liquid crystal director [14], The symbols have their usual meaning 9 and (j) are, respectively, the tilt (measured from the cell normal) and twist angles of the nematic director P is the pitch length of the chiral nematic fluid and D is the dielectric displacement, which is constant through the... 

For those liquid crystals having dielectric anisotropies As > 0, an applied field of the order of a few volts d.c. over a film thickness of 23/im led to homeotropic alignment as schematically indicated in Fig. 2(b), i.e. a Freedericksz transition. In all cases the thickness of the cell was maintained with Mylar spacers. The long carbazole axis was also found to lie perpendicular to the nematic director in the case of homeotropic alignment above the critical field, since the order parameter approached zero and the system appeared to be pseudo-isotropic. All absorption measurements are made normal to the plane of the quartz plates. For K-0327 for which As < 0, and < 2) parallel alignment, there is no change in order... 

The above formulation assumes that the dielectric biaxiality de of the SmC liquid crystal may be neglected. However, it should be pointed out that although in some instances it can be acceptably negligible compared to the uniaxial approximation Ca (especially, in some materials, near the transition temperature to SmA [158, p.212]), it may nevertheless have a dominant role to play and has been incorporated into many models of cell switching, for example by Maltese, Piccolo and Ferrara [191] and Brown, Dmm and Jones [28]. Measurements of dielectric biaxiality for several SmC materials have been reported by Jones and Raynes [139]. Convenient references on the definition of these parameters for SmC may be found in the text by Lagerwall [158, p.l87] or the article by Jones and Raynes [139]. 

In the author s group dielectric cells were developed for the investigation of liquids and plastic crystals. A scheme of the setup is presented in Figure 4. The autoclave A is pressurized with compressed argon that is created with a pump station up to a maximum pressure of 300 MPa. A heating and cooling jacket around the autoclave serves to adjust the temperature. The impedance is measured automatically with a Hewlett-Packard analyzer HP 4192, controlled by a personal computer. 

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