Conoscop

Fig. 15 Conoscopic observation of the alignment of azobenzene mesogens as a function of change in incident angle. From A to F before irradiation, after irradiation at incident angles of...

Figure 25.9 Polarizing optical micrographs of the equimolar mixture of 14 and 16 in the smectic A liquid crystalline state (a) homeotropically aligned monodomain is formed on a glass substrate when the sample is cooled slowly from the isotropic state at the rate of 3° C min Inset is a conoscopic image (b) unaligned polydomains are formed on a glass substrate when the sample is cooled from the isotropic state at the rate of 20°C min ...

Figure 11.18 Predictions of the tumbling parameter A as a function of reduced concentration C/C2 from the Smoluchowski equation for hard rods with the Onsager potential. The exact result from the spherical-harmonic expansion is shown, compared to approximate results from an analytic formula and from the perturbation expansion of Kuzuu and Doi. The open circles (O) are estimates from the periods of shear stress oscillations in transient shearing flows for PEG solutions (see Walker et al. 1995), and the closed circle ( ) is from a direct conoscopic measurement of Muller et al. (1994).

The direction of oscillation can be experimentally determined with a polarizing microscope in the conoscopic mode. Colored rings are observed. The direction of curvature points perpendicular to the x-axis of the crystal, that is, perpendicular to the direction of oscillation. 

In studies of the optical properties of the liquid crystal phase the microscope may be arranged for orthoscopic observations between crossed polars, or it may be arranged for conoscopic observations. These two arrangements are shown in Figure 4.4. 

Figure 4.4. Polarizing microscope for orthoscopic (a) and conoscopic (b) observations.

While in orthoscopic studies the parallel incident lights are desirable, in conoscopic examinations high-aperture condenser and objective are used so that a wide-angle cone of light is allowed to pass through the sample. The cone is composed of bundles of parallel rays along different directions. 

Figure 4.11. Determination of optical sign by conoscopic examination with insertion of a compensator.

The often observed textures for Sa are the focal-conic fan texture and the homeotropic texture. When the molecular orientation is homeotropic, the optic axis is perpendicular to the film surface so that the preparation appears black (pseudoisotropic) on a crossed polarizing microscope. As with the homeotropic nematics, the homeotropic Sa phase can be differentiated from the true isotropic phase by conoscopic observations. Further more, if the cover slip of the homeotropic Sa preparation is slightly moved, the orientation is disturbed resulting in oily streaks in form of bright bands. The homeotropic Sa phase may thus be distinguished from the homeotropic nematic phase. 

The deformation of the director can be probed by optical transmission, electric capacitance, conoscopic pattern, etc. The experimental set-up... 

Conoscopic studies suggest that this compound in fact exhibits a biaxial nematic phase although these experiments indicated only a small opticd biaxiality. Indeed, more recently, NMR spectroscopy using selectively deute-rated materials has shown that for other materials the assignment of a biaxial nematic phase on the basis of conoscopy alone is not necessarily reliable [118], This is discussed further in Sect, 3,4,... 

Fig. 4.30. Conoscopic method with the help of a microscope and with the naked eye...

Fig. 6.6.2. Conoscopic figure demonstrating the biaxiality of the nematic phase of...

Figure 25. Eliipsoidofindicesforapositive(nj>/io) uniaxial LC. The typical conoscopic images with average beam direction perpendicular to the plates are shown for (a) homeotropic, (b) planar and (c) oblique orientation (31].

The conoscopic interference patterns are also quite useful to identify uniaxial and biaxial nematics. For a uniaxial specimen, the conoscopic image consists of a dark cross... 

E.K. Tidey, L.A. Parry-Jones and S.J. Elston, Determination of the difference of flexoelectric coefficients in a nematic liquid crystal using a conoscopic technique, Liq. Cryst. 34(2), 251-255, (2007). 

Observations of the conoscopic images formed in a monochromatic beam converging in the sample are needed to check the uniformity of alignment (Figs 22 and 23). From conoscopic images, evidence has been found for the optically positive uniaxial behavior of... 

Fig. 22. Conoscopic interference pattern given by homeotropic texture of nematic phase of copolyester prepared from a 50/25/25 (mole %) mixture of terephthalic acid, pyrocate-chol and methylhydroquinone ...

With the aid of conoscopic observations, Finkelmann has also proved the positive uniaxial character of comb-like polymers ... 

A thin film of the LC-elastomer with m=3 (x=0) has been stretched approximately 20 % at room temperature (KT ) in the LC-state and then observed in the polarizing microscope using the conoscopic method. 

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