Orientation of Samples

Contrary to nematic liquid crystals there is no standard method for preparing uniform defect-free FLC orientation. In the smectic C phase both the director and smectic layers should be oriented in a proper way. We are going to discuss the various techniques of FLC alignment which provide defect-free samples with high optical quality. 

One of the standard techniques often used for this purpose is the preparation of a perfectly aligned smectic A sample. Liquid crystal often undergoes the following phase sequence isotropic phase (nematic) smectic A = smectic C. The nematic phase (if it exists) could be oriented by traditional methods, such as the rubbing of polymer films or the evaporation of inorganic films at an oblique angle ( 10 ) [135]. The quality of orientation improves for lower values of the specific heat at the nematic smectic A transition and the minimum amount of crystallization centers. 

The Langmuir-Blodgett films (LB films) of polyimides deposited on substrates prove to be useful for aligning FLC molecular axes parallel to the transfer direction without rubbing. High-quality alignment promoting the appearance of bistable states has been reported [136]. 

FLCs can be aligned using a parallel shift of one cell substrate with respect to the other. The smectic layers tend to orient perpendicular to the substrates ( book-shelf geometry ) when mechanical shear is applied perpendicular to the rubbing direction [21]. This technique is one of the most rehable and allows us to orient the smectic A phase in a broad range of cell thicknesses over an area of several square centimeters [140]. 

FLC mixtures having no smectic A phase can be oriented by cooling down the nematic phase in an a.c. electric field which defines the direction 

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Figure 9.17 Geometry of second-harmonic generation from chiral surface. Two orientations of sample are related by coordinate transformation x -> x, y - —y, and z -> —z.

Figure I. ESR spectra of gamma-irradiated Mylar film after postirradiation exposure to air (3). Dose rate 3 X 10 J Mrad min. Dose 10 Mrad. Spectra in a and b obtained at mutually perpendicular orientations of sample in magnetic field...

Proper orientation of sample probe for liquids or gases... 

FIGURE 25. Intensity of parallel component of fluorescence, Iii, as function of orientation of sample in unUxially stretched poly(vinyl alcohol) film at draw ratios of (1), 1, (2) 1.08,... 

Orientation of sample relative to longitudinal axis of model and bone ((f> = cos y)... 

A key point to gather from the textured mixtures above is that the orientation of sampling can have a large impact on mixing measures as well as the sample size. Sampling perpendicular to a striated mixture must be undertaken with care to ensure that the sample size is smaller than the striation size to obtain accurate measures of mixture quality (or lack thereof). 

Since the largest intensity of each band is observed when the beam is polarized in the direction for which the change in dipole moment for that vibrational mode is greatest, important information on the orientation of samples can be derived by installing a polarizer in the beam. For example, one of the more important applications of the use of polarized light is the measurement of the orientation of the chains in drawn or extmded polymers. A more esoteric application is the estimation of the angle at which surfactants are adsorbed on the surface of water. 

Suspension in liquid crystals is an easy-to-perform technique similar to the preparation of samples in Nujol however, the nematic substance, such as a nematic liquid crystal, which is mesomorphic at room temperature, is used as a medium (see later). The suspension of low viscosity containing approximately 5 mg of the substance studied is deposited between preliminarily roughened KBr plates as described earlier. Consequently, the obtained thin layer is additionally powdered into the plates in the direction of their roughened channels. Satisfactory and, in some cases, even good orientation of samples can be achieved by this manipulation. 

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