Twist direction

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Figure 8.31 Modern Kenics mixers with alternating twist directions a) cutaway view, and b) schematic with mixing striations (courtesy of Kevin G. Walsh of Chemineer, Incorporated)...

Furthermore, a cage-type peptide cyclophane bearing D-valine residues in place of L-valine moieties shows a CD band at 244 nm with of —1.1 x 10 in HEPES buffer at 30.0 °C. It becomes apparent that the four pyridinium moieties bound to the chiral o-valine residues are similarly twisted in the same direction, but the twisted direction of bridging components in the host is opposite to that evaluated for the host bearing L-valine residues. A similar asymmetric character... 

Inspection of the fabric attribute dimensions for all location points revealed that certain attributes were either absent or unrecognizable. These attributes are fiber bundle, fiber single, fiber crimp, fiber surface, fiber pattern, yarn type, and yarn twist direction. Therefore, these attributes were deleted from the statistical analysis. The remaining attributes were either present at all location points or exhibited variation. These attributes included the three (layers, 1/1, and float) whose interactive effects had direct relevance to the research hypothesis, and the following fiber and yarn attributes paired fibers, 0 twist, combined yarns, yarn system A, yarn systems A + B, yarn systems A + B + C + D, fabric distortion, red, green, and black. 

Figure 13. Re-plied yam two-ply, Z-twisted yarns plied with another of the same in an S-twist direction, No. 1156, Mound C, Etowah, 1.5 mm diameter of single yam, 12X.

The Etowah people, like the Spiro people, incorporated feathers into yams with structural permutations beyond what might have been used by the earlier Tunacunnhee-Hopewell people. Unlike the Spiro people, the Etowah people may not have spun down as described by King and Gardner (8), but rather they may have used the perhaps older tradition of core-feather-binder yam to achieve their purposes. The feathers found in the yarns of No. 1145 show no twist or twist direction. 

Twist in the yarn Most yams have filaments that are twisted. The main reason for this is that an untwisted yam is difficult to weave or knit. Two types of twists can be given to the yam, a counterclockwise twist or S twist and a clockwise twist or a Z twist. Figure 2.1 shows these twists. We can also make a ply yam by using reverse twist directions. This serves to balance out residual stresses. We can also twist together two or more plies to make a cord. Commonly, yam designation on a fiber spool provides information such as name, linear density, number of fibers and fiber type. 

S-twist See twist, direction of yam. styrene A colorless liquid with a strong order. It polymerizes rapidly when exposed to heat, light, or a peroxide catalyst polymerization may cause it to become explosive. It is toxic by ingestion and inhalation and presents a moderate fire risk. It is used in the plastic production of polystyrene, styrene butadiene rubber, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, and reinforced thermoset polyester plastic. See catalyst polymerization polystyrene plastic styrol. 

Z-twist See twist, direction of yam. zymoplastic A substance that is not an enzyme but is beheved to participate in the formation of enzymes. See catalyst, enzyme decomposition degradation, zymurgy The branch of applied chemistry that has to do with fermentation processes. See decomposition waste. 

The problem that has to be solved then is How might a right-handed supermolecular structure be formed whose bases have to be right-handed helical conformations It can be simply visualized that, for example, a left-handed threefold helix will easily turn into a right-handed threefold one, if the dimer is considered as the building unit of the chain conformation. Only the pitch has to be doubled. Different positions of 06 at adjacent residues or solvent attached to the chain may provide such dimers and lead to a reversal of the twisting direction of the cel-lulosic chain. 

These covered yams consist of elastomeric multifilament yam and a covering made from conventional (hard) multifilament yams or fibers such as polyamide, viscose, and so forth (Petmlis, 2004). In this process, the elastomeric fiber, drawn at a constant factor, is fed through a hollow spindle and covered with a covering yam from a flanged bobbin. The produced yam has a particular differential-twist stracture, caused by the different twists and twist directions between the wrapping yam and the core yam. 

The LC molecules orient on the alignment layer in a predefined in-plane and out-of-plane direction. The out-of-plane (polar) orientation is called the pretilt angle. The directional orientations and pretilt angles on both surfaces in combination with a chiral dopant added to the LC mixture determine the twist direction of the LC molecules in an LCD. Most commonly left-handed dope molecules that cause a left-handed twist sense are added to the LC mixture. Both the in-plane and out-of-plane directions are important to obtain the proper switching behaviour upon driving the display. 

Thermal optical microscopy also finds extensive use in the analysis of optically active mesophases where the molecules can form macroscopic helical structures. For instance, the pitch length for helical phases can be determined by measuring the distances between the defect lines formed through the interaction of the helical structure and the surface of the preparation. The microscope can also be used as polarimeter to determine helical twist direction for homeotropically aligned helical mesophases. 

Twist direction n. The direction of twist in yarns and other textile strands is indicated by the capital letters S and Z. Yarn has S-twist if when it is held vertically, the spirals around its central axis slope in the same direction as the middle portion of the letter S, and Z-twist if they slope in the same direction as the middle portion of the letter Z. When two or more yams, either single or plied, are twisted together, the letters S and Z are used in a similar manner to indicate the direction of the last twist inserted. 

Chiral materials, not necessarily liquid crystalline, must be added to nematic mixtures for some display devices (see Chapter 13). For example, in the twisted nematic display a quarter-helix (90°) twist is caused by the perpendicular molecular alignment at the top and bottom plates and there are two possible twist directions. In order to ensure the same twist direction throughout the device, a very small quantity of a chiral material (e.g, compound 3) is added to the nematic mixture. Significantly too, a chiral material is required in the so-called supertwisted nematic (STN) displays where a twist in the nematic director of more than 90° is employed (usually between 180° and 240°). In this case, a chiral material with an appropriate helical pitch length (P) is chosen in conjunction with a particular cell spacing (d) for example, a d/P ratio of 0.75 induces a twist angle of 270°. 

A twist cell is usually formed by placing orienting glasses on top of each other. Then twist directions at angles tt/2 and —tt/2 are equally probable. Regions with different signs of twist are observable by interference methods... 

Neel walls, twist- and tilt-reverses are the three major disclinations in actual LCDs. A Neel wall disclination may be caused by weak azimuthal anchoring. A twist-reverse is a defect in which the twist direction is opposite to the chirality of the added chiral agent. A tilt-reverse is a defect in which the inclination direction induced by applying a voltage is the opposite of the intended direction. These tilt-reverse defects may be caused by the lateral electric field generated by the fringe field of the pixel electrodes. 

To fix the twist direction of the twisted nematic (TN) display mode, several methods are used. One is to add a small amount of a chiral agent. A typical chiral pitch for the 5 pm gap TN mode is 100 pm. The other method involves the rubbing direction. The rubbing direction determines the inclination direction of the LC, and it is selected so as to avoid a splay alignment as shown in Fig. 2.32. If the pretilt... 

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