Twist-reverse

Bourn (107) as 10.8 kcal mole-1 (AH4 temperature range 156—246 K solution of cyclohexane in carbon disulfide). By applying an interesting trapping technique Anet and coworkers (110) have recently measured a value of 5.3 kcal mole-1 for the barrier of the reverse twist/chair process (AH4 temperature range 72.5—74.0 K solid state). Our calculated (for the gas phase) activation enthalpies AH4 for the former process are at 73,... 

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. 

This transient effect manifests itself in a direct way in the behaviour of a twisted nematic cell (see 3.4.2). When the external field (assumed to be sufficiently strong) is switched off, the light transmission shows an optical bounce effect , i.e., it does not decrease monotonically but rises again to a peak before decaying to its off value. Calculations have confirmed that the peak in transmission corresponds approximately to a perpendicular alignment of the director in the central portion of the cell. This is caused by fluid motion, which also gives rise to a reverse-twist. ... 

As the incidence angle is made smaller, the irregular shape of the disclination lines changes. At 45° beam incidence (Fig. 5) all disclinations run in the beam direction. Again, turning the cell between crossed polarizers changes the transmission only slightly. It appears that two preferred orientations are established. The director is either in the beam direction or perpendicular to it. The parallel disclinations separate areas of reverse twist. 

Plied yams IVisted yams plied with reverse twist on standard textile ply fi ames. B to K fiber, up to /,g ply— many fiber and yardage variations. Starch. Weaving industrial fabrics and tapes in many different cloth styles also heavy cordage. 

Twisted yams plied with reverse twist on standard textile ply frames. 

If the electrodes of a cell which have been pretreated by rubbing are further coated with a layer of surfactant (e.g., lecithin), the resulting orientation of the molecules of the nematic liquid crystal will be homeotropic. A cell formed in such a way that the directions of initial rubbing are perpendicular to each other transmits light under parallel polars. In an electric field a nematic liquid crystal with negative dielectric anisotropy (e.g., MBBA) must reorient itself into a twist structure, and transmission through the cell decreases to zero (reverse twist effect) [51]. 

To avoid the occurrence of domains with reverse tilt and domains with reverse twist in a TN-LCD, the use of an alignment layer with a pretilt angle more than about 5° and the addition of cholesteric dopants to the nematic liquid crystal mixture are preferable (see Section 2.1.1). 

Fig. 4.7.9 LC molecular configuration of splayed-twist state (a) and reverse-twist state (b).

A doped dichroic mixture (0.15% cholester-ylnonanoate) produces an esthetically better cell, especially with a 90° twist, as it is free from reverse twist patches. There is little change in contrast or transmission, but the turn-off time off without cholesteryl-nonanoate was found to be about 15% shorter ( 50 ms instead of 60 ms). 

The majority of current commercial liquid crystal displays are based on the twisted nematic electrooptic effect using active matrices to give, for example, complex computer lap-top screens [13]. It is often overlooked that these displays are based on long pitch chiral nematic materials to remove so-called reverse twist, and the original papers [254, 255] referred to the effect as a positive planar cholesteric to nematic phase... 

E. P. Raynes, Twisted Nematic Liquid Crystal Electro-Optic Devices with Areas of Reverse Twist. Elec. Lett., 9, p. 101 (1973). 

Cordless drill. Look for a variable speed, reversible, twist-lock model with a good set of bits. In addition to a kit of standard bits in a range of sizes, you might want a couple of spade bits, a masonry bit, a hole cutter, and bits used for driving screws (Phillips and flathead). 

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