Pitch unwinding

The results presented in this subsection show that almost all predicted nonlinear effects were observed and studied in CLCs. The results are very promising for applications in information techniques and photonics. However, many physical mechanisms, especially concerning processes at the molecular level, are still to be solved. Another challenge is the light-induced pitch unwinding in nonabsorbing mirrorless CLC structures which is not clarified, yet. 

Intercalating agents are hydrophobic, planar structures that can fit between the DNA base pairs in the center of the DNA double helix. These compounds (ethidium bromide and actinomycin D are often-used examples) take up space in the helix and cause the helix to unwind a little bit by increasing the pitch. The pitch is a measure of the distance between successive base pairs. 

Scheme 17. A representation of the chiral columnar aggregates formed by d(pG), d(GpG), d(GpGpG), and d(GpGpGpGpGpG). The filled circles represent the sugar units (for clarity, only one sugar per tetramer is shown). The screw thread is obtained by joining the filled circles the screw pitches are in the order d(GpG) > d(GpGpG) > d(GpGpCpGpG), indicating parallel unwinding of the helical structure. Reproduced from ref. 362 (Gottarelli et al., Comprehensive Supramolecular Chemistry 1996, Vol. 9, p. 483) with permission from Elsevier Science.

PBLG is a para-magnetic material with positive magnetic susceptibility anisotropy ya. It exhibits the cholesteric phase in various solutions with a helical pitch ranging from ten to tens of microns. The unwinding effect is an approach to measure the twist elastic constant K22-... 

Ferroelectric liquid crystals (FLC) are of great interest due to their fast electro-optical response which is about 1,000 times faster than conventional twisted nematic cells [131]. The geometry used is called a surface stabilized FLC cell which utilizes a very thin gap (=2 pm) to unwind the FLC supramolecular pitch (=1-2 pm) since the bulk FLC materials do not show macroscopic polarization. This very thin gap, however, leads to difficulties in manufacturing large panels and very poor shock resistance. Researchers have proposed the concept of microphase stabilized FLC [79,109, 130] using FLC-coil diblock copolymers for electro-optical applications as shown in Fig. 15. This concept takes advantage of ferroelectric liquid crystallinity and block copolymer microphase separation since the block... 

For unwinding the helical structure, Eq. (12.29) relates the threshold coherence length to a characteristic size of the system, namely, the pitch of the helix = Po/n3-... 

Figure 11 shows the dependence of the critical unwinding voltage Vf on the cell thickness-to-pitch ratio for a cholesteric layer with perpendicular boundary orientation. For these measurements two reusable cells, having thicknesses of 7.8 and 10.9 ym, were filled with various mixtures of the nematic liquid crystal RO-TN-103 and the chiral substance CB 15.We used the Cano wedge technique to determine the pitch of the mixtures and found the simple inverse relationship... 

Fig. 11. Measured dependence of the critical unwinding voltage Vf in volts upon the cell thickness-to-pitch ratio. Homeotropic boundary orientation. Dashed line is theoretical curve for infinite medium without boundaries. Host substance is RO-TN-103.

F. 5.32 Striped texture of the lyotropic SmC analog phase in a sample of C50 with a 59 wt% of water at 36 °C and b 32 wt% of formamide at 30 °C. Due to the occurrence of unwinding lines, the pitch p corresponds to the distance between two stripes instead of only one (adapted from [20], Copyright 2013 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim. Reproduced with permission.)... 

New ferroelectric mixtures with the helix pitch Rq < 0.3 /xm and tilt angle 9 > 30° have recently been developed for the DHF effect [22, 98-101]. The helix unwinding voltage was about 2-3 V. Later on, we will show that short-pitch FLC mixtures could be also used to obtain bistability in FLC samples [103]. 

Pitch, Ro Polarization, Ps Helix unwinding field Switching time... 

Results of experiments performed by Durand et al. [5] are presented in Figure 2.15. They show excellent agreement with the theory. These authors have also verified that the critical field varies as the inverse of the pitch p. The helical unwinding can also be observed when an electric field is applied to a cholesteric liquid crystal with a positive dielectric anisotropy [6],... 

In CLCs with positive dielectric anisotropy, an electric field-induced cholesteric-nematic phase transition was theoretically predicted [45], [46] and experimentally observed [47], [48]. If the electric field E is applied perpendicular to the helix axis hot a CLC, the helix unwinds like in a magnetic field (Chapter 2). At sufficiently high field strengths, the homeotropic nematic structure is stabilized (Figure 6.3). The critical field strength E = Ecn depends on the pitch P, the dielectric anisotropy As, and the twist elastic constant K22 ... 

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