Bistable cholesteric

Yang DK (2005) Flexible bistable cholesteric reflective displays. J Display Technol 2 32-37... 

Reflective Bistable Cholesteric/Polymer Dispersion Display... 

FIGURE 8.46 Structure of the reflective bistable cholesteric LCD fabricated with a black PPy film as the electrode and the light absorbing layer. Relative reflectance as a function of applied AC voltage for the (a) chiral nematic mixture (CNM) and (b) CPD cells initially in either the planar (open symbols) or focal conic (filled symbols) state. Reflectance at 510 and 560 nm for the CNM and CPD cells, respectively, was measured after 200 Hz-AC field was applied for 20 ms and then removed. (From Kim, Y.C., et al.. Mol. Cryst. Liq. Cryst, 327,157,1999. With permission.)... 

Kim, Y.C., et al. 1999. Reflective bistable cholesteric/polymer dispersion display with a black polypyrrole electrode cast from the solution. Mol CrystLiq Cryst 327 157. 

D. W. Berreman and W. R. Heffner, New bistable cholesteric liquid-crystal display, Appl. Phys. Lett., 37, 109 (1980). 

D.-K. Yang, X.Y. Huang, and Y.-M. Zhu, Bistable cholesteric reflective displays material and drive schemes. Annual Review of Materials Science, 27, 117 (1996). 

B. Taheri, J.W. Doane, D. Davis, and W.D. St John, Optical properties of bistable cholesteric reflective displays, SID Digest 96, 39-42 (1996). 

J. Gandhi, D.-K. Yang, X.-Y. Huang and N. Mfller, Gray scale drive schemes for bistable cholesteric reflective displays, Asia Display 98, 127 130 (1998). 

In addition to the TN and IPS modes in nematic displays, himdreds of other displays methods were invented and tested over the last 30 years. Out of them the bistable nematic development based on surface flexoelectric interactions, the bistable cholesterics displays based on switching between planar and focal conic textures and the polymer dispersed liquid crystal displays found some applications. None of them, however, offer better than a few milliseconds switching time. 

Wu, S.-T., Yang, D.-K. Reflective Liquid Crystal Displays. Wiley, New York (2001) Yang, D.-K., Huang, X.-Y., Zhu, Y.-M. Bistable cholesteric reflective displays materials and drive schemes. Annu. Rev. Mater. Sci. 27, 117-146 (1997)... 

Fig. I. Schematic diagrams of the contrast versus the applied field for (a) a twisted nematic, (b) a cholesteric guest-host bistable display (Wysocki ei al., 1972 Ohtsuka and Sukamoto, 1973), and (c) a bistable LCD (Boyd et al., 1982).

D.-K. Yang, J.L. West, L.-C. Chien, J.W. Doane, Control of reflectivity and bistability in displays using cholesteric liquid-ciystals. J. Appl. Phys. 76, 1331-1333 (1994)... 

I. Dozov, A. Boissier and T. Laboureau, Nemoptic s bistable nematic liquid-crystal technology - Cholesterics or ferroelectrics are not necessary to make a bistable LCD, and using conventional nematics offers some substantial advantages, Information Display 18(1), 10 13, (2002). 

The concept of defects came about from crystallography. Defects are dismptions of ideal crystal lattice such as vacancies (point defects) or dislocations (linear defects). In numerous liquid crystalline phases, there is variety of defects and many of them are not observed in the solid crystals. A study of defects in liquid crystals is very important from both the academic and practical points of view [7,8]. Defects in liquid crystals are very useful for (i) identification of different phases by microscopic observation of the characteristic defects (ii) study of the elastic properties by observation of defect interactions (iii) understanding of the three-dimensional periodic structures (e.g., the blue phase in cholesterics) using a new concept of lattices of defects (iv) modelling of fundamental physical phenomena such as magnetic monopoles, interaction of quarks, etc. In the optical technology, defects usually play the detrimental role examples are defect walls in the twist nematic cells, shock instability in ferroelectric smectics, Grandjean disclinations in cholesteric cells used in dye microlasers, etc. However, more recently, defect structures find their applications in three-dimensional photonic crystals (e.g. blue phases), the bistable displays and smart memory cards. 

The limitations on multiplexing any rms-responding monostable liquid crystal effect have been mentioned in Section II.A. Active matrix addressing, described in Sectin IV.A, is one way of overcoming these limitations. Another is to consider alternative liquid crystal effects that are bistable, or at least non-rms responding. With such effects, the maximum number of rows that can be multiplexed is usually determined by the ratio of the frame time (the time period during which the whole picture must be refreshed or updated) to the line time (the time required to address one row of pixels). This is quite demanding of the line time a frame time of 40 msec (only 25-Hz frame rate) would require a line time of 40 /xsec for 1000 lines. Bistable behavior is associated with smectic and cholesteric phases, both of which in completely different ways have translational symmetries added to nematiclike orientational order. In this section, the ferroelectric tilted smectic devices are reviewed, while (untilted) smectic A and cholesteric devices are described in Section IV.C. 

When cholesteric liquid crystals are encapsulated in droplet form, the bistability can be preserved when droplet size is much larger than the pitch [64]. There arc two methods which are used to encapsulate Ch liquid crystals phase separation and emulsification. In phase separation [69], the Ch liquid crystal is mixed with monomers or oligomers to make a homogeneous mixture. The mixture is coated on plastic substrates and then another substrate is laminated on. The monomers or oligomers are then polymerized to induce phase separation. The liquid crystal phase separates from the polymer to form droplets. In the emulsification method [70-73], the Ch liquid crystal, water, and a water dissolvable polymer are placed in a container. Water dissolves the polymer to form a viscous solution, which does not dissolve the liquid crystal. When this system is stirred by a propeller blade at a sufficiently high speed, micron-size liquid crystal droplets are formed. The emulsion is then coated on a substrate and the water is allowed to evaporate. After the water evaporates, a second substrate is laminated to form the Ch display. 

The state of a cholesteric liquid crystal is mainly determined by smface anchoring, cell thickness, and apphed fields. The hquid crystal can be switched from one state to another by applying electric fields. There are many possible transitions among the states, as shown in Figure 10.24 [50,54]. In order to design drive schemes for the bistable Ch reflective display. 

In bistable Ch reflective display applications, it is desirable that the threshold of the transition from the planar state to the focal conic state be high, so that the cholesteric liquid crystal can remain in the planar state and the display does not exhibit flicker under colimin voltage in addressing. 

As discussed in previous sections, cholesteric liquid crystals exhibit two bistable states at zero field the reflecting planar state and the non-reflective focal conic state. They can be used to make multiplexed displays on passive matrices. In this section, we consider the drive schemes for the bistable Ch displays. 

R.Q. Ma and D.-K. Yang, Polymer stabilized bistable black-white cholesteric reflective display, SID Inti. Symp. Digest Tech. Papers, 28, 101 (1997). 

D.-K Yang, Z.J. Lu, L.C. Chien, and J. W. Doane, Bistable polymer dispersed cholesteric reflective... 

X-Y. Huang, D.-K. Yang, and J.W. Doane, Transient dielectric study of bistable reflective cholesteric displays and design of rapid drive scheme, Appl. Phys. Lett. 69, 1211 (1995). 

The effects of bistability and hysteresis in supertwisted nematic layers were first investigated in [122]. To obtain twist angles larger than 90 , nematics were doped with a small amount of an optically active material. Thus a cholesteric (or chiral nematic) with a large pitch P was created, so that the pitch value had to adjust the boundary conditions for the directors on the substrates. The corresponding texture was first discovered by Grandjean and is discussed in Chapter 6. In 1984 the display based on the Supertwist Birefringent Effect (SBE) was proposed [123]. 

The bistable states observed in nemato-cholesteric mixtures (Ae > 0) could be divided into two parts [57, 58] transitions between two ordered states of minimum energy separated by an energy barrier [59] and order-disorder transitions with the switched-off state in the form of an almost transparent spiral structure or a scattering structure [60-64]. 

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