Orientation of liquid crystals

The widespread use of liquid crystals for displays in digital watches, pocket calculators, and computer screens hinges on the fact that the orientation of liquid-crystal molecules is extremely sensitive to the presence of small electric fields and to the nature of nearby surfaces. As shown schematically in Figure 10.31, a typical liquid-crystal display (LCD) contains a thin layer of nematic liquid-crystal molecules sandwiched between two glass sheets that have been rubbed in different directions with a thin nylon brush and then layered with tiny transparent electrode strips made of indium/tin oxide. The outside of each glass sheet is coated... 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

Orientation of liquid crystals depends sensitively on small electric and magnetic fields. This is the basis of liquid crystal displays for digital information. 

K. Ichimura, H. Akiyama, N. Ishizuki, and Y. Kawanishi, Command surfaces. 6. Azimuthal orientation of liquid crystals photocontrolled by an azobenzene pendent polymer, Macromol. Chem., Rapid Commun. 14, 813-917 (1993). 

Assembly and directional orientation of liquid crystals of 5CB (4-cyano-4 -pentylbiphenyl) under magnetic field provide another example of this category of self-assembly. ... 

The fact that self-assembly of thiols on gold makes it possible to create well-defined surfaces with controlled properties was used extensively to understand the forces controlling orientation of liquid crystals adsorbed at surfaces545 - 549. 

The atoms in crystals are not always fixed in a perfectly ordered position except at the absolute temperature Due to the thermal movement, atoms more or less deviate from their equilibrium position. For the same reason, the orientational order in liquid crystals is not perfect either. Because of the thermal fluctuation, the orientation and position of liquid crystals vary constantly. If the positions and orientations of liquid crystal molecules are frozen at a moment in time, the picture should look like that shown in Figure 1.5. The molecules tend to align along a preferred direction, but imperfectly. This preferred direction is defined as the director n. Because the molecules are moving all the time, they are not fixed at a constant... 

To avoid confusion, it is often desirable to use a quantity other than a to express the orientation tendency of a distribution. One possibility is the orientational order parameter S [48,49], which traditionaUy describes the mean orientation of liquid crystals. Assuming a surface-normal director. 

The orientation of liquid crystals can be influenced by an electric field. This property makes LCPs attractive for LCDs. Some liquid crystals undergo a spontaneous electric polarization even in the absence of an external electric field. This phenomenon is referred to as a ferroelectricity. 

This technique has been used before for the characterization of the orientation of liquid crystal molecules [8,9]. 

Y. J. Lim, J. H. Song, Y. B. Kim, and S. H. Lee, Single gap transflective liquid crystal display with dual orientation of liquid crystal, JptL J. Appl. Phys., part 2, 43, L972 (2004). 

Figure 11.19 Orientation of liquid crystal inside the PDLC droplets in the field-off and field-on states. Drzaic 1996. Reproduced with permission from the American institute of Physics.

Polymer networks formed in liquid crystals are anisotropic and affect the orientation of liquid crystals. They tend to align the liquid crystal in the direction of the fibrils. They are used to stabilize desired liquid crystal configurations and to control the electro-optical properties of liquid crystal devices. Polymer networks have been used to improve the performance, such as drive voltage and response times, of conventional liquid crystal devices such as TN and IPS displays. 

Figure 3. Schematic representations of the molecular orientations of liquid-crystal molecules in various types of spherulitic morphologies originating at an s=+l disclination. The ordinary and extraordinary refractive indices are designated no and ne, respectively. (Reproduced from Ref. 11 Copyright American Chemical Society).

Electric-Field-Induced Orientation. Despite the fact that electric field orientation of liquid crystals is an important technology used, for example, in liquid crystal displays, there has been almost no work on the electric field alignment of LCTs. This may be because electric fields are effective only for thin films the high field strengths required, on the order of 10 V/cm or greater, can lead to dielectric breakdown in thick samples. Nevertheless, although bulk orientation cannot be obtained via electric fields, use of electric fields does provide a complement to the surface-induced techniques described above. 

Fig. 3. Model calculations of the magnetic field orientation of liquid crystals. Calculations were conducted using equations 2 and 3. In (a) comparison is made between a nonreactive liquid crystal with a constant viscosity and a reactive liquid crystal with an exponentially...

Liquid crystals are widely used in electrically controlled liquid crystal display (LCD) devices in watches, calculators, and computer screens, as illustrated in Figure 12.2. These applications are possible because an applied electrical field changes the orientation of liquid crystal molecules and thus affects the optical properties of the device. 

Orientation of Liquid Crystals Under Mechanical Forces... 

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