A detailed analysis is given for three techniques used to determine the tilt bias angle of a nematic liquid crystal in contact with a boundary surface. Two of these methods, the crystal rotation and capacitive methods, have severe disadvantages, such as a restricted range of application, an insufficient accuracy, and sometimes require knowledge of nematic material constants which have to be determined by separate experiments. A third and more useful method, a magnetic null method, makes it possible to determine the tilt bias angle directly with only one measurement to an accuracy of 0.1°, regardless of the size of the angle or the nature of the nematic liquid crystal. Using the magnetic null method we have investigated the tilt bias angles of nematics in contact with glass substrates onto which a coating of SiO had been obliquely evaporated. We find that the tilt bias angle decreases with increasing evaporation angle (both angles measured from sample plane) until a critical evaporation angle of ?14° is reached, whereupon the director reorients out of the plane of incidence of the evaporation beam and assumes a direction normal to this plane with a 0° tilt bias. We also find that the tilt bias angle varies from compound to compound for the same evaporation parameters and is influenced by temperature and trace impurities.
Simple derivations of the elastic theory of liquid crystals with uniaxial molecular order are given using a molecular Cauchy-like approach and a more general phenomenological approach. In a second-order approximation, both methods lead to the same expression for the elastic energy density, except for the existence of one “Cauchy relation” between the elastic constants in the molecular case. Our phenomenological equation contains nine elastic constants which is two more than in Frank's earlier derivation. Our molecular equation is in agreement with Oseen's original expression. Differences between the molecular and the phenomenological approach and the justification of neglecting terms which do not contribute to the Euler–Lagrange differential equation are discussed. Higher than second-order terms are given for cholesteric liquids. In addition, the consequence of curvature-induced electric polarization for the equilibrium structure of liquid crystals is discussed.
A new, highly multiplexable liquid crystal display is described, which has a superior image quality than a twisted nematic display multiplexed at the same high level. The display cell consists of a chiral-doped nematic layer with tilted boundaries and a twist angle of ∼270°. It operates in a birefringent optical mode between two ‘‘nonconventionally’’ oriented polarizers. Performance characteristics presented for a 120×240 dot matrix panel multiplexed at a 1/120 duty cycle include driving voltages compatible with complementary-metal-oxide-semiconductor technology, 300-ms response times, a contrast ratio of 10:1 at normal incidence, and ≥4:1 inside a viewing cone of 45° from the vertical.
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