Behavior Of Nematic Liquid Crystals Research Articles

Bestimmung elastischer Konstanten von MBBA und EBBA mit der elektrischen Leitfähigkeitsmethode / Determination of the Elastic Constants of MBBA and EBBA by Electric Conductivity Measurements

Abstract Several methods of studying the elastic behaviour of nematic liquid crystals by means of measuring the electric conductivity are discussed. The temperature dependence of the ratio of the splay and bend elastic constants to the anisotropy of the magnetic susceptibility is determined for N-(p-methoxybenzylidene) -p-n-butylaniline (MBBA), N-(p-ethoxybenzylidene) -p-n-butylaniline (EBBA) and mixtures of both.

Theory of electrohydrodynamic behaviour of nematic liquid crystals in a constant field

A theory is suggested for predicting the threshold voltage for dynamic scattering in nematic liquid crystals subject to a constant electric field. Because of experimental results showing similar instabilities in isotropic liquids, an isotropic model is used. The electrical behaviour of the liquid is assumed to have a charge carrier dissociation-recombination process in equilibrium. Under these assumptions, typical ion density curves are obtained and a non-zero charge density is found near the electrodes. The resulting electrical forces result in an instability. Calculations are performed to predict the threshold voltage for the instability. Order-of-magnitude calculations give results consistent with experimental data in liquid crystals.

Electric and optic behavior of aligned samples of nematic liquid crystals

The orientation behavior of nematic liquid crystals was studied by measurements of I vs V and I vs T combined with observation of their optical behavior. The measurements were performed at low dc voltages in the region around the Helfrich threshold for the formation of domains. A voltage region in which the electric conductivity decreases by increasing the electric field is found. It is also observed that in certain cases the whole crystal orientates itself through a cooperative motion. The change in conductivity with increasing electric field can be partially accounted for by the anisotropy properties of the material, although other phenomena, such as the flow of the material and charge injection at the electrodes, may play a role. The temperature behavior of I gives an indication that charge carrier velocity is governed by the same processes as viscosity.

A new optical method for studying the viscoelastic behaviour of nematic liquid crystals

Simple shear flow of the nematic liquid crystal MBBA is observed between crossed polarizers. The optical anisotropy is found to be a pure function of the product of shear velocity and film thickness. This agrees with the continuum theory of Leslie. The constants k 11/ k 33, γ 1/γ 2, and (γ 2-γ 1)/ k 33 are determined.

Correlation between Electrical Properties and Optical Behaviour of Nematic Liquid Crystals

Abstract The results of simultaneous electrical and optical investigations in p-azoxyanisole and di-n-heptoxy-azobenzene are presented. The role of fluid flows is discussed in connection with their importance for domain formation. The mechanism of domain formation and turbulence occurrence is found to be different for d.c. and very low frequencies than it is for higher frequencies. For strong electric fields the behaviour of azoxyanisole is very different from that of the azo compound, a fact attributed to the different sign of the dielectric anisotropy.

Optical Transient Behavior of Nematic Liquid Crystals in an Electric Field

Optical Transient Behavior of Nematic Liquid Crystals in an Electric Field