Polarization field and orientational order in liquid crystals

Optical birefringence is a conveniently measurable quantity that is often used for determining the orientational order in uniaxial liquid crystals. The orientational order parameter is given by S = αe − α0/ α|| − α⊥ where αe and α0 are the polarizabilities of the medium corresponding to the extraordinary and ordinary refractive indices, α|| and α⊥ are the corresponding values for a perfectly oriented medium with S = 1

Role of molecular dipoles in liquid crystals

A vast majority of compounds exhibiting liquid crystalline phases have polar molecules. The nematic director is apolar, even when the molecules have cyano or nitro end-groups, as neighbouring molecules tend to have an antiparallel short-range order. This leads to a partial bilayer structure in the smectic A phase of rod-like molecules as also many interesting phase transitions. The latter can be understood on the basis of a change over to a polar short-range order as the density of the medium is increased. If the molecules have bent-cores and cyano end-groups, a partial bilayer biaxial smectic A phase is also found, implying the formation of quartets with an apolar structure. Lateral dipolar components lead to a tilting of molecules in smectic phases. This can be understood on the basis of the off-axis locations of such dipoles. A mean field theory of smectic C liquid crystals based on this mechanism gives results which reflect experimental trends. In appropriate cases, the possibility of occurrence of a cone-phase consisting of double-tilt cylinders is also pointed out

Studies on high strength defects in nematic liquid crystals?

We recently reported the discovery of stable disclinations of strength ±3/2 and ±2 in mixtures of several nematogenic compounds with a non-mesomorphic compound having plate-like molecules. We describe here further detailed observations on these defects, particularly on pairs of high strength disclinations

On the occurrence of point and ring defects in the nematic-nematic coexistence range of a binary mixture of rod-like and disc-like mesogens

We report some observations on the director configurations in the coexistence range of two nematic phases, NR and ND in a binary mixture of rod-like and disc- like nematogens. The director alignment is continuous across the boundary of the NR and ND phases. Point defects of strength + 1 in NR droplets in an otherwise continuous ND medium are found to be annulled by singular ring disclinations of strength - ½ Further, the NR drops which orient homeotropically at the glass plates have a point defect of strength + 1 only if their size is less than about 7 μm. Larger drops have ring disclinations of strength + ½ instead of + 1 point defects. This changeover can be understood on the basis of simple energy considerations

Statistical theory of orientational order in nematic liquid crystals

Assuming a model based on dispersion and repulsion interactions, it is shown that the orientational potential energy of a molecule in a nematic liquid crystal is expressible as, where Ui= −(u0 + u2 cos2 θi + u4 cos4 θi + u6 cos4 θi + ....), u0 = w00 + w02 cos2 θ + w04 cos4 θ + .... u2 = w20 + w22 cos2 θ + w24 cos4 θ + ...., etc., Wmn = Wnm, and θ is the angle which the long axis of the molecule makes with the uniaxial direction of the medium. Using a slightly simplified form of this function, a statistical theory of long range orientational order in the nematic state is developed. The thermodynamic properties of the ordered system are evaluated relative to those of the completely disordered one, and the conditions of equilibrium are discussed. The constants of the potential function are determined for p-azoxyanisole that lead to a theoretical curve for the degree of orientational order and a volume change at the nematic-isotropic transition point in good agreement with observations. However, the predicted latent heat of the nematic-isotropic transition is significantly higher than the experimental value suggesting that a certain degree of short range order persists in the liquid phase. The calculated latent heat of transition as well as the specific heat and the compressibility of the liquid crystal fit the experimental data when a correction factor is included in the theory to allow for the effect of short range order. The magnetic birefringence of the liquid phase gives an independent estimate of the short range order which confirms the previous calculations

Component orientational order parameter in binary mixtures

We have used the IR dichroism technique to measure the component order parameters of two different cyanobiphenyl compounds in two binary mixtures. The measurements clearly show that smectic-like short range order considerably enhances the orientational order in the nematic phase. Further, the temperature variation of the orientational order is also increased by virtue of that of the smectic-like short range order

Elasticity and orientational order in some cyanobiphenyls: Part IV. Reanalysis of the data

We present revised absolute values of the elastic constants and order parameters of several cyanobiphenyls. The calculations make use of our earlier measurements of Freedericksz threshold fields and refractive indices, and corrected values of normalizing constants, particularly in respect of the anisotropy of magnetic susceptibility. The new values agree well with some recent measurements

Molecular theory of nematic liquid crystals

The thermodynamic conditions of nematic stability are discussed on the basis of the molecular statistical theory of orientational order developed in previous papers. Theoretical calculations of the order parameter in the nematic phase and the volume change at the nematic-isotropic transition point are presented for 2-4-nonadienic acid, 2-4-undecadienic acid, p-azoxyanisole, p-azoxyphenetole and anisaldazine. The significance of the results are discussed briefly

Effect of strong electric fields on phase transitions in some liquid crystals

An electric field couples to the orientational order in liquid crystals through dielectric anisotropy and is conjugate to the order. However the ionic flow produced by the field causes a dissipation of energy in the medium rendering an experimental study of the effect of the field on the order difficult. We have devised a method of measuring the local temperature of the sample, and both the dielectric constant whose value depends on the orientational order and the conductivity which is sensitive to the short range order in the medium. We report the electric field temperature phase diagrams of pentyl cyanobiphenyl (5CB) and octyloxy cyanobiphenyl (8OCB). The results include (a) the field induced enhancement of the orientational order (b) the critical field beyond which the nematic paranematic transition is continuous and (c) the effect of the field on the nematic-smectic A transition in 8OCB. The results are analysed using the appropriate Landau theories