Zvi Yaniv

Twist impeded self-diffusion in a cholesteric☆

Abstract Molecular self-diffusion along the pitch axis of a twisted nematic is measured by its motional averaging effect on the deuterium quadrupole interaction. The diffusion constant is found to decrease as the pitch length is decreased. This result is explained by simple phenomenological considerations and is consistent with published viscosity measurements.

Reentrant Cholesteric Phase

Abstract We report on a variant of the reentrant phenomena whereby a chiral compound has been added to a liquid crystalline material to form the sequence of phases cholesteric smectic A cholesteric as the temperature is varied. We present here a phase diagram which shows the effects of different concentrations of chiral additive and describe an unusual texture which appears under a polarizing microscope in the vicinity of the transition from the smectic A phase to either the reentrant cholesteric or the cholesteric phases.

2H NMR measurements of the molecular orientational ordering in a reentrant liquid-crystalline mixture☆

Abstract Deuterium NMR measurements of the molecular orientational ordering (nematic order) are reported for a reentrant binary mixture of some alkoxy-eyanobiphenyls ( n OCB) to which small amounts of perdeuterated p -xylene have been added. The results indicate that the degree of orientational order is enhanced at the smectic A to reentrant nematic phase transition. This effect is shown to be in agreement with the predictions of a Landau-type theory of the reentrant phase transition where-by this phase transition is explained as the result of a coupling between the smectic and the nematic order parameters.

NMR Spectral patterns in magneto-aligned biaxial liquid crystals

Abstract Unique NMR spectral patterns can be obtained from biaxially ordered smectic phases where only one of the principal axes of the time averaged nuclear spin interaction is aligned by the magnetic field. It is shown how the asymmetry parameter of the interaction can be obtained directly from edge singularities in these patterns.

New Variant of Reentrant Polymorphism in Liquid Crystals

Abstract X-ray diffraction patterns and observations under the polarizing microscope made on binary mixtures of 4-cyano-4′-n-hexyloxybiphenyl (60CB) and 4-cyano-4′-n-octyloxy-biphenyl (80CB) to which the chiral material 4″-(2-methylbutylphenyl)-4′-(2-methyl-butyl)-4-biphenylcarboxylate (CE2) has been added, have revealed the occurrence of a new variant of liquid crystalline reentrant polymorphism: isotropic-cholesteric (twisted nematic)-smectic A-smectic E-cholesteric (twisted nematic)-solid. X-ray measurements of the layer spacings in the smectic A phase of such samples and similar ones where chiral CE2 has been replaced by chiral 4-cyano-4′-(2-methyl)butylphenyl (CB-15) are found to be consistent with the predictions of a recent microscopic model of the reentrant phenomena whereby the formation of molecular association pairs is an important factor determining the occurrence of the reentrant phase.