PACS 61.30.-v-Liquid crystals PACS 61.30.Gd-Orientational order of liquid crystals; electric and magnetic field effects on order PACS 78.30.-j-Infrared and Raman spectra Abstract-Raman Scattering was used to investigate biaxiality in the nematic phase formed by the bent-core material, C5-Ph-ODBP-Ph-OC12. Linearly polarised light was normally incident on a homogeneously aligned sample, and the depolarisation ratio was measured over a 360 • rotation of the incident polarisation for the Raman-active phenyl stretching mode. By modeling the bent-core structure and fitting to the depolarisation data, both the uniaxial (P200 and P400) and biaxial (P220 , P420 and P440) order parameters, are deduced. We show unequivocally the presence of a uniaxial to biaxial nematic phase transition approximately 30 • C above the underlying smectic phase. Further, we report the temperature evolution of the biaxial and uniaxial order parameters, which increase in magnitude continuously with reducing temperature, reaching values of 0.1, −0.15 and −0.18 for P220 , P420 and P440 , respectively.

Liquid crystals are intriguing electrically responsive soft matter systems. We report previously unexplored field-induced changes in the structures of some frustrated liquid crystal phases and describe them theoretically. Specifically, we have discovered using resonant x-ray scattering that the four-layer intermediate smectic phase can undergo either a transition to the ferrielectric (three-layer) phase or to the ferroelectric phase, depending on temperature. Our studies of intermediate phases using electric fields offer a way to test theories that describe ferroelectricity in self-assembling fluids.

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While the rotation of smectic layers under an applied field may at first appear to be a relatively simple problem, the dynamic processes involved are rather complex. An applied field produces a torque on the liquid crystal director, but has no direct influence on the smectic layers. If the director is reoriented significantly, however, the layers must also reorient in order to accommodate this (the layered structure is produced by short-range molecular interactions). Indeed, if the liquid crystalline order is not maintained during the realignment then matters become even more complex. In this paper we use time-resolved x-ray scattering to investigate the realignment of smectic- A layers in thin-film devices using a magnetic field. No evidence is found for continuous rotation of the smectic layers under any circumstances in such devices, a result that is not found when using bulk samples. No evidence indicating the formation of the nematic phase is observed during realignment. A molecular-dynamics technique is used to model the system which indicates that the sample becomes significantly disorganized during the realignment process when large angular rotations are induced.

Articles you may be interested inThe effect of confinement on the stability of field induced states and on supercooling in antiferro-ferroelectric phase transitions in chiral smectic liquid crystalsThe field-induced transitions between ferri-, antiferro-, and ferroelectric liquid crystal phases are interesting because although there are only small thermodynamic differences between them, each of these phases has different electrical and optical properties. We report an irreversible field-induced transition from an antiferroelectric phase to the ferrielectric phase in a liquid crystal device, and compare it to a system in which the transition is reversible. The two systems differ mainly in their spontaneous polarization ͑120 nC cm −2 for the former and 60 nC cm −2 for the latter͒ while the optical tilt is comparable ͑29°and 25°, respectively͒. We explain the observed transitions based on the relative magnitudes of the discrete flexoelectric and spontaneous polarizations.