V. Jayalakshmi

Fast Responding Robust Nematic Liquid Crystalline Gels Formed by a Monodisperse Dipeptide: Electro-Optic and Rheological Studies

Realization of mechanically robust electrically fast responding liquid crystal devices with low operating voltage is one of the current research interests. Here we report a gel system comprising a commercially available nematic liquid crystal material and a new monodisperse dipeptide liquid crystalline organogelator that results in these properties. The gels exhibit nearly 2 orders of magnitude faster switching response than the pure nematic liquid crystal while having 3 orders of magnitude higher zero shear rate viscosity, and with the attractive feature that the switching threshold voltage is hardly altered. Electro-optic and rheological studies of this system are described here.

Understanding the observation of large electrical conductivity in liquid crystal-carbon nanotube composites

We report the electrical conductivity and dielectric constant measurements in composites of single-walled carbon nanotubes (CNTs) with nematic liquid crystals, employing electrical and magnetic fields as reorienting fields. Our studies demonstrate that the large magnitude of enhancement in the electrical conductivity reported in voltage-driven reorientation measurements is not due to the inherent property of CNTs, and that dielectric breakdown and local heating effects play an important role. The calculated magnitude of the local heating effect is in agreement with the experimental observation including the nematic-isotropic transition caused by such Joule heating.

Effect of aerosil dispersions on the photoinduced nematic–isotropic transition

We report differential scanning calorimetric (DSC) and dielectric measurements on the nematic-isotropic transition in the bulk and aerosil composites of a liquid-crystal mixture having a photoactive guest azobenzene compound in a non-photoactive host, 4-n-heptyl cyanobiphenyl (7CB). The DSC scans taken at different cooling rates show that, at slower rates, the bulk displays a single peak across the transition, whereas the composites in the soft gel regime exhibit a double-peak profile. Such a double-peak profile, although seen in high-resolution ac calorimetric studies, has been observed for the first time in DSC experiments. The temperature range of the region between the two peaks is comparable to that seen in ac calorimetric experiments and has similar features. This observation is significant since the appearance of the low-temperature peak in ac calorimetric data has been explained to be due to a crossover from the random-dilution to the random-field limits. This work also constitutes the first experiments on the photoisomerization driven isothermal phase transitions in liquid-crystal-aerosil composites. The studies carried out in the absence and presence of a low-magnitude UV radiation not only bring out the standard features now established for such photostimulated phase transitions, but display a few surprises. Notable among them are that (i) the photoinduced shift in the transition temperature is a non-monotonic function of the aerosil composition and appears qualitatively similar to the dependence of the transition temperature itself, and (ii) the thermal anomaly mentioned above characterizing the crossover is also seen in the temperature-dependent as well as the temporal variation of the sample capacitance for a composite in the soft gel regime. We have also evaluated, using the temporal variation of the capacitance, the different response times associated with the UV-on photochemical process as well as the UV-off thermal back-relaxation process; the response times appear to have a similar dependence on the aerosil concentration as the transition temperature.

Probing the pore structure of a chiral periodic mesoporous organosilica using liquid crystals

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers. By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics. Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism. We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals. The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts. Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out. The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.

Magnetic anisotropy and microscopy studies in magnetostrictive Tb-(Fe,Co) thin films

This paper reports the effect of the film thickness on the magnetostrictive behavior of (Fe,Co) rich Tb-(Fe,Co) films grown on Si ⟨100⟩ by electron beam evaporation. Magnetostriction was found to decrease with an increase in film thicknesses. To understand the variation of magnetostriction with the film thickness, detailed structural, microstructural, magnetization, and magnetic microscopy studies were carried out. X-ray diffraction studies indicated the presence of two phases, viz., Tb2 (Fe, Co)17 and Fe-Co phases, for all the films. With the increase in the film thickness, the peak intensity of the Tb2 (Fe, Co)17 phase decreased and that of the Fe-Co phase increased. Magnetization studies showed the presence of strong in-plane anisotropy for all the films. In addition to this, the presence of the out-of-plane component of magnetization was also observed for the films grown with higher thicknesses. This anisotropic behavior was also validated through magnetic microscopy studies carried out along the in-plane and out-of-plane directions employing magneto-optic Kerr microscopy and magnetic force microscopy, respectively. The decrease in magnetostriction was explained on the basis of dual phase formation and complex interplay between the in-plane and out-of-plane magnetic anisotropies present in the film.

Thickness dependent structural and magnetic properties of Fe-Co-Si-B thin films

Effect of film thickness on the magnetic behavior of Fe-Co-Si-B thin films has been studied. Structural studies showed a transition from amorphous to partial crystallization with increase in film thicknesses. Magnetization measurements demonstrated presence of strong in-plane magnetic anisotropy for all the films. Angular dependent hysteresis measurements indicated presence of weak magnetic anisotropy at lower thicknesses. Transformation from weak to uni-axial in-plane magnetic anisotropy has been observed with increase in film thicknesses owing to partial crystallization and enhanced stresses.

High-Genus nematic liquid crystal droplets

We will discuss the defect structures that originate in nematic droplets with two or more handles. In these cases, the topology of the bounding surface requires the presence of defects. Our experiments elucidate where do these defects locate and how many of them populated the ground state of the system.

Electroclinic Effect in Axially Chiral Organosiloxane Liquid Crystals

The synthesis, phase properties and electro-optical response of two new axially chiral organosiloxane liquid crystalline materials are presented. The materials form broad smectic A* phases extending below room temperature and exhibit large electroclinic effects at room temperature: a 20° tilt of the optic axis can be achieved with a field of 2 V μm−1.