R. Pratibha

Elasticity and Orientational Order in Some Cyanobiphenyls: Part IV. Reanalysis of the Data

Abstract 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.

Tunable optical metamaterial based on liquid crystal-gold nanosphere composite

Effect of the surrounding anisotropic liquid crystal medium on the surface plasmon resonance (SPR) exhibited by concentrated suspensions of gold nanospheres has been investigated experimentally and compared with the Mie scattering theory. The observed polarization-sensitive SPR and the red-shift in the SPR wavelength with increasing concentration of the gold nanospheres in the liquid crystal matrix have been explained using calculations based on the Maxwell Garnet effective medium theory. Agglomeration of the gold nanospheres that could also lead to such a red-shift has been ruled out using Atomic force microscopy study of thin nanoparticle-doped smectic films obtained on solid substrates. Our study demonstrates feasibility of obtaining tunable optical bulk metamaterials based on smectic liquid crystal - nanoparticle composites.

Colloidal gold nanosphere dispersions in smectic liquid crystals and thin nanoparticle-decorated smectic films

We demonstrate that the layer structure and elasticity stabilize dispersions of colloidal nanoparticles in smectic liquid crystals. We use surface plasmon resonance spectra of gold nanospheres to probe their spatial distributions in the bulk of smectic lamellae. The average interparticle distances between the well-separated nanoinclusions in thin (<100 nm) smectic films are probed by atomic force microscopy. We show that limited motion of nanoparticles across layers due to the one-dimensional quasi-long-range solid-like structure and their elasticity-mediated interactions preclude irreversible aggregation and enhance the stability of the ensuing nanoscale dispersions in thermotropic smectic liquid crystals.

Evidence of columnar structure in compounds composed of banana-shaped molecules

Abstract The synthesis and mesomorphic properties of several achiral compounds composed of banana-shaped molecules are described. These compounds belong to three different homologous series. They have been examined by polarized light microscopy, differential scanning calorimetry, x-ray diffraction studies and electro-optic investigations. In the homologous series containing a lateral methyl group, three different mesophases viz. B6, B1 and B2 have been observed. In the compounds containing a chloro group, the B1 phase predominates, while the fluoro substituted compounds show the B2 phase.

Biaxial smectic A phase in homologous series of compounds composed of highly polar unsymmetrically substituted bent-core molecules

Several compounds belonging to three new series of five-ring banana-shaped esters, which are unsymmetrically substituted with respect to the central phenyl ring, have been synthesised. One of the arms of these bent-core molecules has been terminally substituted with a highly polar cyano group while the other end contains an alkoxy chain. The mesophases exhibited by these compounds have been characterised using a combination of optical polarising microscopy, differential scanning calorimetry and X-ray diffraction studies. These studies indicate that most of the compounds show two smectic A mesophases which have a partial bilayer ordering. Conoscopic experiments clearly reveal that the lower temperature smectic A phase is biaxial in nature. A structural model has been proposed for the lower temperature biaxial smectic A phase. It is argued that quartets of molecules, which are conducive to the formation of the biaxial phase, can form in the layers. The in-layer birefringence of the biaxial smectic A phase has also been measured as a function of temperature for one of the compounds.

An experimental investigation of electromechanical coupling in cholesteric liquid crystals

Abstract Cholesteric liquid crystals which have a helical arrangement of oriented chiral molecules are expected to show novel cross couplings between fluxes and forces. The most convincing demonstration of these couplings is through a dynamical effect on the structure. Though Lehmann found a rotation of the structure under a temperature gradient in 1900, there has been no subsequent experiment confirming the same. We argue that it is very difficult to obtain a sufficiently weak anchoring of the director at the solid-cholesteric interface which is a necessary condition for the occurrence of Lehmann rotation. In order to achieve a practically zero anchoring energy at the surface, we have devised a simple technique of floating essentially flat cholesteric drops in the isotropic phase. Using this configuration we study the electromechanical coupling which produces a rotation of the structure under the action of a DC electric field. Using measurements on samples with different values of the pitch the relevant ...

Effect of dispersion of gold nanoparticles on the optical and electrical properties of discotic liquid crystal

Dispersion of gold nanoparticles (GNPs) in mononitro-substituted triphenylene-based discotic liquid crystal (DLC), which shows columnar plastic phase (Colp) along with columnar hexagonal phase (Colh) at room temperature, was studied by differential scanning calorimetry, polarising optical microscopy, X-ray diffraction, dielectric spectroscopy, visible absorbance spectroscopy and IR dichroic technique. The experimental results show that inclusion of GNPs in the DLC matrix strains the columnar matrix. More ordered Colp which possesses high charge carrier mobility because of the three-dimensional positional order is still preserved even by insertion of GNPs into the columnar matrix. However, we observed a decrease in orientational order parameter (S) and increase in relaxation time (τ) for disc motion with GNPs in Colp. But there is enhancement in dc electrical conductivity by several orders of magnitude at ambient conditions with GNPs. These GNP–DLC composite systems hold great promises in many organic semiconductor device applications.

Electromechanical Coupling in Cholesteric Liquid Crystals

We report the first observation on the ‘Lehmann rotation’ of the director configuration of flat cholesteric drops subjected to a DC electric field. This allows us to determine both the sign and the magnitude of the electromechanical coupling coefficient νE of the chiral liquid crystal, which is ≃ −2 × 10−5 q in cgs units, where q is the wavevector of the helix, whose sign is positive (negative) for a right (left) handed cholesteric.

Studies on High Strength Defects in Nematic Liquid Crystals

Abstract 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.

Occurrence of a transition between two polar orthogonal smectic phases in a new homologous series of novel bent-core compounds

Two new homologous series of isomeric achiral five-ring bent-core compounds containing 3,4-disubstituted biphenyl moiety have been synthesised and the mesophases that they exhibit characterised. Compounds of series A are obtained from 4-hydroxybiphenyl-3-carboxylic acid while those in series B are prepared by reversing the position of the hydroxyl and carboxylic groups. This change seems to have a profound effect on the mesophase properties. Interestingly, for compounds of series A containing the long terminal chain, a transition between two polar orthogonal smectic phases has been observed. The observed mesophases have been characterised using a combination of polarised light optical microscopy, differential scanning calorimetry, X-ray diffraction studies and electro-optical measurements. This is the first example of a transition between two partial bilayer polar orthogonal smectic phases. In addition, the effect of a fluoro substituent ortho to the strongly polar terminal cyano group on the mesophase has also been examined.