Shri Singh

Bio: Shri Singh is an academic researcher from Banaras Hindu University. The author has contributed to research in topics: Liquid crystal & Phase transition. The author has an hindex of 18, co-authored 97 publications receiving 1446 citations. Previous affiliations of Shri Singh include University of Guelph.

Studies of dispersed liquid crystals in binary mixtures with ionic liquid and their excitation by electric signals

Abstract: We report the results of dispersion studies of two nematogens (i) 6CHBT (4-(trans-4-n-hexylcyclohexyl)isothiocyanatobenzene) and (ii) PCH5 (4-(trans-4-pentyl-cyclohexyl)benzonitrile) in their binary mixtures with phosphonium based ionic liquid [P6,6,6,14][Ntf2] (trihexyl(tetradecyl)phosphoniumbis(trifluoromethylsulfonyl)amide). The morphology and properties of these mixtures have been investigated by using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and, their excitations by electric signals. For both the liquid crystals, nematic droplets are found to be dispersed in the ionic liquid having a particular anion, i.e., [Ntf2]. These droplets closely resemble the common droplet morphologies observed in polymer dispersed liquid crystals. Thermal and morphological investigations on the mixtures show a decrease in the peak value of the isotropic to nematic transition and broadening of the associated peak with a decrease in the compositions of nematogens in mixtures. As a result, nematic droplets in a wide temperature range ∼20 °C (i.e., from ∼16 °C to ∼36 °C) are observed for various mixtures. Electrical excitations of the mixtures show significant changes in the droplet size and nematic director orientation inside the droplets due to change in the anchoring conditions with the application of external electric pulses. The influence of the electric field on these nematic droplets and their stability over a wide temperature range suggests their potential for various device development applications.

Pressure variation of reentrant transition temperature in liquid crystals.

Abstract: High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature TAN exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (γ, λeff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling λeff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through γ and λeff which justifies the experimental pressure dependence in the reentrant transition temperature $\tilde{T}^{RE}_{AN}$ . The pressure dependence of γ and λeff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.

Effect of Pressure on Spontaneous Polarization and Tilt Angle in Ferroelectric Liquid Crystal Dobambc

Abstract: Using a thermodynamic model based on the generalized Landau free-energy density expansion, we investigate the influence of hydrostatic pressure on the spontaneous polarization and tilt angle of ferroelectric chiral smectic C (Sm C*) phase near the Sm A–Sm C* transition in a ferroelectric mesogen DOBAMBC. It is shown that the pressure dependence of the tilt angle, spontaneous polarization and Sm A–Sm C* transition temperature (Tc) can be described in terms of four pressure-dependent Landau coefficients: two achiral coefficients a and b and two chiral coefficients Λ and d. The pressure dependence of the parameter a is due to the pressure-dependent Tc. With the increasing pressure, the stability of the Sm C* phase decreases. A close agreement between theory and experiment is found.

Thermodynamic model for the electro-optical and structural properties of Sm C*A phase in antiferroelectric liquid crystal MHPOBC

Abstract: We construct a phenomenological model to describe the electro-optical and structural properties of the SmC*A phase of antiferroelectric liquid crystal MHPOBC. The free-energy density of the system is expanded in terms of three degrees of freedom—tensor orientational order Qij, scalar smectic order Ψ, and polarization vector P and couplings between these order parameters. In this expansion the tilt vector ξ is not incorporated because ξ is governed by Qij. The values of Landau coefficients are determined by comparing with the experimental data of helical pitch, tilt angle and spontaneous polarization of MHPOBC in the SmC*A phase. Taking these values we evaluate the temperature variation of the tilt angle, spontaneous polarization and pitch of the SmC*A phase for MHPOBC. It is found that the theoretical results agree very well with the experimental data for antiferroic mesogens.

Discotic liquid crystals: a new generation of organic semiconductors

Abstract: Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given (234 references).

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications.

Abstract: Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photoc...

Gas-phase spectroscopy of biomolecular building blocks.

Abstract: Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

Density-Functional Theory for Complex Fluids

Abstract: Density-functional theory (DFT) and its variations provide a fruitful approach to the computational modeling of the microscopic structures and phase behavior of soft-condensed matter. The methodology takes deep root in quantum mechanics but shares a mathematical similarity with a number of classical approaches in statistical mechanics. This review discusses different strategies commonly used to formulate the free-energy functional of complex fluids for either phenomena-oriented applications or as a generic description of the thermodynamic nonideality owing to various components of intermolecular forces. We emphasize the connections among different schemes of DFT approximations, their underlying assumptions, and inherent limitations. We also address extensions of equilibrium DFT to phenomenological theories for the dynamic properties of complex fluids and for the kinetics of phase transitions. In addition, we highlight the recent literature concerning applications of DFT to diverse static and time-dependent phenomena in complex fluids.