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.

Dielectric and electro-optical properties of polymer-stabilized liquid crystal system

Abstract: In this work, we report the results of dielectric and electro-optical properties as a function of temperature for both pure liquid crystal matrix and polymer-stabilized liquid crystal (PSLC). The threshold and saturation voltages have been determined from transmission–voltage curves. We have studied the polymer domains formation in PSLC with variation of concentration of polymer in liquid crystal matrix. It is observed that the dielectric anisotropy of PSLC is significantly influenced by the polar order present in the polymer domains environment. A delicate interplay between the orientational order of liquid crystal and polymeric domains determines the molecular orientations of PSLC with respect to the director of the LC system.

Thermal and morphological studies of liquid crystalline materials dispersed in a polymer matrix

Abstract: Polymer dispersed liquid crystal (PDLC) films have been prepared by a combination of solvent-induced phase separation and thermal-induced phase separation techniques, for three mesogenic materials: 4-(undecyloxy) benzoic acid (UDBA), 4-(decyloxy) benzoic acid (DBA) and 4-(dodecyloxy) benzoic acid (DDBA) in poly(methyl methacrylate) (PMMA) as polymer matrix and tetrahydrofuran (THF) as common solvent. Differential scanning calorimetry and polarising optical microscopy are used for thermal and morphological investigations of prepared PDLCs, and findings are explained with the help of a suitable phase diagram. It is found that isotropic liquid–nematic transition temperature (TIN ) for these mesogens in the PMMA matrix decreases by about 2.8–5.6°C as compared with the pure mesogens. Methods based on the solubility parameter and the fraction of liquid crystal (LC) in LC-rich domains inside the PMMA matrix at different concentrations have been adopted to achieve an optimised condition for uniform dispersion of ...

Reentrant phase transitions in liquid crystals

Abstract: A comprehensive review of the recent developments regarding the phenomenon of reentrant phase transitions (RPT) in liquid crystals is presented. In addition to liquid crystals this phenomenon has been observed in amazingly diverse systems. A critical assessment of the experimental investigations concerning single and multiple reentrances is given. A brief account of the theoretical efforts is also given. The article ends with the identification of the factors which impede the proper understanding of the phenomenon.

Perturbation Theroy for Nematic Liquid Crystals of Axially Symmetric Molecules: Properties of a Trial System

Abstract: A statistical mechanical perturbation theory for the equilibrium properties of nematic liquid crystals is presented in which the reference potential function is non-spherical and consists of the short-range rapidly varying repulsive part of the pair potential. Calculations are made for a trial system in which molecules are assumed to interact via a pair potential which has repulsive part represented by a repulsion between hard spherocylinders and an attractive part which is function of only r 12 and Ω12 (r 12 is the center of mass distance and Ω12 the relative orientation between the two molecules), and represent approximately the interaction arising from dispersion interaction between two asymmetric molecules. Assuming that the pair correlation function g(r l2, Ω12) for a fluid of hard spherocylinders scales as g[r 12/D(Ω12] where D(Ω12) is angle-dependent range parameter the properties of the reference system and the first order perturbation term are evaluated. The agreement found between the c...

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.