Browsing by Author "Shri Singh"

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Anisotropic screening in polar fluids and the extent of its accommodation by molecular perturbation theories that use sphericalized reference states
(1984) Shri Singh; Jokhan Ram; Saul Goldman
We developed a function that is sensitive to many-body screening of the anisotropy in dense molecular fluids and applied it to dipolar fluids and to ST2 water. For these fluids, the function was found to be oscillatory with distance and to become damped, for dipolar fluids, with increasing strength of the dipole moment. We then compared this function to the screening implied by the Gray-Gubbins and RAM reference state potentials. For ST2 water these comparisons suggested that a composite potential made up of equal proportions of the RAM and the Gray-Gubbins potential might be better than either the RAM or the Gray-Gubbins potentials alone. © 1984 American Institute of Physics.
Applicability of TiO2 nanoparticle towards suppression of screening effect in nematic liquid crystal
(Elsevier B.V., 2015) Satya Prakash Yadav; Mukti Pande; Rajiv Manohar; Shri Singh
We report the preliminary results observed for TiO2 nanoparticles dispersed nematic liquid crystal. We observed that dispersion of TiO2 nanoparticles (NPs) in nematic liquid crystal (NLC) has suppressed the undesired ionic effect. However unpurified nanoparticles lead to increase in the ionic conductivity, due to insertion of some more impurities. The pristine and dispersed NPs in NLC have been examined by means of optical texture, optical transmittance and dielectric spectroscopy. The diminution of ionic effects has been attributed to the strong adsorption of ionic impurities on the surface of TiO2 nanoparticles. Almost all of the ions have been adsorbed on the surface of NPs at a loading of 1 wt.% in NLC. Also we didn't find any considerable change in the transition temperature of NLC, except for 2 wt.% loading. The optical transmittance curve reveals that relaxation processes are faster than the host materials, after the field is turned off. The faster relaxation in dispersed system is ascribed to the decrease in the rotational viscosity and suppressed screening effect. The present study is helpful in understanding the phenomenon of suppressing the screening effect, caused by the ionic movements under applied field of LC based devices, without affecting the transition temperature of liquid crystalline material. © 2015 Elsevier B.V. All rights reserved.
Application of a generalised thermodynamic model to study of the ferroelectric properties of DOBAMBC and DOBA-1-MPC
(2008) Abhilasha Singh; Shri Singh
The temperature variations of tilt angle, spontaneous polarisation, helical pitch, Goldstone mode rotational viscosity and twist elastic constant of the ferroelectric smectic C (SmC*) phase near the smectic A (SmA)-SmC* transition of the ferroic mesogens DOBAMBC and DOBA-1-MPC were calculated using a thermodynamic model based on an extended Landau expansion of the free-energy density. The model free-energy density of the system is written in terms of order parameters (tilt vector ξ, polarisation P and wavevector q of helical pitch) involving eleven mean-field coefficients including bilinear, biquadratic and flexoelectric couplings between ξ and P and coefficients due to coupling between ξ and q. The values of these coefficients were determined by fitting the results of calculation with the experimental data for the pitch, tilt angle and spontaneous polarisation. A detailed analysis of the relative contribution of each individual term appearing in the free-energy density expansion is presented. Taking these values of parameters, in addition to tilt angle, spontaneous polarisation and pitch, we calculate the Goldstone mode rotational viscosity and twist elastic constant as a function of temperature. The theoretical results agree well with the experimental data for both mesogens. © 2008 Taylor & Francis.
Application of thermodynamic model to study the ferroelectric properties of nAL and nHL homologous series
(2013) Shri Singh; Shubha Singh
Using a thermodynamic model developed by us in an earlier paper [A. Singh and S. Singh, Thermodynamic model for the description of smectic C* and smectic A-smectic C* phase transition properties, Phase Trans. 83 (2010), pp. 205-222] for the description of SmC* and SmA-SmC* phase transition properties, we study, in detail, the temperature variation of spontaneous polarization P 0, tilt angle θ0 and helical pitch p of two homologous series nAL(n = 9, 10, 12) and nHL(n = 9, 10, 12, 14, 16) of ferroelectric mesogens. In this model, the free-energy density of a system is expanded in terms of three degrees of freedom-tensor orientational order, scalar smectic order ψ and polarization vector P and their coupling terms. The tilt vector ξ is not considered as an independent order parameter on the plea that governs ξ. The coupled equations for these order parameters are obtained, as usual, by the minimization of the free-energy density with respect to respective order parameters together with the criterion for the thermodynamic stability. Adopting usual method, we evaluate the relative contribution of each individual term of the expansion series and the values of P 0, θ0, and p as a function of temperature for the above-mentioned eight ferroic mesogens. We found that the theoretical results agree very well with the experimental data of all the mesogens. © 2013 Copyright Taylor and Francis Group, LLC.
Application of thermodynamic model to study the phase transition properties of surface stabilized ferroelectric liquid crystals
(Taylor and Francis Ltd., 2019) Shubha Singh; Shubham Mishra; Abhilasha Singh; Shri Singh
Based on a thermodynamic model, as developed by us [Singh A, Singh S. Thermodynamic model for the description of smectic C* and smectic A–smectic C* phase transition properties. Phase Transit. 2010;83:205–222; Singh S, Singh S. Thermodynamic model for the investigation of phase transition properties of homologous series of nAL and nHL ferroicmesogens. Phase Transit. 2014;87:427–437], we have studied relative temperature dependences of spontaneous polarization and tilt angle for three members C10, C11 and C12 of a homologous series, biphenyl alkyloxy benzoate, of surface stabilized ferroelectric liquid crystal (SSFLC) material. We found that the order parameter coupling terms make crucial contributions to the transition parameters and have to be included in free-energy density expansion. The values of these parameters depend on the length of terminal alkyl chain. A close agreement between theory and experiment has been found. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
Application of thermodynamic model to study the phase transition properties of surface stabilized ferroelectric liquid crystals
(Elsevier B.V., 2017) Shubha Singh; Pankaj Kumar Tripathi; Shri Singh
In this work, we have considered the application of thermodynamic model, as developed by us, to investigate, under electric field, the variation of relative temperature(T − Tc), where Tc represents Sm C* − Sm A* transition temperature, and spontaneous polarization with tilt angle of Sm C* phase near Sm A* − Sm C* phase transition in surface stabilized ferroelectric liquid crystals (SSFLCs). Based on the concept of Landau-de-Gennes formalism, the free-energy density of the system has been written as an expansion series in powers of relevant order parameters and their coupling terms. Adopting usual method, we have evaluated the values of spontaneous polarization (Ps) and relative temperature (T − Tc) as a function of tilt angle for seven SSFLCs materials. In case of materials exhibiting second-order transition a wide temperature variation in the tilt angle has clearly been observed. For materials showings first-order transition, the transition enthalpy has been found to be directly related to the Landu coefficient (b) associated with cubic power of orientational order parameter. We have found a close agreement between theory and experiment. © 2017 Elsevier B.V.
Carbon nanotube dispersion in nematic liquid crystals: An overview
(Elsevier Ltd, 2016) Satya Prakash Yadav; Shri Singh
The aim of present review article is to present a comprehensive and current overview of scientific advancement in liquid crystal and carbon nanotube suspension. Particular attention has been paid to the recent developments and fundamental understanding of carbon nanotube dispersion in nematic liquid crystals. The dispersion and interaction of carbon nanotube in liquid crystal matrices and more elaborately the effect of dispersion on the properties of liquid crystalline materials has been extensively discussed. Recent progress has shown that even a very minute concentration of carbon nanotube in liquid crystals can have a reflective impact on the electrical and optical properties of liquid crystals. Liquid crystals provide a distinctive environment for controlling the alignment of carbon nanotubes whereas carbon nanotubes are important for the enhancement and fine-tuning of liquid crystalline properties. Potential applications of liquid crystal and carbon nanotube suspension are briefly discussed. Conclusion and future perspectives of this rapidly emerging field is provided at the end. © 2016 Elsevier Ltd. All rights reserved.
Curvature elasticity in liquid crystals
(Elsevier, 1996) Shri Singh
The study of curvature elasticity in liquid crystals is of great practical significance and is interesting in its own right from the points of view of understanding. The purpose of this review article is twofold. First, we provide a summarising overview till date of the developments in this field. Second, we focus on the molecular theory (SSR theory), as developed by us recently, for the elastic properties of molecular-ordered phases (liquid crystals, plastic crystals and crystalline solids) and its application to various mesophases. The article begins with a brief discussion on the symmetry, structure and types of liquid crystalline phases. This is followed by a description about distribution functions and order parameters which are considered as essential basic input concepts for the study of ordered phases. The elastic continuum theory of uniaxial nematic, smectic A, biaxial nematic, smectic C, twisted nematic, chiral smectic C and columnar liquid crystalline phases are described briefly. The methods used for measuring the elastic constants of various mesophases are examined critically. In the remaining part of the article the current status of theoretical developments are reviewed. The essential ingradients of the Landau-de-Gennes theory and its application to evaluate the elastic constants of liquid crystals are discussed. Since the formation of a liquid crystal depends on the anisotropy in the intermolecular interactions, the question concerning its role has been addressed. The hard-particle, Maier-Saupe and Van der Waals (VdW) types of theories are reviewed. The theories based on the density functional method have been found useful for studying the properties of ordered phases. We have described the application of density functional theory to study the elastostatics of liquid crystals. This includes our recent work (SSR theory) which is based on weighted density functional formalism and derives an expression for the elastic free-energy of molecular ordered phases. The application of this theory to the uniaxial and biaxial mesophases are discussed. The questions related to the elastic constants of polymer liquid crystals and the computer simulation work are also addressed. Finally, a range of unexplored problems, which can be readily addressed using SSR theory, are indicated.
Dielectric and electro-optical properties of polymer-stabilized liquid crystal system
(Springer Verlag, 2016) Mukti Pande; Pankaj Kumar Tripathi; Abhishek Kumar Misra; Shashwati Manohar; Rajiv Manohar; Shri Singh
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. © 2016, Springer-Verlag Berlin Heidelberg.
Dielectric and electro-optical properties of polymer-stabilized liquid crystal. II. Polymer PiBMA dispersed in MBBA
(Springer Verlag, 2016) Pankaj Kumar Tripathi; Mukti Pande; Shri Singh
In continuation of our earlier work (Pande et al. in Appl Phys A 122:217–226, 2016), we report the results of dielectric and electro-optical properties of pure MBBA and PSLC (polymer PiBMA dispersed in MBBA) systems. The polymer networks domains formed are found to be anisotropic and are oriented in the direction of electric field for both the planar and homeotropic alignment cells. The dielectric anisotropy, optical anisotropy, response time, threshold voltage, splay elastic constant and rotational viscosity were observed for both the LC and PSLC systems with electric field. The liquid crystal properties are affected significantly with increasing concentration of polymer in pure LC material. It has been observed that the polymer networks interaction plays a major role in changing the properties of PSLC system. The effect of dielectric loss and dielectric permittivity on both pure LC and PSLC systems is also discussed. © 2016, Springer-Verlag Berlin Heidelberg.
Dielectric and electro-optical properties of zinc ferrite nanoparticles dispersed nematic liquid crystal 4’-Heptyl-4-biphenylcarbonnitrile
(Taylor and Francis Ltd., 2020) Fanindra Pati Pandey; Ayushi Rastogi; Rajiv Manohar; Ravindra Dhar; Shri Singh
In this work, comprehensive results on dielectric and electro-optical properties of pristine and zinc ferrite (ZnFe2O4) nanoparticles (NPs) dispersed in host nematic 4ʹ-Heptyl-4-biphenylcarbonnitrile (7CB) systems are reported. The dielectric and EO properties of pristine and NPs dispersed (concentrations 0.05, 0.1, 0.15, 0.2, 0.3 and 0.5 wt%) into the host nematic system have been studied. We have seen uniform NPs dispersion into nematic 7CB up to 0.15 wt% concentration of NPs by the POM. The NPs aggregation starts at 0.2 wt% dispersion. It is shown that the dielectric and electro-optical properties are strongly influenced by the concentration of NPs in host nematic and applied electric field. The dielectric anisotropy is maximum for the NPs dispersed system of 0.15 wt% concentration. The Birefringence, parallel and perpendicular components of permittivity and conductivity are found to be dependent on temperature and concentration of NPs into host matrix. In pure nematic material, small memory is seen but in NPs dispersed systems noticeable memory is found. In particular, pronounced memory is found for the dispersed system of 0.3 wt% concentration of NPs. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
Dielectric Relaxation Spectroscopy of Liquid Crystal in Nematogenic Mesophase
(Taylor and Francis Inc., 2016) Pankaj Kumar Tripathi; Rajiv Manohar; Shri Singh
In the present study, we report the dielectric behavior of a high temperature nematic liquid crystal (NLC) in both the planar as well as homeotropic alignments. To understand the mechanism of such high temperature NLC, we have performed dielectric spectroscopy in frequency range 100 Hz to 10 MHz. The different dielectric parameters, viz., effective dielectric permittivity and dielectric anisotropy of the sample have been determined with the variation of frequency and temperature. It is found that the dielectric parameters depend on dipole moment and rotation of side chain of molecule. The phase transition of this high temperature NLC has been confirmed by the DSC study. © 2016 Taylor & Francis Group, LLC.
Discotic nematic-isotropic phase transition properties for a disc-like mesogen
(2011) T.K. Lahiri; Shri Singh
A statistical mechanical perturbation theory has been used to describe discotic nematic-isotropic phase transition for a discotic liquid crystal, hexa(p-octyl phenyl ethynyl)benzene. We have employed the decoupling approximation, which introduces anisotropy in the pair correlation function, to investigate the discotic nematic-isotropic transition properties. The variation of second rank orientational order parameter with temperature has been computed and it shows qualitatively similar behavior to that of experimental studies of Aver'yanov [1]. However, the numerical value of order parameter at the transition found in the present work is larger than that of Aver'yanov's. The various thermodynamic properties have also been calculated at the transition. Copyright © Taylor & Francis Group, LLC.
Disorder, complexity and biology
(2004) Shri Singh; S. Dattagupta
[No abstract available]
Effect of alkyl chains length on ferroelectric properties of Km/n and KLLm/n homologous series
(Taylor and Francis Inc., 2014) Shubha Singh; Shri Singh
In this work, a phenomenological thermodynamic model is used to describe the temperature variations of spontaneous polarization P0, tilt angle θ0, and helical pitch p of Sm C∗ phase near Sm A-Sm C∗ phase transition in two homologous series, Km/n and KLLm/n, of ferroelectric mesogens. The free-energy density of the system is written in terms of tensor orientational order parameter Qij, scalar smectic order parameter ψ, vector polarization order parameter P, and the resulting couplings between these order parameters. The tilt vector ξ is not considered an independent order parameter, as that Qij governs ξ. Adopting usual method, we evaluate the relative contribution of each individual term of the expansion series and the values of P0, θ0 and p as a function of temperature for the eleven ferroic mesogens, and found that the theoretical results agree very well with the experimental data of all the mesogens. © Taylor & Francis Group, LLC.
Effect of carbonaceous oil palm leaf quantum dot dispersion in nematic liquid crystal on zeta potential, optical texture and dielectric properties
(Springer Medizin, 2021) Ayushi Rastogi; Fanindra Pati Pandey; Avanish Singh Parmar; Shri Singh; Gurumurthy Hegde; Rajiv Manohar
In the present work, the dispersion of oil palm leaf based carbonaceous quantum dots (OPL QDs) in nematic liquid crystal (NLC) E 48 eutectic mixture has been reported. The dispersed systems with concentrations 0.1, 0.2, and 0.3 wt% are designated respectively, as MIX 1, MIX 2, and MIX 3. The objective of this study is to analyze the results on the zeta potential, optical texture, dielectric constant, dielectric loss, conductivity, dielectric strength, relaxation frequency, specific power loss and total power loss of pure and OPL QDs dispersed nematic E 48 system. Zeta potential measurement has been performed in the solution state to ensure dispersion stability. The optical textures and dielectric results are recorded after filling the respective samples in sample cells. The core findings in the present study show that the zeta potential varies from − 23.43 mV to + 28.07 mV that signifies the stability of OPL QDs suspension in LCs. Specific power loss (SPL) and total power loss (TPL) are found to be least for MIX 1 which shows that the problem of high power consumption in LCDs can be resolved by dispersing a small weight percent concentration of OPL QDs in LC medium (MIX 1). Improved molecular alignment in the dispersed system has been observed from the textural study which finds its application in good contrast display devices. The color change in the aligned textures with temperature has been attributed to the birefringence change. The porous nature of carbonaceous OPL QDs has its application in supercapacitors. The benchmark results of this study highlight the effect of temperature and frequency on dielectric parameters for both planar and homeotropic state of E 48 LCs. OPL QDs dispersed system display increased conductivity for MIX 3. The decrease in the activation energy for OPL QDs dispersed system in comparison to pure LC material E 48 is a consequential result of the potential barrier change. The increment in the dielectric strength and relaxation frequency of OPL QDs dispersed system is noticed in comparison to pure E 48. These outcomes open the door for the applicability of present LCs in the field of both display and non-display devices like sensors, supercapacitors, low power consumption displays, energy conversion, and electrical storage devices as well as advanced smart systems. Graphical abstract: [Figure not available: see fulltext.]. © 2021, Islamic Azad University.
Effect of density variation on the Sm A–I phase transition properties
(Taylor and Francis Ltd., 2019) Sunita Kumari; Shri Singh
Based on a thermodynamic model, we investigate how the density variation influences the smectic (Sm A)–isotropic (I) phase transition. We find that the density variation shifts the smectic A–isotropic transition temperature, however, first-order nature of the transition remains the same. Here, we also examine the effect of high pressure on the thermodynamic quantities. The present analysis shows that the pressure strongly influences the enthalpy and density step at the clearing transition. The pressure dependence of the thermodynamic properties is incorporated through the pressure dependent second Landau coefficient and the coefficients which couple the nematic and smectic order parameters to the density. We find a close agreement between theoretical and experimental results. Furthermore, we discuss the implications of the results for achiral liquid crystals. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.
Effect of doping ofcd1−xznxs/zns core/shell quantum dots in negative dielectric anisotropy nematic liquid crystal p-methoxybenzylidene p-decylaniline
(MDPI AG, 2021) Ayushi Rastogi; Fanindra Pandey; Rajiv Manohar; Shri Singh
We report the effect of the doping of Cd1−xZnxS/ZnS core/shell quantum dots (CSQDs) in nematic liquid crystal p-methoxybenzylidenep-decylaniline (MBDA) at 0.05 wt/wt%, 0.1 wt/wt%, 0.15 wt/wt%, 0.2 wt/wt%, 0.25 wt/wt%, and 0.3 wt/wt% concentrations of CSQDs in MBDA. Dielectric parameters with and without bias with respect to frequency have been investigated. The change in electro-optical parameters with temperature has also been demonstrated. The increase in the mean dielectric permittivity was found due to the large dipole moment of CSQDs, which im-pose stronger interactions with the liquid crystal molecules. The dielectric anisotropy changes sign on doping CSQDs in MBDA liquid crystal. It was concluded that the CSQD doping noticeably increased the dielectric permittivity of nematic MBDA in the presence of an electric field. The doping of CSQDs in nematic MBDA liquid crystal reduced the ion screening effect effectively. This phe-nomenon is attributed to the competition between the generated ionic impurities during the as-sembling process and the ion trapping effect of the CSQDs. The rotational viscosity of nematic liquid crystal decreased with increasing concentration of the CSQDs, with a faster response time observed for the 0.05 wt/wt% concentration. The birefringence of the doped system increased with the inclusion of CSQDs in MBDA. These results find application in the field of display devices, phase shifters, LC – gratings, TIR waveguide, industries, and projectors. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
Effect of dye dispersion on the relaxation modes of smectic C* phase
(2013) Satya Prakash Yadav; Mukti Pande; Rajiv Manohar; Shri Singh
We study how the dye dispersion influences the properties of ferroelectric liquid crystal (FLC). The polarisation measurement shows the decreasing linear behaviour with the increasing concentration of dye, indicating the orientation of dye in a direction opposite to FLC molecules. Accordingly, the response time increases with increasing concentration of dye. Dielectric data clearly show the existence of two relaxation phenomena in FLC and dye-dispersed systems. The relaxation strength of the Goldstone mode, which is observed around 200 Hz, has decreased with the dispersion of dye, and the relaxation frequency of this mode shifts towards the lower frequency side, due to increase in the rotational viscosity. The second relaxation mode, observed at ∼30 KHz, arises due to the formation of domains at the surface interface. This domain mode got suppressed with the dispersion of dye into FLC. © 2013 Taylor & Francis.
Effect of pressure on spontaneous polarization and tilt angle in ferroelectric liquid crystal dobambc
(2009) Abhilasha Singh; Shri Singh
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 ASm C* transition in a ferroelectric mesogen DOBAMBC. It is shown that the pressure dependence of the tilt angle, spontaneous polarization and Sm ASm 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. © 2009 World Scientific Publishing Company.