Jayashree Saha

Bio: Jayashree Saha is an academic researcher from University of Calcutta. The author has contributed to research in topics: Liquid crystal & Molecular dynamics. The author has an hindex of 6, co-authored 27 publications receiving 96 citations. Previous affiliations of Jayashree Saha include Visva-Bharati University & Central University, India.

Monte Carlo simulations of spontaneous ferroelectric order in discotic liquid crystals.

Abstract: The demonstration of a spontaneous macroscopic ferroelectric order in liquid phases in the absence of any long-range positional order is considered as an outstanding problem of great fundamental and technological interest. We report here off-lattice Monte Carlo simulations of a system of polar achiral disklike ellipsoids which spontaneously exhibit a novel ferroelectric nematic phase which is a liquid in three dimensions, considering attractive-repulsive pair interaction suitable for the anisotropic particles. At lower temperature, the ferroelectric nematic phase condenses to a ferroelectric hexagonal columnar fluid with an axial macroscopic polarization. A spontaneous ferroelectric order of dipolar origin is established here for the first time in columnar liquid crystals. Our study demonstrates that simple dipolar interactions are indeed sufficient to produce a class of novel ferroelectric fluids of essential interest. The present work reveals the structure-property relationship of achieving long searched ferroelectric liquid crystal phases and transitions between them, and we hope these findings will help in future development of technologically important fluid ferroelectric materials.

A lattice model Monte Carlo simulation of cholesteric liquid crystal with freely rotating molecules

Abstract: An earlier Monte-Carlo simulation of a cholesteric configuration of molecules is generalised in the present work allowing for full rotational degrees of freedom. The effect of temperature on pitch has been studied and the results are compared with the results obtained from mean field and plane rotator model simulation studies.

Role of end chains in the reentrant behavior of a nonpolar system.

Abstract: In this paper we propose to estimate the role of chain conformations in bringing about reentrant polymorphism in a nonpolar sample of mesogens. To this end we also show that the reentrant phenomenon is built in the McMillan model, if one explicitly incorporates the e8'ect of tail-chain conformations in the molecular potential instead of treating the chains as an extension of the rigid part. The model is next utilized to predict the single reentrance I-N-Sm-N„with lowering of ternperature in a nonpolar system. I. INTRQDUCTIOlV Since the discovery of reentrant polymorphism in a binary mixture of two polar compounds, ' a large number of investigations have been carried out in this field. Liquid-crystalline systems exhibiting such reentrant polymorphism I X Sm X„-, a-nd -the double (or triple) reentrant phase sequence consist of organic molecules usually with three or four aromatic rings with ester linkages and having polar cyano or nitro-end groups. Apart from pure compounds, ' such reentrant polymorphism with a lowering of temperature is also exhibited by binary mixtures of polar-polar, ' ' polar-nonpolar, ' or even by nonpolar-nonpolar compounds. "' The last one however only shows the single reentrant phase sequence. Further, as a given homologous series is ascended the reentrant phase sequence is shown by the higher homologs (e.g. , usually by the octyloxy, nonyloxy, etc. members) which are neither very short nor very long. These findings indicate that the dipolar force plays a crucial role in the reentrant polymorphism. There are theoretical models' ' that emphasize this role of dipolar force to bring about the phase sequence. A number of theories assume some sort of bimolecular organization (dimers)' ' or even trimers or n-mers with antiparallel association that compensates (not always fully) the dipole moments. This system of dimers or n-mers together with existing monomers can bring about the desired phase sequence. Such theories can also explain the variation of layer thickness' ' as an interdigitation or reorientation' (in the case of constant layer thickness) of the component systems in a mean-field approach. These theories essentially show that the high-temperature smectic phase is an induced phase and the reentrant nematic phase is brought about by a competition between two incommensurate lengths. This two-length theory is also one of the main ingredients of the Landau theory of the phase transition developed by Prost and Barois. ' A further review of theories and experiments for reentrant nematic phases can be found in Refs. 24 and 25. However, the occurrence of single reentrance I-X-SmX„(Refs. 11 and 12) in a binary mixture of nonpolar compounds cannot possibly be due to such dipolar forces. In fact, for such systems, observation' on layering thickness shows no hint for the kind of dimerization as is found in reentrant systems with polar compounds. The above result, together with the observation that in all the systems discussed the reentrant phase usually occurs for certain members (usually eighth or ninth) of the homologous series, indicate that the tail chains should have some active role in bringing about the reentrance. Dowell proposed a lattice model for condensed phases that predicted reentrance in a single-component nonpolar system. In that model, it is seen that a segregated packing of cores beside cores and chains beside chains occurs with a lowering of temperature leading to a usual smectic phase. With further lowering of temperature, the chains become less flexible, and packing differences between the rigid cores and tail chains decrease. Thus, the need for segregated packing of rigid cores with cores (and tail chains with tail chains) is overcome by entropy of unsegregated packing, leading to the disappearance of the smectic-A phase and the appearance of the reentrant nematic phase. In other words, the Dowell model holds the change in chain configuration responsible for reentrance. We present in this section some results we have obtained on the role of chains in bringing about reentrance in a single-component nonpolar sample. As in the case of polar systems, ' ' our present study is also based on a molecular mean-field approach and the starting point, so to say, is again the McMillan model. Here, however, we show that the reentrant phenomenon is built in the McMillan model, if one explicitly incorporates the efFeet of tail-chain conformations in the molecular potential instead of treating the chains as an extension of the rigid part. The model is used next to predict the single reentrant phase sequence I-iV-Sm-X„with lowering of temperature in an idealized nonpolar system. Our results also indicate that chain conformations alone can give rise to reentrance for certain intermediate members of a homologous series only under some very restrictive conditions.

Minimal Coarse-Grained Modeling toward Implicit Solvent Simulation of Generic Bolaamphiphiles.

Abstract: A dual-site dimer model of bola or bipolar amphiphiles is developed based on an earlier single-site model of monopolar amphiphiles [Dey, S.; Saha, J. Phys. Rev. E 2017, 95, 023315]. This model inco...

A simulation study on multicomponent lipid bilayer

Abstract: Simulation of a multicomponent lipid bilayer having a fixed percentage of cholesterol is done to study phase transition leading to domain formation. The concept of random lattice has been used in simulation to account for the coupling between the internal and translational degrees of freedom of lipid molecules. Considering a canonical ensemble, dissimilar lipid molecules are allowed to exchange their positions in the lattice subject to standard metropolis algorithm. The steps involved in the process effectively takes into account for the movement of sphingolipids and cholesterol molecules helping formation of cholesterol rich domains of saturated lipids as found in natural membranes.

Phase transitions in liquid crystals

Abstract: Mesogenic materials exhibit a multitude of transitions involving new phases. Studies of these phases are of importance in a wide range of scientific fields and as such have stimulated considerable theoretical and experimental efforts over the decades. This review article presents a comprehensive overview until this date of the developments in this subject. An attempt is made to identify the essential key concepts and points of difficulty associated with the study of phase transitions. The article begins with a brief introduction about the symmetry, structure and types of liquid crystalline phases. This is followed by a discussion of the distribution functions and order parameters which are considered as the basic knowledge essential for the study of ordered phases. A brief discussion of the thermodynamic properties at and in the vicinity of phase transitions, which are required to understand the molecular structure phase stability relationship, is given. The most widely used experimental techniques for measuring these transition properties are critically examined. The remaining parts of the article are concerned with the current status of the theoretical developments and experimental studies in this field. The application of the various theories to the description of isotropic liquid-uniaxial nematic, uniaxial nematic-smectic A, uniaxial nematic-biaxial nematic, smectic A–smectic C phase transitions are reviewed comprehensively. The basic ideas of Landau–de Gennes theory and its applications to study these transitions are discussed. Since the formation of liquid crystals depends on the anisotropy in the intermolecular interactions, questions concerning its role in the mesophase transitions are addressed. The hard particle, Maier-Saupe and van der Waals types of theories are reviewed. The application of density functional theory in studying mesophase transitions is described. A critical assessment of the experimental investigations concerning reentrant phase transitions in liquid crystals is made and the factors which impede its proper understanding are identified. A survey is given of existing computer simulation studies of the isotropic to nematic transition, the nematic to smectic A transition, the smectic A to hexatic S B transition, the smectic A to reentrant nematic transition, and transitions to the discotic phase. The current status of the study of phase transitions involving hexatic smectic, cholesteric, polymeric and ferroelectric liquid crystals is outlined. Finally, a range of unexplored problems and some of the areas which are in greatest need of future attention are identified.

First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics.

Abstract: We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

Thermotropic columnar mesophases from N–H⋯O, and N⋯H–O hydrogen bond supramolecular mesogenes

Abstract: The structures of supramolecular columnar mesophases that depend on the presence of either N–H⋯O, or N⋯H–O– hydrogen bonds (subsumed under the term ‘N|H|O-bonds’) are reviewed. The thesis is supported that supramolecular mesogenes bridge different fields of liquid crystal research, offering the opportunity to create a unified picture of thermotropic, and lyotropic mesomorphism. With the examples of N|H|O hydrogen bond mesogenes the molecular structures of the respective mesogenes are presented, and general principles to rationally construct such molecules are discussed. A connection is drawn to systems from isolated columns as found in low molecular weight organogels. Perspectives, and outlooks for possible applications of N|H|O hydrogen bonded columnar phases are given.

Flexible lipid bilayers in implicit solvent

Abstract: A minimalist simulation model for lipid bilayers is presented. Each lipid is represented by a flexible chain of beads in implicit solvent. The hydrophobic effect is mimicked through an intermolecular pair potential localized at the \"water\"/hydrocarbon tail interface. This potential guarantees realistic interfacial tensions for lipids in a bilayer geometry. Lipids self-assemble into bilayer structures that display fluidity and elastic properties consistent with experimental model membrane systems. Varying molecular flexibility allows for tuning of elastic moduli and area per molecule over a range of values seen in experimental systems.

古细菌(Archaebacteria)表面糖蛋白

Abstract: 古细菌是一类独特的生物类群,由于它们的极端生活环境及在生物进化中的特殊地位,近年来越来越引起生物学界的密切关注。许多革兰氏阴性古细菌的细胞没有真细菌所特有的胞壁质囊及胞外膜,质膜外仅有一层