Two-state model for the SmAP_{R} phase with randomized layer polarization exhibited by some compounds with bent-core molecules.

One of the unusual liquid crystalline phases exhibited by some compounds with bent-core (BC) molecules is designated as SmAP_{R}, in which transverse polarization (P) of smectic layers with upright molecules has a random orientational distribution. Most of such compounds undergo a transition to the SmAP_{A} phase with antiferroelectric order of adjacent layers as the temperature is lowered. Second harmonic studies have shown that the medium consists of polarized domains with only a few hundred molecules, the number increasing at lower temperatures. This is in contrast to the random orientations of entire layers predicted by a few phenomenological models which have been proposed for the SmAP_{R} phase. In this paper we show that the two-state model, developed earlier by us to describe modulated phases found in compounds with BC molecules, can successfully account for all the experimental results on the SmAP_{R} phase. It is proposed that polarized domains which are made of ground state conformers, with a large bend of the aromatic cores, nucleate as circular disks in a first order transition from the isotropic phase. Excited state conformers (with a smaller bend) freely rotate about their long axes and surround the disks. The polarization in the disk has a spontaneous splay distortion which generates a texture with an expelled center of a partial disclination. The interdisk electrostatic energy is lower than the thermal energy and the disks are subject to significant rotational fluctuations, resulting in the uniaxial symmetry of the medium. Calculated equilibrium properties of the medium as functions of temperature and electric field reflect the trends seen in experiments. The model overestimates properties such as the radius of the disks as the transition to a lower temperature uniform phase is approached, and physical arguments are given to show that interlayer interactions ignored in the model become important in that regime, and should be included for an improved description of the medium.

Simple molecular model for ferroelectric nematic liquid crystals exhibited by small rodlike mesogens.

Nematic liquid crystals (NLCs) are the prime example of a liquid medium with an apolar orientational order. In the past couple of years, the ferroelectric nematic (FN) phase has been discovered in some compounds with small rodlike molecules with large longitudinal dipole moments and very restricted chemical structures, as the temperature is lowered from the NLC. We propose a simple model in which the molecules are idealized as cylindrical rods with longitudinal surface charge density waves. The usually strong electrostatic inter-rod interactions favoring antiparallel structures are shown to be subdued in magnitude, and those of parallel structures enhanced, by reducing the amplitudes of the half-waves at both ends of the rods. By introducing an additional increased amplitude of one interior wave, the energy per rod of a cluster of molecules with a pseudohexagonal order is shown to favor the ferroelectric order compared to the antiparallel order, below some value of the inter-rod separation. The model broadly accounts for the restriction in molecular structures for a compound to exhibit the FN phase. It is suggested that the weakly first-order nature of the NLC to FN transition arises from a coupling of the polar order and the density of the medium.

Challenges, experiences, and postoperative outcomes in setting up first successful lung transplant unit in India.

BACKGROUND: Lung transplantation (LT) has emerged as a definitive cure for a plethora of end-stage lung diseases (ESLDs). With improvements in immune-suppression protocols, the posttransplantation survival rates have gone up. AIM: The study reported the initial experience of the India's single largest lung transplant program on clinicopathological profile, procedures, challenges encountered, and outcomes. SETTINGS AND DESIGN: A retrospective analysis was done from data available at three centers of Institute of Heart and Lung Transplant, Gleneagles Global Hospitals across Chennai, Bengaluru, and Mumbai. MATERIALS AND METHODS: A total of 132 patients underwent lung (single or bilateral) or combined heart and lung transplant between April 2017 and March 2020. All the participants had 30 days' follow-up. Postoperative complications, graft rejection, and 30-day mortality were reported. Kaplan-Meier survival analysis and logistic regression analysis were performed. STATISTICAL ANALYSIS USED: Kaplan-Meier survival and binary logistic regression was performed. RESULTS: Interstitial lung diseases, 65.91%, were the most common diagnosis. Bilateral LT (81.3%) was the most common type of LT performed. Grade III primary graft dysfunction was observed in 16 (12.1%). Distal airway stenosis (21.97%) was the most common complication followed by anastomotic stenosis (14.30%). Gram-negative bacterial sepsis (52%) was the leading cause of death. Cumulative probability of survival at 1 month was 0.85 (95% confidence interval [CI] 0.80-0.92), and at 1 year, it was 0.78 (95% CI, 0.72-0.86). CONCLUSION: This study establishes the fact that despite multiple challenges, LT is a viable option for selected patients with ESLDs in India and should encourage early referrals to a transplant center.

Some geometrical aspects of packing of bent-core molecules in the triclinic smectic-C_{g} liquid crystals and simple models of the modulated SmC_{gmod} phase.

Organic compounds with bent-core (BC) molecules usually form the layered smectic liquid crystals with tilted molecules and polarization (P) which lies in the plane of the layer. A few such compounds have been found in which P itself tilts out of the plane of the layer, and the medium with general tilt (SmC_{g}) of the molecules has the low chiral triclinic symmetry. We discuss the geometric constraints of molecular packing in this structure to show that projecting groups attached to one of the arms of the BC molecules favors the formation of the SmC_{g} phase. We also extend our model for the modulated phases exhibited by BC molecules to show that the stripe structure made of bilayers shown by a few BC compounds, which is a signature of SmC_{g} layers, is preferentially formed by rotation of the BC molecules about their long axes, rather than about the layer normal. The theoretical results based on the unified model which describes all the modulated phases exhibited by the BC molecules broadly reflect experimental trends.

Curvature elasticity of smectic-C liquid crystals and formation of stripe domains along thickness gradients in menisci of free-standing films.

Smectic liquid crystals with a layering order of rodlike molecules can be drawn in the form of free standing films across holes. Extensive experimental studies have shown that smectic-C (SmC) liquid crystals (LCs) with tilted molecules form periodic stripes in the thinner parts of the meniscus, which persist over a range of temperatures above the transition of the bulk medium to the SmA phase in which the tilt angle is zero. The prevailing theoretical models cannot account for all the experimental observations. We propose a model in which we argue that the negative curvature of the surface of the meniscus results in an energy cost when the molecules tilt at the surface. The energy can be reduced by exploiting the allowed (∇·k)(∇·c) deformation which couples the divergence of k, the unit vector along the layer normal, with that of c, the projection of the tilted molecular director on the layer plane. We propose a structure with periodic bending of layers with opposite curvatures, in which the c-vector field itself has a continuous deformation. Calculations based on the theoretical model can qualitatively account for all the experimental observations. It is suggested that detailed measurements on the stripes may be useful for getting good estimates of a few curvature elastic constants of SmC LCs.

Saddle-splay-induced periodic edge undulations in smectic-A disks immersed in a nematic medium.

We report experimental studies on the phase behavior of binary mixtures of 1″,7″-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB) and 4,4-diheptyloxyazoxybenzene, which exhibit, apart from the nematic (N) and twist-bend nematic (N_{TB}) phases, the induced smectic-A (Sm-A) phase for weight fraction of CB7CB between 0.05 and 0.70. In planar nematic layers, the N_{TB} phase separates as droplets of tactoidlike planform; the chirality of droplets manifests in the optical dissimilarity between their opposite angular ends. Our main result is that, in the appropriate two phase region, Sm-A nuclei with positive dielectric anisotropy change over to disks immersed in the nematic above some electric field, their edges decorated by periodic bright spots, a result which was earlier reported in another binary system exhibiting the induced Sm-A phase [R. Pratibha and N. V. Madhusudana, Physica A 224, 9 (1996)10.1016/0378-4371(95)00311-8]. We develop a simple theory for the threshold of this distortion, which is a periodic undulation of the edge of the disk, demonstrating that it arises from saddle-splay elasticity of Sm-A, the low Sm-A-N interfacial tension unable to suppress the distortion. The observed increases in the number of bright spots with field, and with the radius of the disk at a given field, in both the experimental systems are also accounted for by the model. The distortion, which results in the most direct visualization of saddle splay in Sm-A, is also exhibited by disks nucleating on surfaces treated for homeotropic anchoring.

A two-state model for the modulated B7 liquid crystalline phase exhibited by bent-core molecules.

Liquid crystals (LC) made of bent-core (BC) organic molecules have been intensively studied over the past two decades. The B7 LC consists of smectic layers in which tilted molecules have an in-plane polar packing, the polarization vector ( P ) having a splay distortion forming stripes of tens of nanometers in width, and undulated layers giving rise to 2D rectangular or oblique lattices. The prevailing phenomenological theory attributes this structure to a strong coupling between molecular tilt in the layers and div P . Based on our recent studies on other phases exhibited by BC molecules, we propose a new model in which the physical origin of the stripes arises from a minority (~10%) of the BC molecules having less bent excited state (ES) conformers which can freely rotate about their long axes, and aggregate to form smectic C type walls, in which the tilt angle is relatively small. The more bent ground state (GS) conformers with polar packing in turn form the splayed structure between such walls to lower the free energy. The mismatch in the layer spacing between the domains with ES and GS conformers generates the structures in the B7 phase described above. The model predicts a weak first order transition from B7 to the uniform B2 phase as the temperature is lowered, as experimentally observed in some compounds. We show that the observed slow increase of the stripe width in the LC to micrometer dimensions in free standing films can be attributed to changes in the physical parameters by chemical degradation and absorption of ions by the exposed polarized layers. We also describe different possible structures of the undulated layers, including a stacking of racemic pairs of SmCaPF layers within the stripes. Some possible methods of testing the model are also indicated.

Simple model for the stripe phase in compounds with bent-core molecules which exhibit a lower-temperature ferroelectric smectic-A phase.

Bent-core (BC) molecules usually exhibit polar packing in smectic layers in which the long or bow axes tilt with respect to the layer normal. In many compounds, the tilt angle goes to zero, and typically the polarization (P) of neighboring layers has an antiferroelectric order (SmAP_{AF}). A careful molecular engineering has led to the discovery of the ferroelectric SmAP_{F} phase in a few BC compounds. Detailed experimental studies [Zhu et al., J. Am. Chem. Soc. 134, 9681 (2012)10.1021/ja3009314] have shown that one of the compounds undergoes a weak first order transition to a striped phase (SmAP_{Fmod}), in which the repolarization switching occurs at a threshold electric field, the latter increasing with temperature. The main optical birefringence of the SmAP_{Fmod} phase increases rapidly with the field at lower temperatures of its range. A small (<10%) fraction of the BC molecules can be expected to have less bent conformations in excited states (ESs), and we argue that the ES conformers rotate freely about the bow axes, and aggregate to form regions without any polar order to gain rotational entropy. In turn, a favorable divP term leads to the formation of stripes in the polarized regions made of ground state conformers. Based on this physical picture, we develop a simple phenomenological model to reproduce all the experimental trends qualitatively. In sample cells with insulating layers neighboring stripes prefer to have an antiparallel orientation of the mean P direction. The dielectric constant of the stripe phase increases with a dc bias electric field, a trend which is opposite to that in other ferroelectric liquid crystals.

Two-state model for nematic liquid crystals made of bent-core molecules.

Nematic (N) liquid crystals made of bent-core molecules exhibit unusual physical properties such as an intermediate phase between the N and isotropic (I) phases, a very weak NI transition as inferred from magnetic birefringence measurements in a low field, which is apparently incompatible with a large shift in the NI transition temperature (T_{ni}) measured under a high field. Using our conformational studies on the aromatic cores, we propose that only conformers which are more straightened than those in the ground state (GS) form clusters with a few layers, which persist even in the isotropic phase, as inferred from x-ray and rheological experiments. We present a Landau-de Gennes theory of the medium, including an orientational coupling between the clusters and the GS molecules, which accounts for all the unusual properties. The intermediate phase to isotropic transition is predicted to exhibit critical behavior at a very low magnetic field of <1kG.

Tubular growth and bead formation in the lyotropic lamellar phase of a lipid.

We use fluorescence confocal polarised microscopy (FCPM) to study tubular growth upon hydration of dry DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) in water and water-glycerol mixtures. We have developed a model to relate the FCPM intensity profiles to the multilamellar structures of the tubules. Insertion of an additional patch inside a tubule produces a beaded structure, while a straight configuration is retained if the growth is on the outside. We use a simple model to suggest that reduction in overall curvature energy drives bead formation.

Comment on "Origin of tilted-phase generation in systems of ellipsoidal molecules with dipolar interactions''.

In this Comment, I point out that the physical origin of molecular tilt in the smectic phase, found in the Monte Carlo simulations of systems of rodlike molecules with two terminal antiparallel transverse dipole moments by Bose and Saha [Phys. Rev. E 86, 050701(R) (2012)], is similar to the one proposed by McMillan. In particular, unlike in smectic-C liquid crystals, in which the molecules are known to have practically free rotations about their long axes, the molecular rotations are found to be partially frozen in the simulations. Further, I suggest that the attractive interaction between correlated splay fluctuations of the antiparallel polarized sublayers which lie close to each other in adjacent molecular layers give rise to the tilting, rather than a reduced attractive interaction between dipoles belonging to the two dipolar sublayers within one molecular layer, as proposed by the authors.

A simple model for the rotation of a trapped chiral nematic droplet under the action of a linearly polarized laser beam.

There have been recent reports of continuous rotation of chiral nematic droplets in restricted ranges of diameter/pitch (d /p) values, trapped by a linearly polarized laser beam. We have developed a simple model to calculate the distortion in the helical structure of a set of flat layers, caused by the action of the strong electric field of the propagating laser beam on the dielectric anisotropy of the medium. The resulting change in the polarization state of the beam passing through the sample is then used to calculate the torque on the sample as a function of the azimuthal angle of the first layer. The main results are: i) the torque tends to zero even with circularly polarized beam for samples with thicknesses around integral multiples of 0.5p ; ii) the undistorted sample takes an equilibrium orientation for linearly polarized beam, which jumps by π/2 rad at the same sample thicknesses; iii) these samples will have a nonzero torque at all azimuthal angles of the first slice when the helical structure is distorted by the linearly polarized beam. The calculations show that a propagating accordion mode, in which the helical pitch alternately expands and contracts, gives rise to the nonzero torque. The theoretical predictions are in broad agreement with experimental results.

Modulated phase of liquid crystals: covariant elasticity in the context of soft, achiral smectic-C materials.

Ginzburg-Landau-de Gennes-type covariant theories are extensively used in connection with twist grain boundary phases of chiral smectogens. We analyze the stability conditions for the linear, covariant elasticity theory of smectic-C liquid crystals in the context of achiral materials, and predict an equilibrium modulated structure with an oblique wave vector. We suggest that a previous experimental observation of stripes in smectic-C is consistent with the predicted structure.

Temperature- and electric-field-induced inverse Freedericksz transition in a nematogen with weak surface anchoring.

We report electric field dependence of the anchoring transition in a mesogen on cooling in a cell with perfluoropolymer treated surfaces. Below a crossover voltage V(co) the transition is discontinuous between planar and homeotropic alignments, and as the temperature is lowered, the transition temperature decreases quadratically with the field. Above V(co) the transition is continuous between planar and tilted alignments, the transition temperature decreasing essentially linearly with the rms field. We develop a simple model to account for these results and argue that the higher field regime corresponds to a temperature driven inverse Freedericksz transition in which the director orientation starts tilting at the weakly anchored surfaces while the tilt angle remains zero at the midplane of the cell.

Orientational order in liquid crystals exhibited by some binary mixtures of rod-like and bent-core molecules.

We report measurements of the temperature variations of the optical birefringence in the nematic (N) and partial bilayer SmA (SmA(d)) phases in 4-n-octyloxy 4(')-cyanobiphenyl made of rod-like (R) molecules and five mixtures of this compound with 1,3-phenylene bis[4-(3-methylbenzoyloxy)] 4(')-n-dodecylbiphenyl 4(')-carboxylate, made of bent-core (BC) molecules. The birefringence decreases with the concentration x of the BC molecules but the macroscopic order parameter initially decreases upto 11 mol% of BC molecules and subsequently increases with x. This is attributed to the possible formation of polar clusters of BC molecules. Orientation of BC molecules changes between the N and SmA(d) phases and the birefringence data in the two phases imply that the kink angle of the BC molecules is approximately 90 degrees rather than approximately 110 degrees as obtained from calculations which minimize the energy of the molecule. IR spectroscopic measurements on the mixture with 11 mol% of BC molecules have been used to estimate the molecular order parameter S of the R molecules, and to provide additional support for a relatively small kink angle of BC molecules.

Splay and bend elastic constants in the nematic phase of some disulfide bridged dimeric compounds.

We report measurements of the temperature dependences of the splay (K11) and bend (K33) elastic constants of several homologues and a binary mixture of disulfide bridged symmetric dimers made of alkoxy cyanobiphenyl monomeric units. All of them have an even number m of carbon and sulfur atoms in the linking chain and expected to have linear conformations. Both K(11) and K(33) alternate depending on the parity of x which is the number of carbon atoms in the alkyl chain of a monomer. We will refer to this as 'odd-even' effect. This effect, occurs near the nematic-isotropic transition temperature. T(NI), but monotonically decrease with m at lower temperatures. We argue that these results imply large rotational fluctuations of the two halves of the dimers about the S-S bond giving rise to average bent conformations near T(NI). At lower temperatures those conformations which can adapt better to the given curvature deformation in the medium are favored.

N-methylisatin-beta-thiosemicarbazone derivative (SCH 16) is an inhibitor of Japanese encephalitis virus infection in vitro and in vivo.

BACKGROUND: During the early and mid part of 20th century, several reports described the therapeutic effects of N-methylisatin-beta-Thiosemicarbazone (MIBT) against pox viruses, Maloney leukemia viruses and recently against HIV. However, their ability to inhibit flavivirus replication has not been investigated. Hence the present study was designed to evaluate the antiviral activity of 14 MIBT derivatives against Flaviviruses that are prevalent in India such as Japanese Encephalitis Virus (JEV), Dengue-2 (Den-2) and West Nile viruses (WNV). RESULTS: Amongst the fourteen Mannich bases of MIBT derivatives tested one compound - SCH 16 was able to completely inhibit in vitro Japanese encephalitis virus (JEV) and West Nile virus (WNV) replication. However no antiviral activity of SCH 16 was noted against Den-2 virus replication. This compound was able to inhibit 50% of the plaques (IC50) produced by JEV and WNV at a concentration of 16 microgm/ml (0.000025 microM) and 4 microgm/ml (0.000006 microM) respectively. Furthermore, SCH 16 at a concentration of 500 mg/kg body weight administered by oral route twice daily was able to completely (100%) prevent mortality in mice challenged with 50LD50 JEV by the peripheral route. Our experiments to understand the mechanism of action suggest that SCH 16 inhibited JEV replication at the level of early protein translation. CONCLUSION: Only one of the 14 isatin derivatives -SCH 16 exhibited antiviral action on JEV and WNV virus infection in vitro. SCH 16 was also found to completely inhibit JEV replication in vivo in a mouse model challenged peripherally with 50LD50 of the virus. These results warrant further research and development on SCH 16 as a possible therapeutic agent.

Effect of high electric fields on the nematic to isotropic transition in a material exhibiting large negative dielectric anisotropy.

We report the experimental high electric field phase diagram of a nematic liquid crystal which exhibits a large negative dielectric anisotropy. We measure simultaneously the birefringence (Deltan) and the dielectric constant (epsilon( perpendicular)) at various applied fields as functions of the local temperature of an aligned sample. We also measure the higher harmonics of the electrical response of the medium. The following experimental results are noted: (i) enhancement of orientational order parameter S in the nematic phase due to both the Kerr effect and quenching of director fluctuations; (ii) enhancement in the paranematic to nematic transition temperature (T(PN)) with field; (iii) divergence of the order parameter susceptibility beyond the tricritical point as measured by third harmonic electrical signal; (iv) a small second harmonic electrical signal which also diverges near T(PN), indicating the presence of polarised domains. Our measurements show that DeltaT(PN)(= T(PN)(E)-T(NI)(0)) varies linearly with |E| whereas the Landau de Gennes theory predicts a dependence on E(2). It is argued that the quenching of director fluctuations by the field makes the dominant contribution to all the observations, including the thermodynamics of the transition.

Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rodlike and bent-core molecules.

We report on two anomalous trends in the temperature dependences of the splay (K11) and bend (K33) elastic constants in the nematic (N) phase of mixtures of compounds with rodlike (R) and bent-core (BC) molecules: As the sample is cooled from the isotropic to N transition point, (i) K33 increases, attains a maximum value and then decreases, and (ii) close to the N to smectic A (SmA) transition point, K11 decreases sharply. At higher temperatures the bow axes of BC molecules are aligned along the director n, strongly favoring a bend distortion of n as the orientational order parameter is increased. Close to the N-SmA transition point the smecticlike short-range order builds up, and the arrow axes of BC molecules are aligned along n, facilitating a splay distortion of n. A simple model calculation brings out the anomalous trend in K33.

Self-propulsion of nematic drops: novel phase separation dynamics in impurity-doped nematogens.

We report a novel phase separation dynamics, mediated by self-propelled motion of the nucleated drops, in a mixture of a nematogen and an isotropic dopant. We show that surface flow, induced by the gradient in the concentration of the dopant expelled by the growing drops, provides the driving force for the propulsion of nematic droplets. While the liquid crystal-isotropic transition is used here to demonstrate the phenomenon, self-propulsion should be observable in many other systems in which the dynamics of a conserved order parameter is coupled to a nonconserved order parameter.