Structure and Lehmann rotation of drops in a surfactant-doped bent-core liquid crystal.

The structure of the nematic (cholesteric) drops that form at the clearing temperature of a mixture of the bent-core molecule CB7CB and the rodlike molecule 8CB doped with a surfactant is optically determined. Using experimental observations and numerical simulations, it is demonstrated that the director field inside these drops is not escaped concentric, as previously proposed, but twisted bipolar. The Lehmann rotation of these drops in the presence of a temperature gradient is described. Their rotation velocity is shown to be proportional to the temperature gradient and to the surface twist angle of the director field and inversely proportional to the drop radius, thus revealing a fundamental scaling law for the Lehmann effect of nematic and cholesteric twisted-bipolar droplets.

Microscale Structures Arising from Nanoscale Inhomogeneities in Nematics Made of Bent-Shaped Molecules.

Nanoscale structures in fluid media normally require techniques such as freeze fracture electron microscopy and atomic force microscopy for their visualization. As demonstrated in the present study, the surface modification due to nanoscale clusters occurring intrinsically in nematics made of bent-shaped molecules with either rigid or flexible cores leads to microscale structures, which are visible in an optical microscope. The underlying physical mechanism proposed here involves a quasiperiodic change in anchoring conditions on untreated glass plates for the medium made of islands of clusters surrounded by unclustered molecules. The resulting pattern of stripes outlines the director-normal field around line defects in the well-known schlieren texture. The instability, which is seen over most of the nematic range, with increasing visibility under continued cooling of the sample, sets the nematics made of bent-shaped molecules apart from the classical nematics of rod-shaped molecules.

Instabilities in the electric Freedericksz state of the twist-bend nematic liquid crystal CB7CB.

We report on the instabilities in the Freedericksz state of the twist-bend nematic (NTB) liquid crystal 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB). The quasi homeotropic NTB state, into which a planar (untwisted or 90°-twisted) nematic CB7CB layer transits under a strong electric field, is found to be unstable despite the material being dielectrically positive. Close to the NTB melting point, destabilization occurs through the formation of metastable toric focal conic domains (TFCDs) that, in time, transform into parabolic focal conic domains (PFCDs) with the confocal parabolae in vertical planes through the layer normal. This transformation occurs by a novel process of continued dissociation of TFCDs. We outline how the extended Volterra process could help in a general appreciation of focal conic defects in the NTB phase. At relatively lower temperatures, stripes develop competingly with TFCDs. Identifiable as oily streaks, they are both localized and polarity sensitive; they form close to the substrates; and in low frequency square wave fields, they get suppressed at the cathode and augmented at the anode at each polarity switch. The study also dwells on the N-NTB-N sandwich region, found between the N and NTB states under a small temperature gradient.

Confined Electroconvective and Flexoelectric Instabilities Deep in the Freedericksz State of Nematic CB7CB.

We report wormlike flexoelectric structures evolving deep in the Freedericksz state of a nematic layer of the liquid crystal cyanobiphenyl-(CH2)7-cyanobiphenyl. They form in the predominantly splay-bend thin boundary layers and are built up of solitary flexoelectric domains of the Bobylev-Pikin type. Their formation is possibly triggered by the gradient flexoelectric surface instability that remains optically discernible up to unusually high frequencies. The threshold voltage at which the worms form scales as square root of the frequency; in their extended state, worms often appear as labyrinthine structures on a section of loops that separate regions of opposite director deviation. Such asymmetric loops are also derived through pincement-like dissociation of ring-shaped walls. Formation of isolated domains of bulk electroconvection precedes the onset of surface instabilities. In essence, far above the Freedericksz threshold, the twisted nematic layer behaves as a combination of two orthogonally oriented planar half-layers destabilized by localized flexoelectric distortion.

Interfacial and morphological features of a twist-bend nematic drop.

In this experimental and theoretical study, we examine the equilibrium shapes of quasi-two-dimensional twist-bend nematic (Ntb) drops formed within a planarly aligned nematic layer of the liquid crystal CB7CB. Initially, at the setting point of the Ntb phase, the drops assume a nonequilibrium cusped elliptical geometry with the major axis orthogonal to the director of the surrounding nematic fluid; this growth is governed principally by anisotropic heat diffusion. The drops attain equilibrium through thermally driven dynamical evolution close to their melting temperature. They are associated with a characteristic twin-striped morphology that transforms into the familiar focal conic texture as the temperature is lowered. At equilibrium, large millimetric drops are tactoidlike, elongated along the director of the surrounding nematic fluid. This geometry is explained using a mathematical model that features two dimensionless parameters, of which one is the structural cone angle of the Ntb phase and the other is the relative strength of mismatch elastic energy at the drop's interface. Both parameters are extracted from the observations by measuring the aspect ratio of the equilibrium shapes and the inner corner angle of the cusps.

Results from the Survey of Antibiotic Resistance (SOAR) 2012-14 in Thailand, India, South Korea and Singapore.

OBJECTIVES: To provide susceptibility data for community-acquired respiratory tract isolates of Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae and Moraxella catarrhalis collected in 2012-14 from four Asian countries. METHODS: MICs were determined using Etest(®) for all antibiotics except erythromycin, which was evaluated by disc diffusion. Susceptibility was assessed using CLSI, EUCAST and pharmacokinetic/pharmacodynamic (PK/PD) breakpoints. For macrolide/clindamycin interpretation, breakpoints were adjusted for incubation in CO2 where available. RESULTS: Susceptibility of S. pneumoniae was generally lower in South Korea than in other countries. Penicillin susceptibility assessed using CLSI oral or EUCAST breakpoints ranged from 21.2% in South Korea to 63.8% in Singapore. In contrast, susceptibility using CLSI intravenous breakpoints was much higher, at 79% in South Korea and ∼95% or higher elsewhere. Macrolide susceptibility was ∼20% in South Korea and ∼50%-60% elsewhere. Among S. pyogenes isolates (India only), erythromycin susceptibility (∼20%) was lowest of the antibiotics tested. In H. influenzae antibiotic susceptibility was high except for ampicillin, where susceptibility ranged from 16.7% in South Korea to 91.1% in India. South Korea also had a high percentage (18.1%) of ß-lactamase-negative ampicillin-resistant isolates. Amoxicillin/clavulanic acid susceptibility for each pathogen (PK/PD high dose) was between 93% and 100% in all countries except for H. influenzae in South Korea (62.5%). CONCLUSIONS: Use of EUCAST versus CLSI breakpoints had profound differences for cefaclor, cefuroxime and ofloxacin, with EUCAST showing lower susceptibility. There was considerable variability in susceptibility among countries in the same region. Thus, continued surveillance is necessary to track future changes in antibiotic resistance.

Dislocations and metastable chevrons in the electroconvective inplane normal roll state of a bent core nematic liquid crystal.

We report experimental results on the formation, dynamics, and annihilation of edge dislocations of opposite topological charge in the electroconvective inplane vortex state of a bent core nematic liquid crystal. The approach of paired, oppositely charged defects toward each other is a two-step process. Near constant velocity at large separation and accelerated motion close to annihilation are found, as in the case of nematic rolls belonging to standard electroconvection. Periodic arrays of dislocations of alternating polarity form upon a sudden, strong elevation of the control parameter. Chevron structures that appear between undulatory defect chains are metastable, and their decay with time is accompanied by an exponential reduction in the dislocation density. The initial periodicity of defect chains also drops exponentially with increasing field.