Calorimetric study of the nematic to smectic-A phase transition in octylcyanobiphenyl-hexane binary mixtures

The continuous nematic to smectic-A (N-SmA) phase transition has been studied by high-resolution ac-calorimetry in binary mixtures of the liquid crystal octylcyanobiphenyl(8CB) and a non-mesogenic, low-molecular weight, solvent n-hexane(hex) as a function of temperature and solvent concentration. Heating and cooling scans about the N-SmA transition temperature were repeatedly performed on pure and six 8CB+hex samples having hexane molar concentration ranging from x_{hex}= 0.02 to 0.12. All 8CB+hex samples in this range of x_{hex} remain macroscopically miscible and exhibit an N-SmA heat capacity peak that shifts non-monotonically to lower temperature and evolves in shape, with a reproducible hysteresis, as x_{hex} increases. The imaginary part of heat capacity remains zero up to x^{TCP}_{hex}\simeq 0.07$ above which the distinct peak is observed, corresponding to a jump in both the real and imaginary enthalpy. A simple power-law analysis reveals an effective exponent that increases smoothly from 0.30 to 0.50 with an amplitude ratio A^{-}/A^{+}\rightarrow 1 as x_{hex}\rightarrow x^{TCP}_{hex}. This observed crossover towards the N-SmA tricritical point driven by solvent concentration is consistent with previous results and can be understood as weakening of the liquid crystal intermolecular potential promoting increased nematic fluctuations

Thermodynamic approach to the dewetting instability in ultrathin films

The fluid dynamics of the classical dewetting instability in ultrathin films is a non-linear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the films's dynamics. Alternately, the thermodynamic approach based on equating the rate of free energy decrease to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give similar information. Here we have evaluated dewetting in the presence of thermocapillary forces arising from a film-thickness (h) dependent temperature. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h-dependent temperature. The thermodynamic approach provides an analytical description of this thermocapillary dewetting. The results of this approach agree with those from linear theory and experimental observations provided the minimum value of viscous dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum viscous dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss.Comment: 8 pages, 3 figures. Under revie

Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films

Structural, optical and nanomechanical properties of nanocrystalline Zinc Telluride (ZnTe) films of thickness upto 10 microns deposited at room temperature on borosilicate glass substrates are reported. X-ray diffraction patterns reveal that the films were preferentially oriented along the (1 1 1) direction. The maximum refractive index of the films was 2.74 at a wavelength of 2000 nm. The optical band gap showed strong thickness dependence. The average film hardness and Young’s modulus obtained from loaddisplacement curves and analyzed by Oliver-Pharr method were 4 and 70 GPa respectively. Hardness of (1 1 1) oriented ZnTe thin films exhibited almost 5 times higher value than bulk. The studies show clearly that the hardness increases with decreasing indentation size, for indents between 30 and 300 nm in depth indicating the existence of indentation size effect. The coefficient of friction for these films as obtained from the nanoscratch test was ∼0.4.Financial support in the form of fellowships to MSRNK and SK from the ACRHEM project of DRDO is acknowledged

The influence of the jet opening angle on the appearance of relativistic jets

We reinvestigate the problem of the appearance of relativistic jets when geometrical opening is taken into account. We propose a new criterion to define apparent velocities and Doppler factors, which we think being determined by the brightest zone of the jet. We numerically compute the apparent velocity and the Doppler factor of a non homokinetic jet using different velocity profiles. We argue that if the motion is relativistic, the high superluminal velocities beta_{app} ~ gamma, expected in the case of an homokinetic jet, are only possible for geometrical collimation smaller than the relativistic beaming angle 1/gamma. This is relatively independent of the jet velocity profile. For jet collimation angles larger than 1/gamma, the apparent image of the jet will always be dominated by parts of the jet traveling directly towards the observer at lorentz factors < gamma resulting in maximal apparent velocities smaller than gamma}. Furthermore, getting rid of the homokinetic hypothesis yields a complex relation between the observing angle and the Doppler factor, resulting in important consequences for the numerical computation of AGN population and unification scheme model.Comment: Accepted in MNRAS, 12 pages and 9 Figure

Duration of analgesia, sensory and motor blockade and intra operative hemodynamic changes caused by intrathecal bupivacaine, bupivacaine plus clonidine and bupivacaine plus dexmedetomidine in spinal anesthesia: prospective, comparative study

Background: This is comparative study to differentiate the effects of Dexmedetomidine and clonidine on duration of analgesia, motor and sensory blockade and the intraoperative hemodynamic profile when mixed with Bupivacaine.Methods: Patients aged 15-45 years having ASA 1 and ASA 2 scheduled for elective orthopedic surgeries under spinal anaesthesia were included and divided into 3 groups equally (50 patients each)’ i.e. Group B received only 3.0 ml of 0.5% Bupivacaine (Heavy); Group C received 3.0ml of 0.5% Bupivacaine (Heavy) mixed with Clonidine 30 µg; and Group D administered with 3.0 ml of 0.5% Bupivacaine (Heavy) mixed with 5µg Dexmedetomidine.Results: Majority of the cases enrolled in the study were knee disorders and the rest being fracture femur, tibia. At preoperative interval mean systolic blood pressure and diastolic blood pressure of Group B was slightly higher than Group C and Group D. At the time when spinal anaesthesia was given, a slight fall in DBP of patients was observed in all Group B, Group C and Group D. Mean heart rate of patients at preoperative interval in patients of Group B, Group C and Group D respectively, with Group C and Group D showing slightly lower mean heart rate as compared with that of the mean heart rate of Group B but the values were not statistically significant. There were no any significant changes in the hemodynamic status.Conclusions: Both of used combinations provide prolonged sensory and motor blockade, hemodynamic stability, minimal side effects, and excellent intraoperative and postoperative analgesia

Was the Cosmic Web of Protogalactic Material Permeated by Lobes of Radio Galaxies During the Quasar Era?

Evidence for extended active lifetimes (> 10^8 yr) for radio galaxies implies that many large radio lobes were produced during the `quasar era', 1.5 < z < 3, when the comoving density of radio sources was 2 -- 3 dex higher than the present level. However, inverse Compton losses against the intense microwave background substantially reduce the ages and numbers of sources that are detected in flux-limited surveys. The realization that the galaxy forming material in those epochs was concentrated in filaments occupying a small fraction of the total volume then leads to the conclusion that radio lobes permeated much of the volume occupied by the protogalactic material during that era. The sustained overpressure in these extended lobes is likely to have played an important role in triggering the high inferred rate of galaxy formation at z > 1.5 and in the magnetization of the cosmic network of filaments.Comment: 5 pages, 0 figures, submitted to ApJ Letters; uses emulateapj