Radial and vertical angular momentum transport in protostellar discs

Angular momentum in protostellar discs can be transported either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field. We present a model of steady-state discs where these two mechanisms operate at the same radius and derive approximate criteria for their occurrence in an ambipolar diffusion dominated disc. We obtain "weak field'' solutions - which we associate with the MRI channel modes in a stratified disc - and transform them into accretion solutions with predominantly radial angular-momentum transport by implementing a turbulent-stress prescription based on published results of numerical simulations. We also analyze "intermediate field strength'' solutions in which both radial and vertical transport operate at the same radial location. Our results suggest, however, that this overlap is unlikely to occur in real discs.Comment: 5 pages, 2 figures, 1 table, aastex.cls. Accepted for publication in Astrophysics & Space Scienc

Posteromedial elastic stable intra-medullary nailing (ESIN) in volarly displaced metaphyso-diaphyseal distal radius fractures in child

SummaryIntroductionThe distal metaphyso-diaphyseal fractures of the radius with volar angulation are generally considered unstable. Too distal to be treated with classic elastic stable intramedullary nailing (ESIN) via a laterodistal approach and too proximal to be stabilized with simple conservative treatment, they are sometimes treated by plating.Patients and methodsTo avoid the disadvantages of the open exposure necessary for this latter fixation and to prevent volar angulation while respecting the curvature of the radius, radial elastic nailing with posteromedial distal entry was used in 16 patients. These patients were reviewed with an average follow-up of 4.5months.ResultsSix patients presented a mean volar angulation of 7°, eight a mean posterior angulation of 5.5°, and two were aligned at 0°. On the AP view, seven patients presented a mean residual varus of 6.5°, four a mean residual valgus of 5°, and five were at 0°. The pronating curvature of the radius and the radioulnar index remained intact in all cases. One case of extensor digitorum tenosynovitis was observed and disappeared after wire removal.DiscussionFor the unstable fractures of the distal third of the radius, certain authors propose systematic classic ESIN, but the lateral point of entry inevitably entails a varus misalignment, which is no longer the case if the entry is medial.Level of evidenceLevel 4. Retrospective Study

When Langmuir is too simple: H-2 dissociation on Pd(111) at high coverage

Recent experiments of H2 adsorption on Pd(111) [T. Mitsui et al., Nature (London) 422, 705 (2003)] have questioned the classical Langmuir picture of second order adsorption kinetics at high surface coverage requiring pairs of empty sites for the dissociative chemisorption. Experiments find that at least three empty sites are needed. Through density functional theory, we find that H2 dissociation is favored on ensembles of sites that involve a Pd atom with no direct interaction with adsorbed hydrogen. Such active sites are formed by aggregation of at least 3 H-free sites revealing the complex structure of the "active sites.

Lateral stiffness: A new nanomechanical measurement for the determination of shear strengths with friction force microscopy

We present a technique to measure the lateral stiffness of the nanometer-sized contact formed between a friction force microscope tip and a sample surface. Since the lateral stiffness of an elastic contact is proportional to the contact radius, this measurement can be used to study the relationship between friction, load, and contact area. As an example, we measure the lateral stiffness of the contact between a silicon nitride tip and muscovite mica in a humid atmosphere (55% relative humidity) as a function of load. Comparison with friction measurements confirms that friction is proportional to contact area and allows determination of the shear strength

Calibration of frictional forces in atomic force microscopy

The atomic force microscope can provide information on the atomic-level frictional properties of surfaces, but reproducible quantitative measurements are difficult to obtain. Parameters that are either unknown or difficult to precisely measure include the normal and lateral cantilever force constants (particularly with microfabricated cantilevers), the tip height, the deflection sensor response, and the tip structure and composition at the tip-surface contact. We present an in situ experimental procedure to determine the response of a cantilever to lateral forces in terms of its normal force response. This procedure is quite general. It will work with any type of deflection sensor and does not require the knowledge or direct measurement of the lever dimensions or the tip height. In addition, the shape of the tip apex can be determined. We also discuss a number of specific issues related to force and friction measurements using optical lever deflection sensing. We present experimental results on the lateral force response of commercially available V-shaped cantilevers. Our results are consistent with estimates of lever mechanical properties using continuum elasticity theory

Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope

We have studied the variation of frictional force with externally applied load for a Pt-coated atomic force microscope tip in contact with the surface of mica cleaved in ultrahigh vacuum. At low loads, the frictional force varies with load in almost exact proportion to the area of contact as predicted by the Johnson-Kendall-Roberts (JKR) theory [K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Sec. London Ser. A 324, 301 (1971)] of elastic adhesive contacts. The friction-load relation for a deliberately modified tip shape was proportional to an extended JKR model that predicts the area-load relation for nonparabolic tips, The tip shape was determined experimentally with a tip imaging technique and was consistent with the predicted friction behavior. This demonstrates that the frictional force is proportional to the area of contact between the tip and sample. Using the JKR/extended JKR model, interfacial surface energies and shear strengths can be estimated

A variable temperature ultrahigh vacuum atomic force microscope

A new atomic force microscope (AFM) that operates in ultrahigh vacuum (UHV) is described. The sample is held fixed with spring clamps while the AMF cantilever and deflection sensor are scanned above it. Thus, the sample is easily coupled to a liquid nitrogen cooled thermal reservoir which allows AFM operation from ≈ 100 K to room temperature. AFM operation above room temperature is also possible. The microscope head is capable of coarse x-y positioning over millimeter distances so that AFM images can be taken virtually anywhere upon a macroscopic sample. The optical beam deflection scheme is used for detection, allowing simultaneous normal and lateral force measurements. The sample can be transferred from the AFM stage to a low energy electron diffraction/Auger electron spectrometer stage for surface analysis. Atomic lattice resolution AFM images taken in UHV are presented at 110, 296, and 430 K

Interpretations of J/ψJ/\psi suppression

We review the two main interpretations of $J/\psi$ suppression proposed in the literature. The phase transition (or deconfining) scenario assumes that below some critical value of the local energy density (or of some other geometrical quantity which depends both on the colliding systems and on the centrality of the collision), there is only nuclear absorption. Above this critical value the absorptive cross-section is taken to be infinite, i.e. no $J/\psi$ can survive in this hot region. In the hadronic scenario the $J/\psi$ dissociates due both to nuclear absorption and to its interactions with co-moving hadrons produced in the collision. No discontinuity exists in physical observables. We show that an equally good description of the present data is possible in either scenario.Comment: 12 pages, LaTeX, uses epsfig and ioplppt; review talk given by A. Capella at the International Symposium on Strangness in Quark Matter, Santorini (Greece), April 1997; Figs. 1 and 2 not available but can be found in Refs. 13 and 6 respectivel

Fully Transparent Gas Sensor Based on Carbon Nanotubes

In this paper, we demonstrate the feasibility of realization of transparent gas sensors based on carbon nanotubes (CNTs). Both sensing layer and electrodes consist of CNTs deposited by spray deposition. The transparent sensor—with a transmittance higher than 60% in both sensing layer and electrodes—is characterized towards NH3 and CO2 and compared with a reference sensor with the same active layer but evaporated Au electrodes. In particular, the sensitivity towards NH3 is virtually identical for both reference and transparent sensors, whereas the transparent device exhibits higher sensitivity to CO2 than the reference electrode. The effect of the spacing among consecutive electrodes is also studied, demonstrating that a wider spacing in fully CNT based sensors results in a higher sensitivity because of the higher sensing resistance, whereas this effect was not observed in gold electrodes, as their resistance can be neglected with respect to the resistance of the CNT sensing layer. Overall, the transparent sensors show performance comparable—if not superior—to the traditionally realized ones, opening the way for seamlessly integrated sensors, which do not compromise on quality.This work has been partially supported by the fellowship H2020-MSCA-IF-2017-794885-SELFSENS and the TUM Graduate Schoo