Multiaxial miniaturized load cell for measuring forces acting through a sternotomy

Experimental Techniques - A miniaturized multiaxial load cell was developed and calibrated. The transducer geometry was designed for sternal closure following sternotomy. The overall accuracy was...     

Heterogeneous and anisotropic dynamics of a 2D gel

We report x-ray photon correlation spectroscopy (XPCS) results on bidimensional (2D) gels formed by a Langmuir monolayer of gold nanoparticles. The system allows an experimental determination of the fourth order time correlation function, which is compared to the usual second order correlation function and to the mechanical response measured on macroscopic scale. The observed dynamics is anisotropic, heterogeneous and superdiffusive on the nanoscale. Different time scales, associated with fast heterogeneous dynamics inside 2D cages and slower motion of larger parts of the film, can be identified from the correlation functions. The XPCS results are discussed in view of other experimental results and models of three-dimensional gel dynamics.     

Magnetic properties of nickel and platinum quaternary borocarbides

The zero‐field μ⁺SR technique is employed to study the magnetic properties of the non‐superconducting borocarbides Sm_1-xLu_xNi₂B₂C (x=0.0,\ 0.2) and NdPt_1.5Au_0.6B₂C. Coherent ordering of the Sm electronic moments appears below T_N=9.86(3) and 4.15(3) K for the x=0.0 and 0.2 compositions, respectively. No long range order of the Nd₃₊ moments is found in NdPt_1.5Au_0.6B₂C down to 80 mK. Freezing of the electronic moments into a spin‐glass state occurs at T_f\approx 1.2 K. A stretched exponential decay of the μ⁺ spin polarisation is observed above T_f with the exponent \beta smoothly decreasing on approaching T_f from above, reminiscent of the behaviour in other spin glasses. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microscopic structure of crystalline Langmuir monolayers of hydroxystearic acids by X-ray reflectivity and GID: OH group position and dimensionality effect

Hydroxystearic acid (HSA) molecules at the air-water interface present an interesting bicompetitive adsorption between primary and secondary hydrophilic groups on either end of an alkyl chain, which, depending on the position of the second hydrophilic group, may lead to a sharp transition from an expanded phase to a crystalline condensed morphology as surface pressure is increased. Here we report a set of measurements on a series of hydroxystearic acids in which the position of the secondary competing hydrophilic group position is varied along the whole extent of the alkyl chain from position 2 (i.e., close to the primary hydrophilic group) to positions 7, 9 and 12, the latter being the compounds mostly studied in the literature. We show here direct microscopic evidence, obtained by synchrotron radiation reflectometry and grazing incidence diffraction, that the position of the secondary hydrophilic group not only strongly influences the phase diagram as determined by compression isotherms and ellipsometry but also induces different crystallization patterns in the 2D system of the Langmuir monolayer. In particular, we report for the first time the existence of a turning point in the effects of the hydroxyl position on the monolayers structure at 7-HSA.     

Comparison of uniaxial and triaxial rosette gages for strain measurement in the femur

A major problem of hip prostheses and other bone fixation devices is that they alter the stress distribution in the femur. This phenomenon is named stress shielding, and derives from the bigger stiffness of the implant when compared to the bone. Strain gages are the most common tool used in in vitro stress-shielding investigations. There is a lack of agreement in the literature about which gage type is to be preferred (uniaxial grid, biaxial or triaxial rosette). Rosettes are required when complete information is needed. However, uniaxial gages can be advantageous because they reduce the number of measurement channels required. The aim of this work is to verify if (and under which hypothesis) uniaxial gages can replace triaxial rosettes.     

Photon statistics and speckle visibility spectroscopy with partially coherent X‐rays

A new approach is proposed for measuring structural dynamics in materials from multi‐speckle scattering patterns obtained with partially coherent X‐rays. Coherent X‐ray scattering is already widely used at high‐brightness synchrotron lightsources to measure dynamics using X‐ray photon correlation spectroscopy, but in many situations this experimental approach based on recording long series of images (i.e. movies) is either not adequate or not practical. Following the development of visible‐light speckle visibility spectroscopy, the dynamic information is obtained instead by analyzing the photon statistics and calculating the speckle contrast in single scattering patterns. This quantity, also referred to as the speckle visibility, is determined by the properties of the partially coherent beam and other experimental parameters, as well as the internal motions in the sample (dynamics). As a case study, Brownian dynamics in a low‐density colloidal suspension is measured and an excellent agreement is found between correlation functions measured by X‐ray photon correlation spectroscopy and the decay in speckle visibility with integration time obtained from the analysis presented here.     

Duty cycle and directional jet effects of a plasma actuator on the flow control around a NACA0015 airfoil

This paper reports on the effects of a series of fluid-dynamic dielectric barrier discharge plasma actuators on a NACA0015 airfoil at high angle of attack. A set of jet actuators able to produce plasma jets with different directions (vectoring effect) and operated at different on/off duty cycle frequencies are used. The experiments are performed in a wind tunnel facility. The vectorized jet and the transient of the flow induced by unsteady duty cycle operation of each actuator are examined and the effectiveness of the actuator to recover stall condition in the range of Reynolds numbers between 1.0 × 10⁵ and 5.0 × 10⁵ (based on airfoil chord), is investigated. The actuator placed on the leading edge of the airfoil presents the most effective stall recovery. No significant effects can be observed for different orientations of the jet. An increase of the stall recovery is detected when the actuator is operated in unsteady operation mode. Moreover, the frequency of the on/off duty cycle that maximizes the stall recovery is found to be a function of the free stream velocity. This frequency seems to scale with the boundary layer thickness at the position of the actuator. A lift coefficient increase at low free stream velocities appears to linearly depend on the supply voltage.     

Glass transition and relaxation following PhotoPerturbation in thin polymeric films

In this letter we employ null ellipsometry on Langmuir Blodgett multilayers of a glass forming photosensitive side chain polymer to investigate both the structural changes and the related time relaxation as a function of temperature and sample thickness following isothermal optical perturbation. Below a thickness of a few layers the glassy multilayer system collapses to a smecticlike crystal. This effect is discussed in the context of the glass transition in restricted dimensionality.     

Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.