Structural diversity of sphingomyelin microdomains

In cells plasma membrane, sphingomyelin (SM) plays a key role in the formation of a category of lipid microdomains enriched in cholesterol (Chl) often referred to as rafts. Atomic force microscopy (AFM) was used to analyze the mesoscopic topography of enriched SM microdomains in supported bilayers made of SM/dioleoylphosphatidylcholine (SM/DOPC) and SM/palmitoyl-oleoyl-phosphatidylcholine (SM/POPC) equimolar mixtures, in buffer, at room temperature. Gel–fluid phase separation occurs in both SM/DOPC and SM/POPC bilayers. The gel phase SM-enriched microdomains adopt a variety of size, shape and mesoscopic structure, from homogeneous flat domains of a few hundreds of nanometer in diameter to domains of several micrometers made of closely packed globular structures. Gel–gel phase separation in SM domains is also observed which gives rise to different structures for the diunsaturated and the mixed-saturated PC species. These differences could also extend to the interactions with Chl. This suggests that studies on rafts formation commonly performed using SM/DOPC mixture as a model should also include the physiologically more relevant POPC species.     

Optimal control law for classical and multiconjugate adaptive optics

Classical adaptive optics (AO) is now a widespread technique for high-resolution imaging with astronomical ground-based telescopes. It generally uses simple and efficient control algorithms. Multiconjugate adaptive optics (MCAO) is a more recent and very promising technique that should extend the corrected field of view. This technique has not yet been experimentally validated, but simulations already show its high potential. The importance for MCAO of an optimal reconstruction using turbulence spatial statistics has already been demonstrated through open-loop simulations. We propose an optimal closed-loop control law that accounts for both spatial and temporal statistics. The prior information on the turbulence, as well as on the wave-front sensing noise, is expressed in a state-space model. The optimal phase estimation is then given by a Kalman filter. The equations describing the system are given and the underlying assumptions explained. The control law is then derived. The gain brought by this approach is demonstrated through MCAO numerical simulations representative of astronomical observation on a 8-m-class telescope in the near infrared. We also discuss the application of this control approach to classical AO. Even in classical AO, the technique could be relevant especially for future extreme AO systems.     

Learning eigenfunctions links spectral embedding and kernel PCA

In this letter, we show a direct relation between spectral embedding methods and kernel principal components analysis and how both are special cases of a more general learning problem: learning the principal eigenfunctions of an operator defined from a kernel and the unknown data-generating density. Whereas spectral embedding methods provided only coordinates for the training points, the analysis justifies a simple extension to out-of-sample examples (the Nystr?m formula) for multidimensional scaling (MDS), spectral clustering, Laplacian eigenmaps, locally linear embedding (LLE), and Isomap. The analysis provides, for all such spectral embedding methods, the definition of a loss function, whose empirical average is minimized by the traditional algorithms. The asymptotic expected value of that loss defines a generalization performance and clarifies what these algorithms are trying to learn. Experiments with LLE, Isomap, spectral clustering, and MDS show that this out-of-sample embedding formula generalizes well, with a level of error comparable to the effect of small perturbations of the training set on the embedding.     

On the corrosion resistance of the submerged arc welded titanium in concentrated hydrochloric acid 8 N

The corrosion behaviour of the submerged arc welded T-35 in concentrated 8 N HCl has been investigated. The weld metal is characterized by a martensite type structure, a hardness superior by 60 HV and an oxygen content about two and a half times higher than the base metal. In order to underline the part played by oxygen and structural transformations intervening during the welding operation, an analysis of the anodic current-potential curves by a reaction model involving four determining steps has been carried out. It has been shown that the two reaction rates which determine the passivation and the passivity of the metal depend on the oxygen content as well as the metallurgical structure. In the active dissolution range, whereas one of the two determining rates depends on the oxygen content, the other one is controlled by the metallurgical structure of the metal.     

The implication of sensitivity analysis on the safety and delayed-neutron parameters for fast breeder reactors

The delayed-neutron energy spectra for LMFBRs are not as well known as those for LWRs. These spectra are necessary for kinetics calculations which play an important role in safety and accident analyses. A sensitivity analysis was performed to study the response of the reactor power and power density to uncertainties in the delayed-neutron spectra during a rod-ejection accident. The accidents studied were central control-rod ejections with ejection times of 2, 10 and 30s. A two-energy group and two-precursor group model was formulated for the International Nuclear Fuel Cycle Evaluation (INFCE) reference design MOX-fueled LMFBR.The sensitivity analysis is based on the use of adjoints so that it is not necessary to repeatedly solve the governing (kinetics) equations to obtain the sensitivity derivatives. This is of particular importance when large systems of equations are used.The power and power-density responses were found to be most sensitive to uncertainties in the spectrum of the second delayed-neutron precursor group, resulting from the fission of²³⁸U, producing neutrons in the first energy group. It was found, for example, that for a rod-ejection time of 30 s, an uncertainty of 7.2% in the fast components of the spectra resulted in a 24% uncertainty in the predicted power and power density. These responses were recalculated by repeatedly solving the kinetics equations. The maximum discrepancy between the recalculated and the sensitivity analysis response was only 1.6%. The results of the sensitivity analysis indicate the need for improved delayed-neutron spectral data in order to reduce the uncertainties in accident analyses.     

Altération et rupture d'échantillons d'argilite lors de sollicitations thermomécaniques en conditions humides contrôlées

The objective of this work was to undertake creep tests on shale and describe the microtextural phenomena which initiate internal damage/rupture under thermomechanical loading and relative humidity conditions. The CGI test cell is a system developed at the Centre de Geologie de l'Ingénieur specifically to allow observation of the fabric of a thick slide (50×40×5?mm) at different magnifications during the test. The CGI test cell imposes both an axial and a planar strain on the sample. The first step is to prepare a thick slide from a core and to describe its "initial" state. The second stage is to undertake a creep test to rupture, while the third step is to study the fragments of the ruptured material using XRD (X-ray diffractometry), SEM (scanning electron microscopy) and EDS (energy dispersive spectrometry). The samples studied are from the shales of the Callovo-Oxfordian of the Paris Basin, to the south of Bar-le-Duc in the east of France. These rocks consist of clay minerals (illite and kaolinite), quartz, calcite (22.8%), dolomite, feldspar and pyrite (1%). The pyrite is essentially found in the bioclasts. The creep test is carried out under thermomechanical loading with an axial stress of up to 12?MPa and a temperature rising to 80??°C. The relative humidity is maintained at more than 90%. After 33?days the axial strain reached 0.76%, with rupture of the thick slide occurring 10?days later. The deformation was related to the texture/heterogeneity of the rock and occurred mainly along fissuring. Rupture also occurred along bedding planes, particularly where oxidized pyritic bioclasts were present. The chemical phenomena noted were in part related to the formation of iron hydroxides and sulphuric acid. This reacts with the calcite to form gypsum, which has a lower density than the original pyrite and calcite. The XRD indicated that the iron hydroxides observed on the discontinuities are dominantly limmonite. Examination of the SEM and EDS suggested an increase in porosity, even at some distance from the macroscopic fissures.     

Kinetics of NMBA polymerization in a starved feed reactor

A starved feed reactor (SFR) is a semibatch polymerization reactor in which initiator and monomer are fed slowly into a fixed amount of solvent. The polymerization is carried out isothermally at elevated temperatures. The added initiator decomposes instantaneously, and the added monomer polymerizes immediately. The monomer concentration in the SFR is very low, ideally approaching zero. The reactor is in a starved status for reactants. The molecular weight of the product polymer can be effectively controlled by the feed ratio of monomer to initiator. This article presents a study on the kinetics of NBMA polymerization in an SFR. The model parameters were regressed with the experimental data. Although the mass fraction of polymer in the SFR was high (higher than 50%), viscosity was not high and the “gel effect” was weak because of the low molecular weight of the products. It was found that the termination rate constant is a power function of polymer concentration, and the lumped rate constant, k_p/(k)^1/2, can be modeled by the Arrhenius equation. The initiator efficiency, f, remained constant at a temperature less than 120°C, and increased with the temperature increasing at temperatures greater 120°C. The other parameters remained constant in the experiment range. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2068–2075, 2001