Asymptotic analysis of flow in wavy tubes and simulation of the extrusion process

The paper is devoted to the mathematical modelling of an extrusion process. Usually, an extruder has a very complicated geometry. This generates a lot of difficulties for computations of three‐dimensional flows. In the present paper, we develop and justify the asymptotic domain decomposition strategy in order to parallelize the computational process and reduce the memory. The error estimates are proved for the Stokes steady‐state equation in the two‐dimensional and three‐dimensional cases. Then, the asymptotic domain decomposition procedure is applied for numerical testing and computations of the non‐Newtonian fluid simulating a real process of the polymer extrusion. Copyright © 2007 John Wiley & Sons, Ltd.     

AFM study of the SIMS beam induced roughness in monocrystalline silicon in presence of initial surface or bulk defects of nanometric size

In this paper, the SIMS beam induced roughness (BIR) in monocrystalline Si in presence of initial surface or bulk defects of nanometric size is studied. We follow the development of the BIR by monitoring the increase of Si²⁺ and SiO₂⁺ signals during SIMS sputtering. The topography of the crater bottoms is measured at different steps of the evolution of the roughness using an atomic force microscope (AFM). We show that in presence of nanometric sized defects on the surface or in the bulk, the BIR develops far more rapidly than usual. It appears as soon as the crater reaches the defects and, as reported on Si free from any treatment, the same morphology evidencing waves perpendicular to the sputtering beam develops rapidly. This study of the behaviour of the BIR in presence of voluntarily introduced defects allows us to better understand the basic physical phenomena involved in its apparition.     

Analysis of a semiparametric mixture model for competing risks

Semiparametric mixture regression models have recently been proposed to model competing risks data in survival analysis. In particular, Ng and McLachlan (Stat Med 22:1097–1111, 2003) and Escarela and Bowater (Commun Stat Theory Methods 37:277–293, 2008) have investigated the computational issues associated with the nonparametric maximum likelihood estimation method in a multinomial logistic/proportional hazards mixture model. In this work, we rigorously establish the existence, consistency, and asymptotic normality of the resulting nonparametric maximum likelihood estimators. We also propose consistent variance estimators for both the finite and infinite dimensional parameters in this model.     

Langmuir isotherm analysis of novel branched per-fluorinated surfactants and their interactions with single stranded DNA

The efficient synthesis and surface properties of new fluorinated gemini surfactants are described. The aim of this study was to investigate the relationships between the molecular structure and the Langmuir layer properties of these fluorinated gemini lipids. The electrostatic ssDNA binding interactions of amino groups included on the spacer were also investigated. The synthesis corresponds to the substitution of vinyl fluorine atom of fluoro-unsaturated esters by a diethylene-oxide diamine via a Michael addition followed by a fluoride elimination reaction. For the spread layers, the measurements of surface pressure versus molecular area were performed with or without ssDNA in the subphase. The monolayers characteristics depend on the hydrophobic chain length, the polar-head, the pH of the subphase and the flexibility of the spacer. The introduction of ssDNA in the subphase seems to show a low interaction between the surfactants and the oligonucleotide.     

Ceramide N-acyl chain length: a determinant of bidimensional transitions, condensed domain morphology, and interfacial thickness

Several lipids of biological interest are able to form monomolecular surfaces with a rich variety of thickness and lateral topography that can be precisely controlled by defined variations of the film composition. Ceramide is one of the simplest sphingolipids, consisting of a sphingosine base N-linked to a fatty acid, and is a membrane mediator for cell-signaling events. In this work, films of ceramides N-acylated with the saturated fatty acids C10, C12, C14, and C16 were studied at the air-aqueous interface. The dipole moment contribution (from surface potential measurements) and the surface topography and thickness (as revealed by Brewster angle microscopy) were measured simultaneously with the surface pressure at different molecular areas. Several surface features were observed depending on the asymmetry between the sphingosine and the N-linked acyl chains. At 21 °C, the C16:0 and C14:0 ceramides showed condensed isotherms and the film topography revealed solid film patches (17.3-15.7 ? thick) that coalesced into a homogeneous surface by further compression. On the other hand, in the more asymmetric C12:0 and C10:0 ceramides, liquid expanded states and liquid expanded-condensed transitions occurred. In the phase coexistence region, the condensed state of these compounds formed flowerlike domains (11.1-13.3 ? thick). C12:0 ceramide domains were larger and more densely branched than those of C10:0 ceramide. Both the film thickness and the surface dipole moment of the condensed state increased with ceramide N-acyl chain length. Bending of the sphingosine chain over the N-linked acyl chain in the more asymmetric ceramides can account for the variation of the surface electrostatics, topography, and thickness of the films with the acyl chain mismatch.     

Phase transition of ice precipitated in an aqueous ionic glassy matrix

In an easily forming glass aqueous electrolyte, an ice precipitation appears, that is variable with water to salt mole-ratio R. We here describe the 1 _c → 1 _h phase transition. The initial conditions of these transitions are small crystallites, a condensed form of water, with size varying from clusters to 250 Å. The transition to 1 _h can be considered as healing of the 1_h faults, which is accompanied by an apparent weakening of the cubic structure due to symmetry loss. Order parameter and specific effects, such as branching, are postulated in this growth which appears as a random walk in an ultrametric space.     

The Gold(I)‐Catalysed Protodecarboxylation Mechanism

A mechanistic study of the gold‐catalysed protodecarboxylation is described. Each reaction step has been investigated experimentally and computationally. More specifically, the activation parameters for the decarboxylation step have been determined through kinetic studies. Further experimental studies on the hydrolysis of the arylgold intermediate have revealed that the protodeauration can become competitive with the decarboxylation process at high conversions. This switch in rate‐limiting step has been shown to be pK_a‐dependent. These studies have been supported by DFT calculations and permit a better understanding of which prevalent features of the reaction mechanism account for the decarboxylation process.     

Mirror Electron Microscopy (MEM): Work function and imaging of an electron beam biased junction of silicon (100)

We describe in this paper the use of MEM in ultra high vacuum to visualize surface potential, and to obtain projected shadow images and the work function, applied to a silicon (100) surface. We show that the work function does not depend on the n or p doping of the sample and, simultaneously, we show that it is possible to visualize surface potential of approximately 10 mV, the junction bias being ensured by the electron beam.