Effet d'un champ magnétique transversal sur la stabilité de l'écoulement de Hartmann : les modes tridimensionnels

A numerical study is conducted in order to determine the influence of a transverse magnetic field on the three-dimensional instabilities of the Hartmann flow. We prove that the Squire transformation currently used to characterize such instabilities give satisfactory results only in the case where Ha is weak and 3D modes are slightly deviated from the 2D modes. Such a study is justified by the fact that the two-dimensional instabilities could not always explain successfully the experimental observations relating to the transition phenomenon. To cite this article: M. Jédidi et al., C. R. Mecanique 333 (2005).     

Atomic force microscopy of model lipid membranes

Supported lipid bilayers (SLBs) are biomimetic model systems that are now widely used to address the biophysical and biochemical properties of biological membranes. Two main methods are usually employed to form SLBs: the transfer of two successive monolayers by Langmuir–Blodgett or Langmuir–Schaefer techniques, and the fusion of preformed lipid vesicles. The transfer of lipid films on flat solid substrates offers the possibility to apply a wide range of surface analytical techniques that are very sensitive. Among them, atomic force microscopy (AFM) has opened new opportunities for determining the nanoscale organization of SLBs under physiological conditions. In this review, we first focus on the different protocols generally employed to prepare SLBs. Then, we describe AFM studies on the nanoscale lateral organization and mechanical properties of SLBs. Lastly, we survey recent developments in the AFM monitoring of bilayer alteration, remodeling, or digestion, by incubation with exogenous agents such as drugs, proteins, peptides, and nanoparticles.

Optimal distributed-control of vortices in Navier–Stokes flows

Optimal control of time-dependent fluid-flow governed by the incompressible Navier–Stokes model is considered. A cost functional based on a local dynamical systems characterization of vortices is investigated. The resulting functional is a non-convex function of the velocity gradient tensor. The optimality systems based on a Lagrangian formulation and adjoint equations describing first order necessary optimality conditions is provided. The first and the second derivative of the cost functional with respect to the control are derived. To cite this article: S. Chaabane et al., C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

The Vlasov–Poisson System for Stellar Dynamics in Spaces of Constant Curvature

We obtain a natural extension of the Vlasov–Poisson system for stellar dynamics to spaces of constant Gaussian curvature \({\kappa \ne 0}\): the unit sphere \({\mathbb S^2}\), for \({\kappa > 0}\), and the unit hyperbolic sphere \({\mathbb H^2}\), for \({\kappa < 0}\). These equations can be easily generalized to higher dimensions. When the particles move on a geodesic, the system reduces to a 1-dimensional problem that is more singular than the classical analogue of the Vlasov–Poisson system. In the analysis of this reduced model, we study the well-posedness of the problem and derive Penrose-type conditions for linear stability around homogeneous solutions in the sense of Landau damping.     

Efficient Solution Phase Synthesis of PPII Helix Mimicking Ena/VASP EVH1 Inhibitors from Proline-Derived Modules (ProMs)

In the search for efficient inhibitors for the enabled/vasodilator-stimulated phosphoprotein homology 1 (EVH1) domain to reduce cell motility in metastatic cancer, we previously developed a toolkit of proline-derived modules (ProMs), which mimic the PPII helix found in the natural −FPPPP− binding motif of EVH1. In this work, we describe the modular assembly of these ProM-based pentapeptidic EVH1 ligands through liquid phase peptide synthesis. We initially used pentafluorophenyl (Pfp) active esters for amide bond formation and built up the growing peptide chain from the C- to the N-terminus. Switching to HATU/DIPEA coupling conditions and changing the directionality of the synthesis from the N- to the C-terminus afforded the target ligands with improved overall yields and purity. Employing a Fmoc-protected (instead of the N-acetylated) phenylalanine derivative as N-terminal building block significantly reduced epimerization. In contrast to the originally used solid phase peptide synthesis (SPPS), the developed solution phase method allowed for a facile alteration of the C-terminal ProM unit and the production of various pentapeptidic ligands in an efficient fashion even on a multigram scale.     

Synthesis and structural study of copolymers of l-lactic acid and bis(2-hydroxyethyl terephthalate)

Poly(ethylene terephthalate)-poly(lactic acid) (PET-PLLA) copolyesters were synthesized by the melt reaction of bis(2-hydroxyethyl terephthalate) (BHET) with l-lactic acid oligomers (OLLA) in the presence of SnCl₂, H₂O-p-toluene sulfonic acid, H₂O catalytic system. The ¹H and ¹³C NMR studies confirm the incorporation of lactate units in PET chains after reaction. Copolyesters containing nearly equimolar terephthalate/lactate ratio are not completely random and present some block-copolymer character, while the microstructure of PET-rich copolyesters is a random one. Due to a longer PET sequence length, the latter exhibit a melting point close to 210 °C while the other ones are amorphous. SEC/MALDI-TOF MS off-line coupling was used to obtain the absolute average molar masses of the copolyesters. The results indicate that the conventional polystyrene calibration method leads to a strong overestimation of PET-PLLA molar masses, while the determined by NMR is much closer to the SEC/MALDI value.     

Microwave-accelerated multicomponent synthesis and X-ray characterization of novel benzothiadiazinone dioxide derivatives,analogues of Monastrol

Research on Chemical Intermediates - We demonstrate here a rapid and comfortable synthetic sequence to benzothiadiazinone dioxide (six-membered cyclic sulfamide) derivatives, analogues of...     

Magnetic control of natural convection in the horizontal Bridgman configuration using a spectral method: transversal plan

The present study investigates the electromagnetic braking of buoyancy convective flows occurring in differentially heated cavities, filled with low Prandtl, dilute, incompressible and electrically conducting alloys, and subjected to a constant horizontal temperature gradient. In practice, such flows known as ‘Hadley circulation’ are relevant in material processing technologies, such as the horizontal Bridgman configuration. A collocation spectral numerical method is developed to solve the two-dimensional Navier–Stokes equations, modelling the flow phenomena occurring in such configurations, using a vorticity–stream function formulation. The two components of the velocity are deduced from the stream function and the temperature distribution is obtained through the resolution of the energy conservation equation. The results in terms of velocity and temperature distributions for a given Grashof number are obtained for various Hartmann numbers and show that as the Hartmann number increases, the electromagnetic braking of the flow is observed. Moreover, the results illustrate the changes affecting the flow structure which becomes quasi-parallel in the core region of the cavity for sufficiently high values of Ha and the onset of the Hartmann and parallel layers along the boundaries. Also, with increasing Ha, the isotherms are less affected by the convective flow and become parallel to the vertical walls indicating that heat transfer is mainly achieved by conduction.     

A double microbeam MEMS ohmic switch for RF-applications with low actuation voltage

In this paper, we propose the design of an ohmic contact RF microswitch with low voltage actuation, where the upper and lower microplates are displaceable. We develop a mathematical model for the RF microswitch made up of two electrostatically actuated microplates; each microplate is attached to the end of a microcantilever. We assume that the microbeams are flexible and that the microplates are rigid. The electrostatic force applied between the two microplates is a nonlinear function of the displacements and applied voltage. We formulate and solve the static and eigenvalue problems associated with the proposed microsystem. We also examine the dynamic behavior of the microswitch by calculating the limit-cycle solutions. We discretize the equations of motion by considering the first few dominant modes in the microsystem dynamics. We show that only two modes are sufficient to accurately simulate the response of the microsystem under DC and harmonic AC voltages. We demonstrate that the resulting static pull-in voltage and switching time are reduced by 30 and 45%, respectively, as compared to those of a single microbeam-microplate RF-microswitch. Finally, we investigate the global stability of the microswitch using different excitation conditions.     

The potent antimalarial peptide cyclosporin A induces the aggregation and permeabilization of sphingomyelin-rich membranes

Cyclosporin A (CsA) is a hydrophobic peptide drug produced by the fungus Tolypocladium inflatum. CsA is commonly used as an immunosuppressive drug, but it also has antimalarial activity. The immunosuppressive activity of CsA is clearly due to its association with specific proteins of immune cells such as cyclophilins. By contrast, the antimalarial properties of this peptide are completely independent of the association with a parasite's cyclophilins. Because of its hydrophobicity, CsA may interact with biological membranes, which may participate in its therapeutic effect. Recently, we have shown a marked preference of CsA for insertion into sphingomyelin (SM) monolayers. In this article, we measure for the first time the ability of CsA to induce permeabilization and aggregation and to change the lipid order, especially in the presence of SM. Calcein-release experiments permitted us to show that CsA causes the leakage of the fluorescent probe from SM-rich liposomes by 40% and PC liposomes by 11%, suggesting a lipid-selective effect. Electron microscopy and dynamic light scattering experiments confirmed the different interaction of CsA with SM and PC vesicles: it formed much larger aggregates with SM than with PC. Our results taken together suggest that CsA could specifically weaken and aggregate SM-rich membranes, which could in turn explain why CsA is efficient in the treatment of malaria. Indeed, CsA could inhibit the development of Plasmodium by permeabilizing and aggregating the SM-rich membrane network formed by the parasite during its intraerythrocytic growth cycle.