Wide-range viscoelastic measurement using resonating sensors

We have developed a specially designed sensor with its associated instrumentation using a so called acoustical near-field technique based on small resonating horns. Contrary to the conventional measuring devices used in the rheology industry, this sensor enables wide-range and continuous viscosity measurements, including liquid to solid material transition. The sensor is composed of a small piezoelectric element which enables the horn to resonate. Shear waves are generated by means of the thinner part of the sensor (dipped into the material) confining the acoustic field to the tip to ensure local measurements. The frequency is scanned across the resonating frequencies in order to perform these measurements with the maximum of speed and efficiency. The shear moduli G' and G' of the material is deduced from the electrical impedance measurement at the resonance frequencies. In case of rapidly changing materials, such as quick setting cements or polyesters, we have developed a specific fast instrumentation based on the impulse response of the sensor, which realizes very fast measurements, typically 30 measurements per second. Modelization and experiments are reported.     

Experimental evidence of 25-fs laser pulse distortion in singlet beam expanders

We report the measurement of spatiotemporal distortions of an ultrashort pulse in singlet beam expanders. With a simple second-order autocorrelator the temporal broadening of the pulse from 23 to 40 fs, due to propagation time difference (PTD), is determined. The delay due to PTD between different parts of the beam is also measured. This effect was theoretically studied for the first time by Bor [J. Mod. Opt. 35, 1907 (1988)]. These experimental results are in good agreement with the calculations of a dedicated three-dimensional ray-tracing program developed to simulate the spatial and temporal transformation of femtosecond pulses in optical systems.     

High-order harmonic generation by nonlinear reflection of an intense high-contrast laser pulse on a plasma

We demonstrate the use of a plasma mirror to obtain 60-fs 10-TW laser pulses with a temporal contrast of 10(8) on a nanosecond time scale and 10(6) on a picosecond time scale, and we use these high-contrast pulses to generate high harmonics by nonlinear reflection on a plasma with a steep electronic density gradient. Well-collimated harmonics up to 20th order are observed for a laser intensity of approximately equal to 3 x 10(17) W/cm2, whereas no harmonics are obtained without the plasma mirror.     

A novel diastereoselective route to alpha-hydroxyalkyl dihydropyrans using an hetero Diels-Alder/allylboration sequence

The development of a new strategy for the synthesis of alpha-hydroxyalkyl dihydropyrans is reported. This approach is based on a tandem hetero[4 + 2]/allylboration reaction.     

Convergent access to ketones,vinyl esters and vinyl bromides by a tin-free radical addition-intramolecular hydrogen atom transfer

Xanthate-mediated intermolecular radical addition, hydrogen atom transfer and sulfonyl radical elimination have been efficiently combined in a new convergent synthesis of ketones and substituted olefins.     

Enumeration of Planar Constellations

The enumeration of transitive ordered factorizations of a given permutation is a combinatorial problem related to singularity theory. Let n ≥ 1, and let σ₀ be a permutation of _n having d_i cycles of length i, for i ≥ 1. Let m ≥ 2. We prove that the number of m-tuples (σ₁,…,σ_m) of permutation of _n such that

• σσ···σ = σ,

• the group generated by σ,…,σ acts transitively on {1, 2,…,},

• ∑ (σ) = ( − 1) + 2, where (σ) denotes the number of cycles of σ

A one-to-one correspondence relates these m-tuples to some rooted planar maps, which we call constellations and enumerate via a bijection with some bicolored trees. For m = 2, we recover a formula of Tutte for the number of Eulerian maps. The proof relies on the idea that maps are conjugacy classes of trees and extends the method previously applied to Eulerian maps by the second author. Our result might remind the reader of an old theorem of Hurwitz, giving the number of m-tuples of transpositions satisfying the above conditions. Indeed, we show that our result implies Hurwitz' theorem. We also briefly discuss its implications for the enumeration of nonequivalent coverings of the sphere.     

Numerical Simulations of Screened Coulomb Systems. A Comparison Between Hyperspherical and Periodic Boundary Conditions

Numerical simulations or Coulomb systems can be performed in various geometries, for instance in a cube with periodic boundary conditions ( ³) or on the surface of a hypersphere or on the surface of a hypersphere ( ³). We show how to extend these methods of simulations to the case of screened (Yukawa) potentials. We make a detailed comparison between the properties of Yukawa systems in these two geometries and derive the correct configurational energies of some models such as the Yukawa restricted primitive model and the Yukawa one component plasma.     

Bioactive peptides grafted silicone dressings: A simple and specific method

The need for bioactive dressings increases with the population aging and the prevalence of chronic diseases. In contrast, there are very few dressings on the market which are designed to display a chosen bioactivity. In this context, we investigated the surface-functionalization of silicone wound dressing with bioactive peptides. One of the challenges was to avoid multistep grafting reactions involving catalysts, solvents or toxic reagents, which are not suitable for the fabrication of medical devices at an industrial scale. In the other hand, a covalent bonding was necessary to avoid the loss of the biological effect by progressive removal of the peptide in biological fluids generated by the wound. To solve these limitations, we developed a strategy allowing an easy and direct functionalization of silicone. This strategy relies on hybrid silylated bioactive peptides, which chemoselectively react with plasma-activated silicone surfaces. We synthesized three hybrid peptides with wound healing properties, which were grafted on commercially available silicone dressings Cerederm^® and Mepitel^®. Grafted dressings were evaluated in vitro and enabled a quicker scare recovery and extracellular matrix deposition with human dermal fibroblasts. These results were confirmed by in vivo studies showing an enhanced wound-healing of the pig skin. By this simple method, we transformed inert dressing into bioactive dressing which showed properties of wound healing.     

Pont entre les « micro » et « méso » mécaniques des composites stratifiésRelationships between ‘micro’ and ‘meso’ mechanics of laminated composites

A relatively complete bridge is built between the micromechanics of laminates for which there are numerous theoritical and experimental works and the mesomechanics of laminates introduced and developped in particular at Cachan for more than 15 years. At the ‘meso’ scale, the ply and the interface are homogeneized. To cite this article: P. Ladevèze, G. Lubineau, C. R. Mecanique 331 (2003).     

Quasistatic Crack Growth in Nonlinear Elasticity

In this paper, we prove a new existence result for a variational model of crack growth in brittle materials proposed in [19]. We consider the case of n-dimensional nonlinear elasticity, for an arbitrary n1, with a quasiconvex bulk energy and with prescribed boundary deformations and applied loads, both depending on time.