The influence of surface forces on shear-induced tracer diffusion in mono and bidisperse suspensions

The shear-induced self-diffusivity of tracer particles of radius a _t = λa in a suspension of particles having a radius, a , is calculated by Stokesian dynamics for different values of the size ratio, λ , both in 2 and 3 dimensions in the binary-collision regime. The self-diffusion is found to decrease strongly when the size ratio becomes quite different from unity. On the other hand, for the same average distance of contact between two spheres, the presence of a soft force always increases greatly the diffusion compared to the effect of a hard shell which is used to model the roughness. This is particularly true for tracer particles smaller than the bath particles, where the shear-induced diffusion can be increased by many order of magnitudes in the presence of a soft force. For suspensions of monodisperse particles we show that, for low volume fraction, the diffusion coefficient is much smaller than the one predicted by the binary collision model, due to the existence of a layered structure. On the contrary at higher volume fraction, many-body collisions strongly enhance the diffusion and it appears that the value of the diffusion is quite sensitive to the presence of clusters of particles which are themselves determined by the range of interparticle forces.     

Hierarchy of One-Dimensional Models in Nonlinear Elasticity

By using formal asymptotic expansions, we build one-dimensional models for slender hyperelastic cylinders submitted to conservative loads. According to the order of magnitude of the applied loads, we obtain a hierarchy of models going from the linear theory of flexible bars to the nonlinear theory of extensible strings.

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Résumé On construit, à l'aide de développements asymptotiques formels, des modèles unidimensionnels de cylindres hyperélastiques élancés soumis à des forces conservatives. Suivant l'ordre de grandeur des forces appliquées, on obtient une hiérarchie de modèles allant de la théorie des poutres flexibles jusqu'à la théorie des fils élastiques.

     

Laser-assisted local patterning of ZnO-based spots for mirror-less lasing

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.     

The first chemical synthesis of the core structure of the benzoylhydrazine-NAD adduct, a competitive inhibitor of the Mycobacterium tuberculosis enoyl reductase

[reaction: see text] An isoniazid-NAD adduct has been recently proposed as the ultimate metabolite responsible for the antituberculous activity of isoniazid (INH). Its structure results from binding of the isonicotinoyl radical at C4 position of the nicotinamide ring of NAD with further possible and debated cyclization to form a cyclic hemiamidal derivative. Replacing the pyridine cycle of INH in INH-NAD adduct by a phenyl cycle (BH-NAD adduct) was shown previously to still retain the activity. On these bases, the core structure (4-benzoyl-1,4-dihydronicotinamide ribonucleoside) of the BH-NAD adduct and a series of analogues have been synthesized by using 3,4-pyridinedicarboximide as starting material. Depending on the nature of the substituent (pyridine or aryl) and on the oxidized or the reduced state of the nicotinamide nucleus, they were found either in a cyclized hemiamidal or an opened form or were shown to exist in equilibrium under cyclized or opened forms. Although none of these compounds could significantly inhibit activity of the InhA or MabA reductases (two possible targets of isoniazid), they represent attractive targets to develop potential second-generation inhibitors, including the total chemical synthesis of the bioactive BH-NAD adduct.     

Laser induced formation of periodic nanostructures in silicon covered by SiO2

Three-dimensional (3D) and two-dimensional (2D) periodic silicon nanostructures formed by polarized focused Nd:YAG laser irradiation (532 nm) with spot size less than 3 μm on Si covered by SiO₂ are presented in this paper. We observed that at a low laser intensity I range, from I=0.9 to 1.08 W, 2D periodic coexisting of liquid and solid exists, while for 1.08<I<1.44 W, 3D periodic ripples were formed. However, when the light intensity is out of those ranges, either no melting was created (I<0.9 W) or the periodicity was destroyed (I>1.44 W). The periodicity of these periodic structures is 359 nm related to the wavelength of frequency doubledNd:YAG laser and the index of refraction of SiO₂. We propose a model based on the fact that as the oxygen is diffusing locally from SiO₂ into the melted Si, thus forming SiO_β with a lower melting point, successive pulses melt preferentially these regions giving rise to a positive feedback. This dynamic nanoscale modeling, based on variations of melting points of Si and dielectric and reflection coefficient, confirms the experimental results. PACS 81.07.–b; 81.10.Fq; 61.80.Ba; 66.10.Cb     

Effect of Scanning Beam Profile to Fabricate Fused Fiber Tapers by CO2 Laser Irradiation Method

Beam uniformity is a crucial building block of CO2 experiments aimed at fusing and stretching optical fibers in a lossless manner. When the irradiation beam is expanded through a galvanometer mirror, ways to achieve beam uniformity are investigated.     

Access a divers dichalcogenophenylenes metallocenes (M = Ti, Zr, Hf). Premier exemple d'un ditellurophenylene-zirconocene

Elementary sulfur and selenium combine (in boiling heptane) with [(tBuCp)₂-Zr(C₆H₄R)₂] (Cp = η⁵-C₅H₄; R = OCH₃) to give the corresponding dichalcogenophenylenezirconocene. With tellurium, the reaction proceeds only at lower temperature (in boiling hexane), affording the first ditellurophenylenezirconocene. As no metallacycle was obtained with the Cp ligand or when the metal is Hf, complexes of the general type [(RCp)₂MSe₂C₆H₄-o] (M = Ti, Zr, Hf; R = H, t-Bu, (CH₃)₅) have been synthesized by allowing metallocene dichlorides to react with potassium benzenediselenolate, prepared by cleaving [(t-BuCp)₂ZrSe₂C₆H₄-o] with t-BuOK.