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The effective wavenumber of the coherent wave propagating in a fluid containing parallel porous cylinders randomly distributed in space is derived at the Rayleigh limit for (i) explicit formulas: Independent Scattering Approximation (ISA), Waterman and Truell (WT) and Linton and Martin (LM) and (ii) for implicit formulas: Coherent Potential Approximation (CPA) and Generalized Self Consistent Method (GSCM) applied to WT and to LM. The effective mass density and bulk modulus are also derived. The validity of all the effective quantities is checked by recovering, when the porosity of the scatterers tends to zero, the case of an inhomogeneous medium of elastic cylinders. © 2016 Elsevier B.V. Source


Duchemin B.J.C.,CNRS Laboratory of Waves and Complex Media
Green Chemistry | Year: 2015

In this study, mercerisation of native cellulose I was achieved in an aqueous sodium hydroxide solution at a concentration of only 1 wt% NaOH by processing at temperatures below 0 °C. This represents a tenfold reduction in the use of NaOH to accomplish this very common transformation. The cellulose sample was a form of hydrolysed cotton with a high crystallinity. The samples were mixed with aqueous sodium hydroxide at various concentrations and stored at -17 °C. The samples were then defrosted, neutralised and dried before being analysed by Fourier-transform infrared spectroscopy, wide-angle X-ray diffraction and field-emission scanning electron microscopy. In the route described here, transformation from cellulose I to cellulose II was possible without greatly affecting the crystallinity or the microstructure of the samples. © The Royal Society of Chemistry 2015. Source


Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2015

We report nearly exact quantum time-independent calculations of rate coefficients for the collisional (de-)excitation of H2 by H, from low to high temperatures. Our calculations are based on a highly accurate global potential energy surface. The reactive hydrogen exchange channels are taken into account rigorously. New collisional data are obtained for the rovibrational relaxation of highly excited H2 (with internal excitation up to ≃22 000 K) for temperatures ranging from 100 to 5000 K. We also provide a comparison with the available experimental rate coefficients at room temperature. The good agreement between theory and experiment is an illustration of the accuracy of the present calculations. The new results significantly differ from previous data presently used in astrophysical models, especially at low temperatures, the difference being essentially due to the inclusion of the reactive channels. The impact of these new data in astrophysics is discussed. In particular, the coolingmechanism will have to be reviewed for several astrophysical media. © 2015 The Author. Source


Roueff E.,Laboratoire Univers et Theories | Lique F.,CNRS Laboratory of Waves and Complex Media
Chemical Reviews | Year: 2013

Collisional excitation is the basic process, and its efficiency relies on the composition of the medium providing the density of the main perturbers and the temperature, which establishes the degree of excitation and, consequently, drives the intensities of the radiated emission. Nevertheless, other excitation mechanisms can be at work such as radiative or chemical pumping. The computation of collisional inelastic rate coefficients usually takes place within the Born-Oppenheimer approximation for the separation of electronic and nuclear motions. Scattering cross sections are thus obtained by solving the motion of the nuclei on an electronic potential energy surface (PES) that is independent of the masses and spins of the nuclei. Recent studies have demonstrated that computational techniques employing advanced treatments for both the electronic and nuclear motion problems can rival experimental measurements, in terms of the achieved accuracy. Source


Faure A.,CNRS Grenoble Institute for Particle Astrophysics and Cosmology Laboratory | Lique F.,CNRS Laboratory of Waves and Complex Media
Monthly Notices of the Royal Astronomical Society | Year: 2012

Nuclei with non-zero spin induce hyperfine splittings in the rotational spectrum of many commonly observed interstellar molecules. Radiative transfer modelling of such species requires in general a good knowledge of hyperfine selective collisional rate coefficients. We investigate in this work the impact of collisional rate coefficients on the molecular hyperfine excitation. The approximate sudden and statistical (proportional) methods are first compared to the almost exact recoupling approach. Rate coefficients are presented for a large number of CN and HCN transitions, with para-H 2(j = 0) as a collider. The sudden approximation and the recoupling approach, which both predict the propensity rule Δj = ΔF, are found to agree within a factor of 3 or better. Radiative transfer calculations are then performed using the large velocity gradient approximation. At low and moderate total optical depths (τ ≲ 10), where the relative hyperfine populations are close to the statistical weights, both the sudden and the statistical approximations are shown to provide accurate alternatives to the recoupling approach. At higher total opacities, however, the hyperfine propensity rule appears to matter and the sudden method is found to be significantly superior to the statistical approach. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

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