Probing the dynamics of CO2 and CH4 within the porous zirconium terephthalate UiO-66(Zr): a synergic combination of neutron scattering measurements and molecular simulations

Quasi-elastic neutron scattering (QENS) measurements combined with molecular dynamics (MD) simulations were conducted to deeply understand the concentration dependence of the self- and transport diffusivities of CH(4) and CO(2), respectively, in the humidity-resistant metal-organic framework UiO-66(Zr). The QENS measurements show that the self-diffusivity profile for CH(4) exhibits a maximum, while the transport diffusivity for CO(2) increases continuously at the loadings explored in this study. Our MD simulations can reproduce fairly well both the magnitude and the concentration dependence of each measured diffusivity. The flexibility of the framework implemented by deriving a new forcefield for UiO-66(Zr) has a significant impact on the diffusivity of the two species. Methane diffuses faster than CO(2) over a broad range of loading, and this is in contrast to zeolites with narrow windows, for which opposite trends were observed. Further analysis of the MD trajectories indicates that the global microscopic diffusion mechanism involves a combination of intracage motions and jump sequences between tetrahedral and octahedral cages.     

Free convolution operators and free Hall transform

We define an extension of the polynomial calculus on a W^?$W^{?}$-probability space by introducing an algebra C{_Xi:i∈I}$\mathbb{C}\{X_i:i\in I\}$ which contains polynomials. This extension allows us to define transition operators for additive and multiplicative free convolution. It also permits us to characterize the free Segal–Bargmann transform and the free Hall transform introduced by Biane, in a manner which is closer to classical definitions. Finally, we use this extension of polynomial calculus to prove two asymptotic results on random matrices: the convergence for each fixed time, as N   tends to ∞, of the ?-distribution of the Brownian motion on the linear group _GLN(C)${\mathit{GL}}_N(\mathbb{C})$ to the ?-distribution of a free multiplicative circular Brownian motion, and the convergence of the classical Hall transform on U(N)$U(N)$ to the free Hall transform.     

Macroscopic model of multicomponent fluids in porous media

This article is about a macroscopic model simulating the heat and mass transfer of a fluid containing numerous species in porous media. In order to use it in the modelling of petroleum fields, the physical phenomena studied are gravitational segregation, thermal diffusion, convection and diffusion. The originality of this approach is to simulate the movement of each component in order to deduce the species distribution and mixture flow. The mass flux calculated by an entropic balance is compared with the Darcy equation to evaluate the phenomenological coefficients. After presenting the possible origins of composition variations and the ways that they are usually studied, the article describes the developments that have led to the new model and finally reports the first results obtained.     

Hyper‐Raman and Raman scattering in paratellurite TeO2

Raman spectra of the tetragonal structure of paratellurite TeO₂ have been revisited avoiding anomalous polarization‐selection‐rules violations previously observed and due to optical activity. We present a complementary hyper‐Raman scattering study of paratellurite. Wavenumber and symmetry assignments are given for all expected 21 Raman active optical branches, except one LO component (out of the eight expected TO–LO pairs) of the polar doublet E modes. Also, the four expected hyper‐Raman active A₂ (TO) modes have been observed. Moreover, we have observed a strong Kleinman‐disallowed hyper‐Rayleigh signal, which is tentatively assigned as a first evidence of hyper‐Rayleigh optical activity. Copyright © 2013 John Wiley & Sons, Ltd.     

Hydration of clays at the molecular scale: the promising perspective of classical density functional theory

We report here how the hydration of complex surfaces can be efficiently studied, thanks to recent advances in classical molecular density functional theory. This is illustrated on the example of the pyrophyllite clay. After presenting the most recent advances, we show that the strength of this implicit method is that: (1) it is in quantitative or semi-quantitative agreement with reference all-atom simulations (molecular dynamics here) for both the solvation structure and energetics, and (2) the computational cost is two to three orders of magnitude less than in explicit methods. The method remains imperfect in that it locally overestimates the polarisation of water close to hydrophylic sites of the clay. The high numerical efficiency of the method is illustrated and exploited to carry out a systematic study of the electrostatic and van der Waals components of the surface–solvent interactions within the most popular force field for clays, CLAYFF. Hydration structure and energetics are found to weakly depend upon the electrostatics. We conclude on the consequences of such findings on future force-field development.     

Ultrasound propagation in trabecular bone: A numerical study of the influence of microcracks

The accumulation of microdamage in trabecular bone tissue is suspected of being a predictive indicator of osteoporosis diagnosis. To quantify this microdamage, the Dynamic AcoustoElastic Testing (DAET) method measures the Time Of Flight (TOF) and amplitude variations of transmitted ultrasound (US) pulses, while the bone sample is submitted to a low frequency sinusoidal hydrostatic pressure (opening/closing of microcracks). However, DAET is both sensitive to viscoelastic properties changes and microcracks density. To verify the microcracks density contribution on DAET results, a numerical approach is proposed. Multliple configurations of microdamaged trabecular bone-tissue-like mesh have been simulated. A 2D pseudo-spectral time domain numerical model was then developed to simulate linear wave propagation in heterogeneous solids. The influence of the microcracks number and orientation on the US TOF was particularly investigated. Results are discussed and compared with experimental data obtained from DAET measurements in trabecular bone samples.     

The GRANIT spectrometer

The existence of quantum states of matter in a gravitational field was demonstrated recently in the Institut Laue–Langevin (ILL), Grenoble, in a series of experiments with ultra cold neutrons (UCN). UCN in low quantum states is an excellent probe for fundamental physics, in particular for constraining extra short-range forces; as well as a tool in quantum optics and surface physics. The GRANIT is a follow-up project based on a second-generation spectrometer with ultra-high energy resolution, permanently installed in ILL. It has been constructed in framework of an ANR grant; and will become operational in 2011.