Polarized Raman spectra of brushite (CaHPO4.2H2O) crystal. Investigation of the phosphate stretching modes,study of the LOTO splitting

Polarized Raman measurements were recorded on a monoclinic brushite (CaHPO₄.2H₂O) crystal in the 800–1200 cm⁻¹ spectral range, corresponding to the P O stretching modes. This study is a continuation of the investigation of the phosphate stretching modes observed in polarized infrared reflectance spectra of brushite crystal. In such ionic non‐centrosymmetric crystals, the splitting between the transverse and longitudinal components of the optic vibrations was observed in the polarized Raman spectra recorded at several scattering geometries. A″ symmetry modes of the brushite crystal were measured. Using a simple model based on the symmetry of the PO₄ group, the Raman intensities of the stretching modes are calculated and compared with experimental bands. Copyright © 2016 John Wiley & Sons, Ltd.     

X‐ray fluorescence induced by standing waves in the grazing‐incidence and grazing‐exit modes: study of the Mg–Co–Zr system

The characterization of Mg–Co–Zr tri‐layer stacks using X‐ray fluorescence induced by X‐ray standing waves, in both the grazing‐incidence (GI) and the grazing‐exit (GE) modes, is presented. The introduction of a slit in the direction of the detector improves the angular resolution by a factor of two and significantly improves the sensitivity of the technique for the chemical characterization of the buried interfaces. By observing the intensity variations of the Mg Kα and Co Lα characteristic emissions as a function of the incident (GI mode) or detection (GE mode) angle, it is shown that the interfaces of the Si/[Mg/Co/Zr]_×30 multilayer are abrupt, whereas in the Si/[Mg/Zr/Co]_×30 multilayer a strong intermixing occurs at the Co‐on‐Zr interfaces. An explanation of this opposite behavior of the Co‐on‐Zr and Zr‐on‐Co interfaces is given by the calculation of the mixing enthalpies of the Co–Mg, Co–Zr and Mg–Zr systems, which shows that the Co–Zr system presents a negative value and the other two systems present positive values. Together with the difference of the surface free energies of Zr and Co, this leads to the Mg/Zr/Co system being considered as a Mg/Co_xZr_y bi‐layer stack, with x/y estimated around 3.5.     

Rheology of highly concentrated anionic surfactants

Surfactant solution flow behavior is of great importance to both the chemical and consumer product industries. Most studies on the flow behavior of surfactant solutions, however, have focused on the dilute regime. Seldom reported is rheology in the highly concentrated regime where typically these surfactants are processed and delivered. First, we present here the phase diagram for the ternary system: water and two anionic surfactants (sodium salt of lauric and oleic acid) at different temperatures. Then, we present both linear viscoelastic and steady shear flow results in the high (70 to 90%) surfactant regime. We find that high values of the shear modulus are directly dependent on the quantity of surfactant crystals and that the formation of a lamellar liquid crystal phase at 45°C affects both modulus and flow of the system. Lamellar crystals create a stiff network resulting in wall slip at large shear strain. Using serrated plates removes slip at the wall and we find a shear rate where microfractures localize in a preferential plane and the material flows. This behavior is reversible.     

Large-amplitude internal solitary waves in a two-fluid modelOndes solitaires internes de grande amplitude dans un modèle à deux fluides

We compute solitary wave solutions of a Hamiltonian model for large-amplitude long internal waves in a two-layer stratification. Computations are performed for values of the density and depth ratios close to oceanic conditions, and comparisons are made with solutions of both weakly and fully nonlinear models. It is shown that characteristic features of highly nonlinear solitary waves such as broadening are reproduced well by the present model. To cite this article: Ph. Guyenne, C. R. Mecanique 334 (2006).     

On the continuum modeling of carbon nanotubes

We have recently proposed a nanoscale continuum theory for carbon nanotubes. The theory links continuum analysis with atomistic modeling by incorporating interatomic potentials and atomic structures of carbon nanotubes directly into the constitutive law. Here we address two main issues involved in setting up the nanoscale continuum theory for carbon nanotubes, namely the multi-body interatomic potentials and the lack of centrosymmetry in the nanotube structure. We explain the key ideas behind these issues in establishing a nanoscale continuum theory in terms of interatomic potentials and atomic structures.     

Numerical and experimental study of two turbulent opposed plane jets

The turbulent interaction between two opposed plane jets separated by a distance H is experimentally studied by using a PIV (Particle Image Velocimetry) method and numerically investigated by means of a finite volume code. Two turbulence models have been tested: the standard k-ɛ model and a second-order model. The validation of the numerical study was performed by comparing the results with experimental data obtained for the case of two interacting opposed jets at ambient temperature (isothermal case). The effect of the angle of inclination of the jets is studied. Conclusions of the validation are then used to study the interaction between two jets, one being maintained at ambient temperature whereas the other is heated. Results show that the stagnation point moves towards the heated jet. It is shown that the heating induces a stabilizing effect on the flow.     

High-speed imaging of an ultrasound-driven bubble in contact with a wall: “Narcissus” effect and resolved acoustic streaming

We report microscopic observations of the primary flow oscillation of an acoustically driven bubble in contact with a wall, captured with the ultra high-speed camera Brandaris 128 (Chin et al. 2003). The driving frequency is up to 200 kHz, and the imaging frequency is up to 25 MHz. The details of the bubble motion during an ultrasound cycle are thus resolved, showing a combination of two modes of oscillations: a radius oscillation and a translation oscillation, perpendicular to the wall. This motion is interpreted using the theory of acoustic images to account for the presence of the wall. We conclude that the bubble is subjected to a periodic succession of attractive and repulsive forces, exerted by its own image. Fast-framing recordings of a tracer particle embedded in the liquid around the particle are performed. They fully resolve the acoustic streaming flow induced by the bubble oscillations. This non-linear secondary flow appears as a tiny drift of the particle position cycle after cycle, on top of the primary back and forth oscillation. The high oscillation frequency accounts for a fast average particle velocity, with characteristic timescales in the millisecond range at the lengthscale of the bubble. The features of the bubble motion being resolved, we can apply the acoustic streaming theory near a wall, which provides predictions in agreement with the observed streaming velocity.     

Nonlinear Hyperbolic Systems: Nondegenerate Flux,Inner Speed Variation,and Graph Solutions

We study the Cauchy problem for general nonlinear strictly hyperbolic systems of partial differential equations in one space variable. First, we re-visit the construction of the solution to the Riemann problem and introduce the notion of a nondegenerate (ND) system. This is the optimal condition guaranteeing, as we show it, that the Riemann problem can be solved with finitely many waves only; we establish that the ND condition is generic in the sense of Baire (for the Whitney topology), so that any system can be approached by a ND system. Second, we introduce the concept of inner speed variation and we derive new interaction estimates on wave speeds. Third, we design a wave front tracking scheme and establish its strong convergence to the entropy solution of the Cauchy problem; this provides a new existence proof as well as an approximation algorithm. As an application, we investigate the time regularity of the graph solutions (X,U) introduced by LeFloch, and propose a geometric version of our scheme; in turn, the spatial component X of a graph solution can be chosen to be continuous in both time and space, while its component U is continuous in space and has bounded variation in time.     

Effect of Wetting on Gravity Drainage in Porous Media

A series of benchmark experiments on the effect of the wetting state on the flow properties in porous media were performed, allowing us to relate the wetting properties at the pore scale to the macroscale hydrodynamics. Drainage of n-alkanes (oils) displaced by air in a model porous medium consisting of water-wet sand was studied using gamma-ray densitometry and weight measurements. The enormous advantage of our system is that we know and control the wetting properties perfectly: we can tune the wetting properties by changing the salinity of the water. This allows us to perform porous medium flow experiments for the different wetting states without changing the transport properties (viscosity and density) of the oil. Drainage is found to be more efficient, and consequently oil recovery more important for partial wetting.     

Far-Wake of a Sphere in a Stably Stratified Fluid: Characterization of the Vortex Structures

This article discusses the structure of the far-wake of a towed sphere in a saline stratification. We compare very low Froude number experiments to existing results at higher Froude numbers and investigate the vertical structure of the far-wake in terms of their vorticity and density fields. We show that the vertical propagation of vorticity is viscously dominated and propose a simple three-dimensional model for the quasi-equilibrium of the structure in terms of the density field.