The "strange term" in the periodic homogenization for multivalued Leray-Lions operators in perforated domains

International audienceUsing the periodic unfolding method of Cioranescu, Damlamian and Griso, we study the homogenization for equations of the form -\Div d_\varepsilon=f,\text{ with }\bigl(\nabla u_{\varepsilon , \delta }(x),d_{\varepsilon , \delta }(x)\bigr) \in A_\varepsilon(x) in a perforated domain with holes of size $\varepsilon \delta$ periodically distributed in the domain, where $A_\varepsilon$ is a function whose values are maximal monotone graphs (on $\R^{N})$. Two different unfolding operators are involved in such a geometric situation. Under appropriate growth and coercivity assumptions, if the corresponding two sequences of unfolded maximal monotone graphs converge in the graph sense to the maximal monotone graphs $A(x,y)$ and $A_0(x,z)$ for almost every $(x,y,z)\in \Omega \times Y \times \R^N$, as $\varepsilon \to 0$, then every cluster point $(u_0,d_0)$ of the sequence $(u_{\varepsilon , \delta }, d_{\varepsilon , \delta } )$ for the weak topology in the naturally associated Sobolev space is a solution of the homogenized problem which is expressed in terms of $u_0$ alone. This result applies to the case where $A_{\varepsilon}(x)$ is of the form $B(x/\varepsilon)$ where $B(y)$ is periodic and continuous at $y=0$, and, in particular, to the oscillating $p$-Laplacian

Illite K–Ar dating and crystal growth processes in diagenetic environments: a critical review

K–Ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting. In spite of a great number of specialized papers, some problems remain unresolved. These are mostly due to a misunderstanding of the argon accumulation process during illitization. Criteria for identifying detrital–authigenic mineral mixtures, crystal ripening, fast precipitation or continuous nucleation-growth processes are discussed using K–Ar data available in the literature. Using different parameters, such as Dage (age K–Ar ) age strati), D cryst (diagenetic ageK–Ar ) age strati) or D frac (age K–Arfraction ) age K–Ar finest), it is shown that the K–Ar age significance depends on the illite nucleation–growth processes. A ‘diagenetic age’ is obtained when these processes are rapid (the K2O accumulation period is shorter than 2r). If lower than this value, the K–Ar ratio depends on the proportions of new and old particles, respectively, which are controlled by the relative rates of nucleation, crystal growth and ripening

Advances in characterization of the soil clay mineralogy using X-ray diffraction: from decomposition to profile fitting

International audienceStructural characterization of soil clay minerals often remains limited despite their key influence on soil properties. In soils, complex clay parageneses result from the coexistence of clay species with contrasting particle sizes and crystal-chemistry and from the profusion of mixed layers with variable compositions. The present study aimed at characterizing the mineralogy and crystal chemistry of the < 2 μm fraction along a profile typical of soils from Western Europe and North America (Neo Luvisol). X-ray diffraction (XRD) patterns were nterpreted using i) the combination of XRD pattern decomposition and indirect identification from peak positions commonly applied in soil science and ii) the multi-specimen method. This latter approach implies direct XRD profile fitting and has recently led to significant improvements in the structural characterization of clay minerals in diagenetic and hydrothermal environments. In contrast to the usual approach, the multi-specimen method allowed the complete structural characterization of complex clay parageneses encountered in soils together with the quantitative analysis of their mineralogy. Throughout the profile, the clay paragenesis of the studied Neo Luvisol systematically includes discrete smectite, illite and kaolinite in addition to randomly interstratified illite-smectite and chlorite-smectite. Structural characteristics of the different clay minerals, including the composition of mixed layers, did not vary significantly with depth and are thus indicative of the parent material. The relative proportion of the < 2 μm fraction increased with increasing depth simultaneously with smectite relative proportion. These results are consistent with the leaching process described for Luvisols in the literature

Outstanding magnetorheological effect based on discontinuous shear thickening in the presence of a superplastifier molecule

International audienceWe present experimental results showing an increase of stress of about 150kPa for a weak applied magnetic field (H<10kA/m) in an aqueous suspension of carbony iron particles coated with a superplasticizer molecule used in cement industry. These values, which are several orders of magnitude larger than those classically obtained with magnetorheological suspensions at such low field, can open the way to new applications. These high values result from the triggering of a discontinuous shear thickening induced by the magnetic field. A phase diagram is presented for a volume fraction of carbonyl iron particles of 62%, showing two domains in the plane, magnetic field versus shear rate. The lower one is liquid of quite low viscosity and the upper one corresponds to a jammed phase where the particles are in frictionnal contacts and can only move under very high stresses. The transition between the two states is monitored by the ability of the superplasticizer molecule to resist to the compression forces both hydrodynamic and magnetic

Behavior of nanoparticle clouds around a magnetized microsphere under magnetic and flow fields

When a micron-sized magnetizable particle is introduced into a suspension of nanosized magnetic particles, the nanoparticles accumulate around the microparticle and form thick anisotropic clouds extended in the direction of the applied magnetic field. This phenomenon promotes colloidal stabilization of bimodal magnetic suspensions and allows efficient magnetic separation of nanoparticles used in bioanalysis and water purification. In the present work, size and shape of nanoparticle clouds under the simultaneous action of an external uniform magnetic field and the flow have been studied in details. In experiments, dilute suspension of iron oxide nanoclusters (of a mean diameter of 60 nm) was pushed through a thin slit channel with the nickel microspheres (of a mean diameter of 50$\mu$m) attached to the channel wall. The behavior of nanocluster clouds was observed in the steady state using an optical microscope. In the presence of strong enough flow, the size of the clouds monotonically decreases with increasing flow speed in both longitudinal and transverse magnetic fields. This is qualitatively explained by enhancement of hydrodynamic forces washing the nanoclusters away from the clouds. In the longitudinal field, the flow induces asymmetry of the front and the back clouds. To explain the flow and the field effects on the clouds, we have developed a simple model based on the balance of the stresses and particle fluxes on the cloud surface. This model, applied to the case of the magnetic field parallel to the flow, captures reasonably well the flow effect on the size and shape of the cloud and reveals that the only dimensionless parameter governing the cloud size is the ratio of hydrodynamic-to-magnetic forces - the Mason number. At strong magnetic interactions considered in the present work (dipolar coupling parameter $\alpha \geq 2$), the Brownian motion seems not to affect the cloud behavior

Dynamics of a 2D Vibrated Model Granular Gas in Microgravity

We are reporting an experimental study performed on a granular gas enclosed into a 2D cell submitted to controlled external vibrations. Experiments are performed in microgravity during parabolic flights. High‐speed optical tracking allows to obtain the kinematics of the particles and the determination of all inelastic parameters as well as the translational and rotational velocity distributions. The energy into the medium is injected by submitting the experimental cell to an external and controlled vibration. Two model gases are studied beads and disks; the latter being used to study the rotational part of the particle’s dynamics. We report that the free cooling of a granular medium can be predicted if we consider the velocity dependence of the normal restitution coefficient and that the experimental ratio of translational versus rotational temperature decreases with the density of the medium but increases with the driving velocity of the cell. These experimental results are compared with existing theories. We also introduce a model that fairly predicts the equilibrium temperatures along the direction of vibration

Tunable discontinuous shear thickening with magnetorheological suspensions

International audienceDiscontinuous shear thickening is a phenomenon observed in concentrated suspensions where, at a given applied stress, the flow becomes suddenly partially blocked and the shear rate begins to decrease and to oscillate when the stress is increased above the critical one. In this work, we show that it is possible to control with a magnetic field this abrupt transition from a flowing state to a jammed state, using a suspension of magnetic particles coated with a superplastifier molecule. In the case of experiments made at constant velocity, the transition to the jammed state corresponds to a very high jump of stress which can reach several hundred pascals for fields as low as a few kiloampere per meter. We present also results obtained in microgravity on a magnetic powder, showing that the solid friction between particles plays a key role in the jamming phenomenon

Magnetic Resonance Imaging findings in a Positron Emission Tomography-Positive thoracic aortic aneurysm

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Effect of mid-term adaptation on pure-tone detection

International audiencePerformance in detecting a 50 ms pure-tone signal in a longer tone-burst masker was evaluated as a function of the delay between masker and signal onset. Delays from 500 ms to 5 s were used, and detection was measured in terms of sensitivity d'. Performance was also mt;asured for a continuous masker. In a first experiment, the masker and the signal had the same frequency (500, 1000, 2000, 4000 and 8000Hz). In this case, sensitivity increased with delay for high frequencies and for high masker levels. In a second experiment, the masker and the signal had different frequencies; the masker was set at 4 kHz and the signal frequency was inside and outside the critical band of the masker. For this second case, the results depended on masker level and signal frequency: increasing the delay for a 50 dB SL masker did not improve detection; at 80 dB SPL, however, delaying the signal improved detection for signal frequencies close to the masker frequency. The assumption is made that the improved detection for long durations of the masker originates from a long-term decrease in the firing rate of the auditory nerve fibers