L'IRM s'aventure hors des sentiers du monde médical

National audienceL'agroalimentaire et le génie civil constituent deux domaines d'applications très particuliers pour l'imagerie par résonance magnétique (IRM). Essentiellement axées sur les problèmes d'élaboration, de conditionnement et d'évolution de produits et de matériaux en contexte industriel, les études doivent souvent être menées sur des échantillons massifs. Elles mettent en oeuvre des imageurs de grande taille et réclament le développement de méthodologies spécifiques par rapport aux applications médicales. Des systèmes originaux d'instrumentation et de sollicitation in situ des matériaux ont pu être conçus dans ce cadre, nourris par des collaborations étroites entre différents corps de métiers. Deux laboratoires, l'Unité Technologie des équipements agroalimentaires de l'Irstea et le Laboratoire Navier de l'Université Paris-Est, livrent leur témoignage, fruit de plusieurs années d'expertise. Food-processing and civil engineering are very particular application fields for magnetic resonance imaging (MRI). Studies mainly focus on preparation, packaging, and further evolution of products and materials in industry. They are often carried out on massive samples. They involve the use of large MRI facilities, and require the development of specific methodology as compared with routine medical studies. Stimulating close collaborations between various technical specialties, they saw the raise of original experimental setups for in situ measurements and sample solicitations. Two French laboratories, the " Technologie des équipements agroalimentaires " Unity at Irstea and " Laboratoire Navier " at Paris-Est University, report on their long standing expertise

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the MRI device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology

Franchissement d'interface et enrobage d'une sphère

International audienceThe passage of a sphere through a liquid-liquid interface is studied experimentally in the particular case of a highly viscous upper liquid. For low values of the modified Bond number β the motion of the sphere consists of two distinct regimes: a slow draining stage of the upper liquid film trapped between the sphere and the interface, and a rapid one where a liquid tail bridges the sphere to the upper phase. It is found that the volume of liquid carried away by the sphere varies as ln β and that the time necessary for breakthrough varies as l/ln β.Le passage d'une sphère millimétrique à travers une interface liquide-liquide est étudié expérimentalement dans le cas où le liquide supérieur est très visqueux. Pour de faibles valeurs du nombre de Bond modifié β, le mouvement de la sphère peut être décomposé en deux phases: une phase lente de drainage du film de liquide supérieur piégé entre la sphère et l'interface et une phase rapide durant laquelle la sphère est reliée au liquide supérieur par un fil liquide. Le volume de liquide emporté par la sphère varie globale-ment comme In β et la durée du franchissement varie comme l/ln β

Spike-removal filter for NMR-measurements in the presence of external electric devices

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Liquid Bridge between Two Moving Spheres: An Experimental Study of Viscosity Effects

International audienceThe effects of viscosity on the mechanical response of a liquid bridge are investigated in the case of small amounts of liquid axially strained between two moving spheres. An experimental setup allows the measurement of capillary and viscous forces exerted on the spheres as a function of the spheres separation distance and the spheres velocity. The experimental results are found to be accurately described over a large range in spheres velocity and liquid viscosity by a simple closed-form expression. In addition, the bridge rupture distance is found to increase like the square root of the separation velocity. C 2000 Academic Pres

Conception et fabrication d'un rhéomètre insérable dans un imageur à résonance magnétique.

National audienceUn rhéomètre est un appareil de laboratoire capable de faire des mesures relatives à la rhéologie d'un matériau. Il applique un cisaillement ou une oscillation à l'échantillon. Généralement de faible dimension caractéristique, il permet d'observer la façon dont s'écoule un liquide, une suspension ou une pâte en réponse à une force appliquée

Micromechanics of Unsaturated Granular Media

International audienceThe homogenization method is used to study the properties of the strength criterion of a granular material in the dry, saturated, and unsaturated situations. Adopting a periodic description of the granular material at the microscopic level, the main features of the up-scaling technique are recalled. Then, a general definition of the strength criterion at the macroscopic scale is given in the framework of yield design homogenization theory. This approach makes it possible to find again classical properties of the strength criterion as a function of the properties of the granular material at the microscopic level. Furthermore, some experiments are presented in order to assess the accuracy of the model used to describe the unsaturated granular medium at the microscopic level

Cardinal series to sort out defective samples in magnetic resonance data sets

International audienceNMR signals are unavoidably impaired with noise stemming from the electronic circuits of the spectrometer. This noise is most often white and Gaussian and can be greatly reduced by applying low pass analogue and digital filters. Nevertheless, extra noise with other statistics than Gaussian may interfere with the signal, e.g. when auxiliary electrical devices are placed near the magnet of the NMR spectrometer. This paper reports on how one can make use of this difference in statistics to remove the noise caused by electrical devices before any further data processing. The algorithm is based on the use of a new linear low pass filter, which consists in fitting NMR data in the time domain with a cardinal series and whose spectral width can be controlled. Over other filtering methods such filter has the advantage of not distorting the signal neither at the beginning nor the end of the acquisition period. The performance of the method is demonstrated by applying it to a data set collected in a flow PGSE experiment and impaired with noise emanated from a brushed DC electric motor

Study of freezing and melting of water in porous materials by NMR

We intend to study mechanical effects of freezing and thawing on bulky samples of concrete by nuclear magnetic resonance (NMR) high resolution spectroscopy and imaging (MRI). Variously functionalised three dimensional images of liquid water in the porous media can be obtained by MRI (see Figure 1). NMR spectroscopy, on the other hand, can be used to directly measure the total quantity of liquid water as a function of temperature. Moreover, measurements of magnetic spin relaxation rates and diffusion can provide information about distribution of pore sizes (see Figure 2) and their connectivity as well as the flow of water through porous media. Knowledge of all these properties is required to fully understand behaviour of porous materials under freezing and thawing. We have designed and built a cooler (see the figure opposite and overleaf) that can be inserted into the probe of an NMR spectrometer to bring the temperature a balky cylindrical sample down to ­100°C. The present version of the cooler is intended first and foremost for experiments in which the NMR signal of water hydrogen will be registered. Therefore, the part of the device that will be placed in the active zone of the NMR probe (see Figure 2) is made of non­magnetic non­conductive materials containing no hydrogen isotope to avoid that it emits any extra signal that could interfere with that emanating from water protons of the sample. Furthermore, a cryostat, pump, air compressor and several other auxiliary components of the device were all put outside the Faraday cage built around the NMR spectrometer to minimise noise that they can induce in the NMR probe. Optical fiber sensors were built into the cell to provide for monitoring temperature and mechanical deformations by the means other than NMR. The cooler was designed to use a commercial coolant 'Galden' for heat transfer and has advantage of maintaining the radio frequency (RF) field emitter/receiver coil of the spectrometer at room temperature