Blade-coating of pastes (yield stress fluids) consists of coating the fluid by pushing and spreading it over a solid substrate. It is a widely used process in civil engineering (mortars, cement pastes, rendering, paints) and in cosmetics (coating of gels or creams). Here we study this process experimentally by displacing horizontally a vertical plate partially immersed in the initially uniform layer of model yield stress fluid (Carbopol gel). We look at the impact of the initial fluid thickness, the scratched layer thickness, the fluid yield stress, and the blade velocity. The analysis is completed by rough PIV measurements. We show that the displacement of the blade at a constant distance from the solid plane induces, behind the blade, the formation of a uniform layer of thickness equal to 1.09 times this distance. In front of the blade the fluid forms a heap of growing volume. It appears that this heap advances over the initial uniform layer which induces a shear in a band between two almost undeformed blocks. It is shown that the shape of this heap results from the momentum balance on the fluid volume submitted to gravity and shear stress (along the shear-band). Nevertheless the force to apply on the blade is larger than the stress along the band times the band surface by a factor larger than 1.5, which might result from normal force due to heap weight and elastic deformation in the upstream static layer. (C) 2016 Elsevier B.V. All rights reserved.

We study the emergence of discontinuous shear thickening (DST) in cornstarch by combining macroscopic rheometry with local magnetic resonance imaging measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region is not DST but, strikingly, is often shear thinning. Our data thus show that the stress jump measured during DST, in cornstarch, does not capture a secondary, high-viscosity branch of the local steady rheology but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.

The internal flow characteristics of a kaolin suspension were examined via magnetic resonance imaging (MRI) in a flow regime for which the material behaves as an almost pure plastic material. The paste also showed a significant tendency toward liquid phase migration under squeezing tests. The MRI measurements produced a 2D slice of the 3D velocity field near the entrance of the cylindrical extrusion die, as well as the relative liquid–solid ratio within the cylindrical extrusion cartridge. Finite element simulations were also carried out using a simple yield stress fluid model, i.e. Herschel–Bulkley model, which was fit to rheometrical data obtained from independent experiments. A relatively good agreement between numerical simulations and the measured velocity field was obtained. In addition, the clay concentration of the paste within the extrusion column was imaged before and after extrusion experiments displaying a migration of water from the dead zones toward the entrance of the die. It is remarkable, however, that this phenomenon does not significantly affect the velocity distribution.

L'agroalimentaire et le genie civil constituent deux domaines d'applications tres particuliers pour l'imagerie par resonance magnetique (IRM). Essentiellement axees sur les problemes d'elaboration, de conditionnement et d'evolution de produits et de materiaux en contexte industriel, les etudes doivent souvent etre menees sur des echantillons massifs. Elles mettent en oeuvre des imageurs de grande taille et reclament le developpement de methodologies specifiques par rapport aux applications medicales. Des systemes originaux d'instrumentation et de sollicitation in situ des materiaux ont pu etre concus dans ce cadre, nourris par des collaborations etroites entre differents corps de metiers. Deux laboratoires, l'Unite Technologie des equipements agroalimentaires de l'Irstea et le Laboratoire Navier de l'Universite Paris-Est, livrent leur temoignage, fruit de plusieurs annees 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 equipements agroalimentaires Unity at Irstea and Laboratoire Navier at Paris-Est University, report on their long standing expertise.

MRI ventures into non-medical world 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 equipements agroalimentaires" Unity at lrstea and "Laboratoire Navier" at Paris-Est University, report on their long standing expertise.

Ce document est tire de travaux de recherche effectues dans le cadre de l'operation de recherche de l'Ifsttar, 'physico-chimie des milieux poreux', au cours de la periode 2006-2009. Ces travaux ont porte sur l'etude de transformations physiques et chimiques a l'echelle des pores et de leurs consequences sur la durabilite et les degradations mecaniques des materiaux poreux du genie civil. Quatre phenomenes ont ete particulierement etudies : la cristallisation de sels dans les pierres de construction et leur deterioration, le gel de l'eau dans les materiaux cimentaires et la resistance aux cycles de gel/degel, la carbonatation des materiaux a base de ciment et l'evolution de la microstructure, le suivi par IRM de l'hydratation et sechage des materiaux cimentaires.

Ce travail s'integre plus largement dans une thematique de l'equipe Imagerie et Materiaux du laboratoire Navier, concernant les materiaux poreux au sens large (materiaux cimentaires, roche, sol, poreux modeles...) et plus precisement le role et le deplacement de l'eau dans ces structures complexes. Le spectrometre-imageur est arrive en 1999, et les premiers travaux sur materiaux cimentaires ont debute fin 2003, notamment avec l'equipe Physique et Mecanique des Milieux Poreux du laboratoire Navier. L'etude que nous avons menee s'inscrit dans la comprehension des mecanismes de structuration des materiaux cimentaires et de leur durabilite en utilisant la technique RMN. Le travail avait pour objectif d'utiliser le spectrometre-imageur comme outil de caracterisation des materiaux cimentaires. Une premiere etape etait de situer l'IRM par rapport aux techniques classiques utilisees en genie civil. La seconde etape etait de mettre en application cette technique pour sonder les materiaux cimentaires sur des suivis d'hydratation et d'hydratation couplee au sechage.

Un rheometre est un appareil de laboratoire capable de faire des mesures relatives a la rheologie d'un materiau. Il applique un cisaillement ou une oscillation a l'echantillon. Generalement de faible dimension caracteristique, il permet d'observer la facon dont s'ecoule un liquide, une suspension ou une pâte en reponse a une force appliquee.

We intend to study mechanical effects of freezing and thawing on bulky samples of concrete by nuclear resonance high resolution spectroscopy and imaging (MRI). Variously functionalised three dimensional images of liquid water in the porous media can be obtained by MRI. 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 the figure below) 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.

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 magnetic resonance imaging 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.

We study the flow of a typical thixotropic material subjected to very different deformation histories (squeeze, shear, and extrusion) with either local (proton NMR and magnetic resonance imaging) or macroscopic measurements after different times of rest. Specifically, we measure the velocity fields and the spin-spin NMR relaxation of the material after different flow histories. The relaxation data exhibits a long relaxing component revealing information about the reversible microstructural evolution of the sample during aging-rejuvenation cycles. We show that for each deformation process, the evolution of the viscosity during the solid-liquid transition is similar by a factor related to the initial state of the material. Moreover, results examining the impact of the rate at which the deformation is imposed suggest that the state of the material during this transition may be described by a single parameter reflecting the average size and deformation of the material's flocs. These results also show that localization of flow occurs as a result of a progressive differential evolution of the material in different regions of the flow, and thus are determined by the boundary conditions of the flow.

Extrusion tests were performed by forcing a well-characterized model yield stress fluid from a cylindrical cartridge through various cylindrical extrusion dies using a variety of different piston velocities. In this study the Bingham number within the die ranged from 0.1 to 10. MRI techniques allowed for the non-invasive determination of the local velocity within the extruded material in the range [0.015; 20mms−1]. The velocity profile within a very long die was determined by MRI and agreed very well with the analytical results for the flow of a Herschel–Bulkley fluid within a conduit using parameters determined from independent rheometrical tests, validating both the rheological approach and the accuracy of the MRI techniques. Although the velocity was determined by MRI in the upper and lower zones separately, the intersection of these zones showed great agreement, providing an entire view of the extrusion process. In the range of Bingham number studied, the velocity field for a given contraction ratio appeared similar when scaled by the piston velocity, with a dimpled acceleration zone above the die and lateral dead zones varying negligibly with the piston velocity. For a further analysis the experimental results were compared with the results of numerical simulations. Finite element simulations using an elastic solids model were performed to provide this comparison. It was found that this model did well in representing the characteristics of extrusion flow seen in the experiments; an aspect that was not present in the biviscous simulations. The MRI results show that for the range of values studied, both the piston velocity and the contraction ratio have little effect on the characteristics of the flow, including the size and location of the apparent dead zones. It was found that with an appropriate scaling the central, longitudinal velocity follows a master curve. A decreasing contraction ratio, on the other hand, appears to increase the size of the weak velocity region, in contrast with the simulation results.

NMR 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.

L'endommagement par le gel des materiaux de construction et les moyens de l'eviter sont des questions importantes en genie-civil. La RMN fournit une methode non destructive pour une etude quantitative de la transition eau/glace dans ces materiaux. Nous presentons nos premieres mesures de profils 1D et 2D de teneur en eau non-gelee, comme un premier apercu du potentiel eleve de la methode RMN vis-a-vis de l'etude du gel/degel des materiaux poreux du genie civil. A l'avenir nous emploierons une cellule thermostatee a l'interieur de l'imageur et nous ameliorerons l'installation RMN afin de mesurer les deformations de l'echantillon. (A). (Voir fiche generale F101003 et fiches specifiques F101004 a F101084).

L'Imagerie par Resonance Magnetique, surtout connue comme outil de diagnostic medical, est egalement une technique riche d'applications dans le domaine de l'etude des materiaux solides ou liquide. Depuis son installation en 1999, l'IRM du LMSGC a, entre autres, permis de mettre en place au laboratoire un ensemble de techniques innovantes pour la caracterisation des fluides et des milieux granulaires en ecoulement, notamment a travers la conception de dispositifs experimentaux inserables de mise en sollicitation, et l'adaptation et le perfectionnement de methodes de velocimetrie. Nous proposons un tour d'horizon du savoir faire actuel du LCPC en la matiere, illustre par des applications variees : mesure d'ecoulement sur modele reduit, rheometrie assistee par IRM en geometries Couette et Cone-Plan (ciment, boues, grains, suspensions, ...), dynamique d'ecoulement dans une mousse, ... (A). (Voir fiche generale F101003 et fiches specifiques F101004 a F101084).

Two recent developments using proton MRI velocimetry are reported, that concern both fields of rheology and of fluid mechanics. The first one is a way to get noticeable improvement in the signal-to-noise ratio during velocity measurements in a large-size 'Couette' rheometer. It opens a route to improve the study of complex or thixotrope fluids. The second one deals with the introduction of a model of concrete mixer inside the MRI facility. Thanks to synchronisation between movements of the model and MRI sequences, 3D velocity maps were obtained for a viscoplastic fluid inside the mixer. (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.