Shear thickening and migration in granular suspensions

We study the emergence of shear thickening in dense suspensions of non-Brownian particles. We combine local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometry experiments. In steady state, we observe that the material is heterogeneous, and we find that that the local rheology presents a continuous transition at low shear rate from a viscous to a shear thickening, Bagnoldian, behavior with shear stresses proportional to the shear rate squared, as predicted by a scaling analysis. We show that the heterogeneity results from an unexpectedly fast migration of grains, which we attribute to the emergence of the Bagnoldian rheology. The migration process is observed to be accompanied by macroscopic transient discontinuous shear thickening, which is consequently not an intrinsic property of granular suspensions

Réflectométrie optique dans le domaine fréquentiel pour l’analyse des réseaux locaux domestiques optiques

In order to develop high capacity future-proof home network, the FUI 12 RLDO project suggests passive star network topology using multimode couplers. This topology encounters implementation difficulties due to the non-uniformity of the power distribution in the output ports of multimode couplers. Analyzing this problem shows that the properties of modes propagating in the network elements plays a key role in this non-uniform characteristics of multimode couplers. In order to characterize these modes propagating in the network, we have developed a complex optical frequency domain reflectometry (OFDR) setup. The phenomena limiting the sensitivity to the phase in OFDR – in fact, those related to the non-linear frequency tuning of the laser source - are investigated to contribute to a better understanding of the limiting mechanisms. Then we have tested the intensity and phase measurement performance of the developed setup. Later, we studied the optimal resolution conditions to characterize the modes in a multimode waveguide. We have also analyzed the uncertainty of the measurement of the chromatic dispersion of modes in case where it is impossible to determine whether one or several modes are present in a given dispersion curve. Additionally, we have introduced an adaptive time-frequency method, to obtain the dispersion curves with optimal resolution. This method allows us to show the versatility of the special RLDO multimode fiber (single-mode behavior under single-mode excitation at 1310 nm and 1550 nm). The analysis of the propagation in the optical fibers, associated with a model we have developed to study the behavior of multimode couplers, has permitted to explain encountered difficulties with the experiments of the multimode passive star network topology. This also gives insights to develop a functional prototype of networkLe projet FUI12 RLDO – dans le cadre duquel s’inscrit cette thèse – préconise une solution de réseau de topologie en étoile passive pour la montée en débit des réseaux domestiques. Cette solution de réseau rencontre des difficultés dans son implémentation avec la non-uniformité des puissances des ports de sortie des coupleurs multimodes. L’analyse de ce point nous a permis de comprendre que les propriétés des modes de propagation dans les éléments du réseau jouent un rôle clé dans les problèmes rencontrés. Pour caractériser la propagation dans le réseau, nous avons développé un banc de réflectométrie optique complexe dans le domaine fréquentiel. Les phénomènes limitant la sensibilité à la phase – liés en particulier à la non-linéarité du balayage en fréquence de la source laser – sont étudiés pour contribuer à une meilleure compréhension des mécanismes. Puis les performances de la mesure en intensité et en phase de l’instrument que nous avons mis en place sont testées. Nous avons aussi étudié les conditions de résolution optimales pour caractériser les modes d’un guide multimode et analysé l’incertitude sur la dispersion chromatique dans le cas où il est impossible de déterminer si on a accès à un mode ou plusieurs modes dans un diagramme de dispersion donné. Nous introduisons par la suite une méthode d’analyse temps-fréquence adaptative, permettant d’obtenir les courbes de dispersions avec une résolution optimale. Cette méthode nous a permis de montrer le caractère quasi-monomode, en condition d’excitation monomode, de la fibre multimode spéciale RLDO à 1310 nm et à 1550 nm. L’analyse de la propagation dans les fibres optiques, associée au modèle que nous avons développé pour comprendre le fonctionnement des coupleurs multimodes, a permis d’expliquer les difficultés rencontré avec les premières expérimentations de la topologie de réseau en étoile passive multimode et d’envisager des pistes de réalisation d’un prototype de réseau fonctionne

Décoloniser les sciences sociales en Afrique

L’objet de cet article est d’interroger de manière critique la pratique des sciences sociales en Afrique cinq décennies après les indépendances. Son but est de situer la difficulté des sciences sociales à participer de manière décisive à la reconstruction du continent. Notre discussion aborde la question du point de vue de leur objet qui semble en même temps leur objectif naturel – le « développement » du continent –, avant d’opérer un détour par les malaises que vivent les sciences sociales dans l’univers qui les a vues naître, à savoir l’Occident, afin de mieux réfléchir – par le cas concret de l’étude des représentations sociales et appliquée aux phénomènes d’économie populaire – sur les modalités de leur appropriation en Afrique.The focus of this paper is to critically examine the practice of social sciences in Africa five decades after independence. Its purpose is to emphasize on the difficulties that impede the effective participation of the social sciences in the reconstruction of the continent. Our discussion tackles the issue with regard to their object which also seems to be their natural aim – the « development » of the continent – before enlarging on the discomfort they experience in the universe they were borne, namely the West, to better approach – by the case study of social representations, applied to the phenomena of popular economy – the modalities of their appropriation in Africa

Yield stress and shear-banding in granular suspensions

We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combining local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometric experiments. We show that the competition between gravity and viscous stresses is at the origin of the development of a yield stress in these systems at relatively low volume fractions. Moreover, it is accompanied by a shear banding phenomenon that is the signature of this competition. However, if the system is carefully density matched, no yield stress is encountered until a volume fraction of 62.7 0.3%

Shear thickening of cornstarch suspensions as a re-entrant jamming transition

We study the rheology of cornstarch suspensions, a dense system of non-Brownian particles that exhibits shear thickening, i.e. a viscosity that increases with increasing shear rate. Using MRI velocimetry we show that the suspension has a yield stress. From classical rheology it follows that as a function of the applied stress the suspension is first solid (yield stress), then liquid and then solid again when it shear thickens. The onset shear rate for thickening is found to depend on the measurement geometry: the smaller the gap of the shear cell, the lower the shear rate at which thickening occurs. Shear thickening can then be interpreted as the consequence of the Reynolds dilatancy: the system under flow wants to dilate but instead undergoes a jamming transition because it is confined, as confirmed by measurement of the dilation of the suspension as a function of the shear rate

Gestational diabetes and endothelial function: impact of gestational insulin resistance on reactive hyperhemia index

Our aim was to characterize endothelial function in gestational diabetes by evaluating the reactive hyperemia index (RHI, LnRHI). A prospective, descriptive and comparative study was conducted on a population of pregnant women aged over 20 and under 36, located in the gestational age group 24-38th week of amenorrhea. They were divided into two groups. Group 1 (G1): group of pregnancies without diabetes, consists of pregnant women with no risk factor for gestational diabetes and with normal fasting blood glucose. Group 2 (G2): group of pregnancies with diabetes, includes pregnancies whose oral glucose tolerance tests (OGTT) came back positive. Anthropo-physiological parameters (age, weight, height, blood pressure (PA) and biochemical parameters (glycemia, insulinemia, HOMA-IR, cholesterol, triglycerides) were measured. RHI and LnRHI were determined at Endopat 2000. The two groups were matched for age, weight, heart rate (HR) and blood pressure (BP). Levels of glucose (G1:0.76±0.11; G2:1.11±0.11; p˂0.0001), insulin (G1:7.67±4.35; G2:22.9±3.75; p˂0.0001), HOMA-IR (G1:1.51±0.97; G2:6.29±1.23; p˂0.0001), total cholesterol (G1:1±0.81; G2:2.49±0.74; p=0.002), HDL cholesterol (G1:0.45±0.23; G2: 0.8±0.19; p=0.004, LDL cholesterol (G1:0.42±0.54; G2:1.39±0.6; p=0.004), triglycerides (G1:0.65±0.49; G2:1.48±0.27; p=0.0018), were significantly higher in the diabetic group. Both RHI and LnRHI were negatively correlated with HOMA-IR (respectively, r=-0.8931, p<0.0001; r=-0.8938; p<0.0001). HOMA-IR index was independently associated with levels of RHI and LnRHI (respectively r²=0.797; p<0.0001); (r²=0.804; p<0.0001)). Thus, gestational insulin resistance would be associated with a change in endothelial function such as a decrease in endothelium-dependent vasodilatation reflecting endothelial dysfunction, hence an increase in cardiovascular risk

Rheology of sedimenting particle pastes

We study the local and global rheology of non-Brownian suspensions in a solvent that is not density-matched, leading to either creaming or sedimentation of the particles. Both local and global measurements show that the incomplete density matching leads to the appearance of a critical shear rate above which the suspension is homogenized by the flow, and below which sedimentation or creaming happens. We show that the value of the critical shear rate and its dependence on the experimental parameters are governed by a simple competition between the viscous and gravitational forces, and present a simple scaling model that agrees with the experimental results from different types of experiments (local and global) in different setups and systems