Shear induced drainage in foamy yield-stress fluids

Shear induced drainage of a foamy yield stress fluid is investigated using MRI techniques. Whereas the yield stress of the interstitial fluid stabilizes the system at rest, a fast drainage is observed when a horizontal shear is imposed. It is shown that the sheared interstitial material behaves as a viscous fluid in the direction of gravity, the effective viscosity of which is controlled by shear in transient foam films between bubbles. Results provided for several bubble sizes are not captured by the R^2 scaling classically observed for liquid flow in particulate systems, such as foams and thus constitute a remarkable demonstration of the strong coupling of drainage flow and shear induced interstitial flow. Furthermore, foam films are found to be responsible for the unexpected arrest of drainage, thus trapping irreversibly a significant amount of interstitial liquid.Comment: Published in Physical Review Letters. http://prl.aps.org/abstract/PRL/v104/i12/e12830

Réappropriations contemporaines d’une figure mythologique : Les multiples visages de la Femme double

Prenant pour point de départ le film vidéo Lakota Quillwork, Art and Legend (Jane Nauman, 1990), cet article propose une réflexion sur la fabrication du sacré et, plus précisément, sur les réappropriations subies par la figure de la Femme double dans la culture lakota. Ce personnage apparaît central aussi bien historiquement qu’ethnographiquement, et il est toujours présent dans les pratiques visionnaires, artistiques, ainsi que dans les discours des brodeuses autochtones. L’analyse suit le personnage de la Femme double comme un révélateur possible des définitions et valeurs successives accordées à la féminité au sein des cultures des Plaines et des Prairies. Incarnant l’ambiguïté des rôles féminins, cette figure a ainsi changé de régime de valeur au fil des contextes socio-historiques. Elle fut notamment associée à Marie durant la période d’évangélisation des communautés autochtones. Enfin, face aux recompositions de l’espace social et familial, l’auteure interroge les résonances contemporaines que pourrait avoir cette figure émancipatrice auprès des femmes amérindiennes.Taking as the starting point the video film Lakota Quillwork, Art and Legend (Jane Nauman, 1990), this article proposes an analysis on the making of the sacred and more precisely, on the reappropriations of the figure of Double Woman in the Lakota culture. This character appears central historically and ethnographically, and always present in the visionary and artistic practices, as well as in the aboriginal quillworkers’ speeches. The analysis follows the character of the Double Woman, as a focal point revealing the successive definitions and values granted to femininity within the Plains and Prairie cultures. Incarnating the ambiguity of the feminine roles, this figure has evolved in different historical and social contexts, as for example associated with Mary during the evangelization period of the Native communities. In view of the remaking of the social and family space, the author questions the contemporary meaning this liberating figure could hold for Native women

Coupling of elasticity to capillarity in soft aerated materials

We study the elastic properties of soft solids containing air bubbles. Contrary to standard porous materials, the softness of the matrix allows for a coupling of the matrix elasticity to surface tension forces brought in by the bubbles. Thanks to appropriate experiments on model systems, we show how the elastic response of the dispersions is governed by two dimensionless parameters: the gas volume fraction and a capillary number comparing the elasticity of the matrix to the stiffness of the bubbles. We also show that our experimental results are in good agreement with computations of the shear modulus through a micro-mechanical approach.Comment: submitted to Soft Matte

Rheological behaviour of suspensions of bubbles in yield stress fluids

The rheological properties of suspensions of bubbles in yield stress fluids are investigated through experiments on model systems made of monodisperse bubbles dispersed in concentrated emulsions. Thanks to this highly tunable system, the bubble size and the rheological properties of the suspending yield stress fluid are varied over a wide range. We show that the macroscopic response under shear of the suspensions depends on the gas volume fraction and the bubble stiffness in the suspending fluid. This relative stiffness can be quantified through capillary numbers comparing the capillary pressure to stress scales associated with the rheological properties of the suspending fluid. We demonstrate that those capillary numbers govern the decrease of the elastic and loss moduli, the absence of variation of the yield stress and the increase of the consistency with the gas volume fraction, for the investigated range of capillary numbers. Micro-mechanical estimates are consistent with the experimental data and provide insight on the experimental results.Comment: submitted to Journal of non Newtonian Fluid Mechanic

On the existence of a simple yield stress fluid behavior

Materials such as foams, concentrated emulsions, dense suspensions or colloidal gels, are yield stress fluids. Their steady flow behavior, characterized by standard rheometric techniques, is usually modeled by a Herschel-Bulkley law. The emergence of techniques that allow the measurement of their local flow properties (velocity and volume fraction fields) has led to observe new complex behaviors. It was shown that many of these materials exhibit shear banding in a homogeneous shear stress field, which cannot be accounted for by the standard steady-state constitutive laws of simple yield stress fluids. In some cases, it was also observed that the velocity fields under various conditions cannot be modeled with a single constitutive law and that nonlocal models are needed to describe the flows. Doubt may then be cast on any macroscopic characterization of such systems, and one may wonder if any material behaves in some conditions as a Herschel-Bulkley material. In this paper, we address the question of the existence of a simple yield stress fluid behavior. We first review experimental results from the literature and we point out the main factors (physical properties, experimental procedure) at the origin of flow inhomogeneities and nonlocal effects. It leads us to propose a well-defined procedure to ensure that steady-state bulk properties of the materials are studied. We use this procedure to investigate yield stress fluid flows with MRI techniques. We focus on nonthixotropic dense suspensions of soft particles (foams, concentrated emulsions, Carbopol gels). We show that, as long as they are studied in a wide (as compared to the size of the material mesoscopic elements) gap geometry, these materials behave as 'simple yield stress fluids': they are homogeneous, they do not exhibit steady-state shear banding, and their steady flow behavior in simple shear can be modeled by a local continuous monotonic constitutive equation which accounts for flows in various conditions and matches the macroscopic response.Comment: Journal of Non-Newtonian Fluid Mechanics (2012) http://dx.doi.org/10.1016/j.jnnfm.2012.06.00

Efficient Multi-Party Contact Tracing

Since the beginning of the Covid-19 Pandemic, many Countries adopted Contact Tracing Apps as an automatic way to trace if someone has been in contact with a Covid-19 patient. However, most existing solutions consider two party settings only. Since many people meet at the same time in real-life scenarios, one cannot extend current schemes to multi-party settings because they will not scale well and burden the device. In this paper, we propose a new contact tracing protocol that works in a multi-party setting. We evaluate our scheme to show its efficiency.M. de Goyon, A. Miyaji and Y. Tian, "Efficient Multi-Party Contact Tracing," in 2021 Ninth International Symposium on Computing and Networking (CANDAR), Matsue, Japan, 2021 pp. 10-18. doi: 10.1109/CANDAR53791.2021.00010

L’obsolescence déprogrammée : Fablabs, makers et repair cafés

Image d’ouverture Atelier imprimante 3D, FOL Lyon © FOL, CC, Dimitri Ferrière Il est établi que les objets ont une vie sociale et sensible dont on peut retracer le parcours bio­graphique (Appadurai 1986). Ils ont parfois une « seconde vie » par l’intermédiaire de dispositifs divers, du recyclage au marché de la « seconde main ». Dans le cadre de cet article, c’est plus précisément la dimension politique de la seconde vie des objets qui est interrogée, à travers la réparation et le bricolage,..

Yielding and flow of foamed metakaolin pastes

Metakaolin is a broadly used industrial raw material, with applications in the production of ceramics and geopolymers, and the partial replacement of Portland cement. The early stages of the manufacturing of some of these materials require the preparation and processing of a foamed metakaolin-based slurry. In this study, we propose to investigate the rheology of a foamed metakaolin-based fresh paste by performing well-controlled experiments. We work with a non-reactive metakaolin paste containing surfactant, in which we disperse bubbles of known radius at a chosen volume fraction. We perform rheometry measurements to characterize the minimum stress required for the foamed materials to flow (yield stress), and the dissipation occurring during flow. We show that the yield stress of the foamed samples is equal to the one of the metakaolin paste, and that dissipation during flow increases quadratically with the bubble volume fraction. Comparison with yielding and flow of model foamed yield stress fluids allows us to understand these results in terms of coupling between the bubbles' surface tension and the metakaolin paste's rheology