Velocity Profiles in Slowly Sheared Bubble Rafts

Measurements of average velocity profiles in a bubble raft subjected to slow, steady-shear demonstrate the coexistence between a flowing state and a jammed state similar to that observed for three-dimensional foams and emulsions [Coussot {\it et al,}, Phys. Rev. Lett. {\bf 88}, 218301 (2002)]. For sufficiently slow shear, the flow is generated by nonlinear topological rearrangements. We report on the connection between this short-time motion of the bubbles and the long-time averages. We find that velocity profiles for individual rearrangement events fluctuate, but a smooth, average velocity is reached after averaging over only a relatively few events.Comment: typos corrected, figures revised for clarit

Shear-Induced Stress Relaxation in a Two-Dimensional Wet Foam

We report on experimental measurements of the flow behavior of a wet, two-dimensional foam under conditions of slow, steady shear. The initial response of the foam is elastic. Above the yield strain, the foam begins to flow. The flow consists of irregular intervals of elastic stretch followed by sudden reductions of the stress, i.e. stress drops. We report on the distribution of the stress drops as a function of the applied shear rate. We also comment on our results in the context of various two-dimensional models of foams

[La microstructure 3D des matériaux polycristallins vue sous la lumière synchrotron]

International audienceSynchrotron radiation X-ray imaging and diffraction techniques offer new possibilities for non-destructive bulk characterization of polycrystalline materials. Minute changes in electron density (different crystallographic phases, cracks, porosities) can be detected using 3D imaging modes exploiting Fresnel diffraction and the coherence properties of third generation synchrotron beams. X-ray diffraction contrast tomography, a technique based on Bragg diffraction imaging, provides access to the 3D shape, orientation and elastic strain state of the individual grains from polycrystalline sample volumes containing several hundred up to a few thousand grains. Combining both imaging modalities allows a comprehensive description of the microstructure of the material at the micrometer length scale. Repeated observations during (interrupted) mechanical tests provide unprecedented insight into crystallographic and grain microstructure related aspects of polycrystal deformation and degradation mechanisms in materials, fulfilling some conditions on grain size and deformation state.Les techniques d'imagerie et de diffraction au rayonnement synchrotron offrent de nouvelles possibilités pour la caractérisation tridimensionnelle et non destructive des matériaux polycristallins. De faibles variations de densité électronique (phases secondaires, fissures, porosités) peuvent êtres détectées grâce à des modes d'imagerie qui exploitent la diffraction de Fresnel ainsi que la cohérence des faisceaux issus des sources synchrotron de troisième génération. La tomographie par contraste de diffraction, autre technique d'imagerie 3D basée sur la diffraction de Bragg, donne accès à la forme, l'orientation et l'état de déformation élastique des grains dans des volumes polycristallins contenant jusqu'à mille grains. La combinaison de ces deux modes d'imagerie permet de caractériser des matériaux polycristallins à l'échelle du micron. Des observations répétées lors d'essais mécaniques (interrompus) permettent d'analyser le rôle de la cristallographie locale sur les mécanismes de déformation et de dégradation dans des matériaux polycristallins, respectant certaines conditions sur la taille de grains, et/ou leur état de déformation

Nonlinear Stress Fluctuation Dynamics of Sheared Disordered Wet Foam

Sheared wet foam, which stores elastic energy in bubble deformations, relaxes stress through bubble rearrangements. The intermittency of bubble rearrangements in foam leads to effectively stochastic drops in stress that are followed by periods of elastic increase. We investigate global characteristics of highly disordered foams over three decades of strain rate and almost two decades of system size. We characterize the behavior using a range of measures: average stress, distribution of stress drops, rate of stress drops, and a normalized fluctuation intensity. There is essentially no dependence on system size. As a function of strain rate, there is a change in behavior around shear rates of $0.07 {\rm s^{-1}}$.Comment: accepted to Physical Review