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Diffusion in soft particle suspensions near jammingCraig E. MaloneyCivil & Environmental EngineeringCarnegie Mellon UniversityPittsburgh, PA 15213-3890, USASuspensions of soft particles exhibit a remarkable bifurcation at the random closepacking volume fraction, [1]. There is a yield stress above but not below. Weperform numerical simulations of soft-particle suspensions under shear near . Wefind that above at sufficiently low shearing rates, the effective diffusion constant,, scales with length of the simulation cell in the flow-gradient direction, . Thisis in agreement with the size-dependent diffusion constant observed in lowtemperatureLennard-Jones glasses. Furthermore, the value of can beunderstood in terms of organized lines of slip and is independent of the form of therepulsive interactions between the particles and the precise way in which the viscousdrag of the suspending fluid is modeled in the particle-scale simulations.[1] Microfluidic Rheology of Soft Colloids above and below Jamming. K. N.Nordstrom, E. Verneuil, P. E. Arratia, A. Basu, Z. Zhang, A. G. Yodh, J. P. Gollub,and D. J. Durian. Phys. Rev. Lett. 105, 175701 (2010)How do yield stress materials start to flow?Sébastien MannevilleUniversité de Lyon, Laboratoire de Physique, École Normale Supérieure de Lyon,CNRS UMR 5672 - 46 Allée d'Italie, 69364, Lyon cedex 07, FranceInstitut Universitaire de France, FranceYield stress materials are viscoelastic solids at rest but behave as viscous liquidswhen stressed above their yield stress. In soft jammed systems, yielding can be seenas an instance of an unjamming transition driven by the shear stress. The question ofwhether this shear-induced fluidization displays universal features, in a way similar tojamming driven by temperature or by volume fraction, has triggered much researcheffort in the recent years. Experimentally, difficulties arise from the need to measuredeformations and flows close to yielding at vanishingly small shear rates withsufficient spatial and temporal resolutions.In this talk, I will first review the current state of research on the steady state reachedby a soft glassy system above yielding. It is now established that some "simple"materials undergo a continuous yielding transition characterized by homogeneousflows while others display flow heterogeneities, e.g. shear bands, at steady state. Iwill then concentrate on the spatiotemporal fluidization dynamics of a "simple" yieldstress material, namely a carbopol microgel, that presents negligible aging andthixotropy. Through long experiments combining standard rheology and ultrasonic21