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Near-wall hydrodynamics

Anne Mongruel

à 11h, en salle C. BROT

In numerous industrial or natural multiphase flows, the grains are subjected to short-range hydrodynamic interactions (lubrication interactions) and to contact interactions, that are highly influenced by the roughness of the solid surfaces. In this context, studying the interaction between a fluid-immersed sphere and a wall is a useful step for the understanding of more complicated situations . In this talk, I will present experiments on the dynamics of a sphere approaching and colliding a textured wall in a viscous fluid. The texture is comprised of arrays of micro-pillars. Using high-frequency laser interferometry, the dynamics of the sphere is resolved in time and space, before and during its collision with the textured wall. For negligible sphere inertia, the velocity of the sphere near the texture is larger than the corresponding value near a smooth wall, and this effect is interpreted in terms of an effective slip over the pillar array. When the sphere inertia is non-neglible, the sphere can either stick or bounce off the wall, and the Stokes number below which no bouncing occurs depends critically on the geometrical parameters of the texture. Finally, the contact dynamics between the sphere and the micro-pillars is described by a modified Hertz model taking into account the geometry of the texture.