Phoresis in cellular flows: from enhanced dispersion to blockage

International audienceIn this article, we study numerically the dispersion of colloids in a 2-dimensional cellular flow in the presence of an imposed mean salt gradient. Due to the additional scalar, the colloids do not follow exactly the Eulerian flow-field, but have a (small) extra-velocity proportional to the salt gradient, v dp = α∇S, where α is the phoretic constant and S the salt concentration. We study the demixing of an homogenous distribution of colloids and how their long term mean velocity V m and effective diffusivity D eff are influenced by the phoretic drift. We observe two regimes of colloids dynamics depending on a blockage criterion R = αGL/ √ 4D c D s , where G is the mean salt gradient amplitude, L the lengthscale of the flow and D c , D s the molecular diffusivities of colloids and salt. When R 1, the compressibility effect due to the phoretic drift is so strong that a depletion of colloids occurs along the separatrices inhibiting cell-to-cell transport