We investigate the dispersion of a layer of dye initially applied at the outer wall of a cylindrical Couette-cell into a sheared suspension of non-Brownian spherical particles. The process is directly visualized and quantified at the particle scale. A “rolling-coating” mechanism is found to convectively transport the dye at a constant rate directly from the wall towards the bulk. The fluid velocity fluctuations, u′, measured with particle image velocimetry, and the imposed shear-rate, γ ̇ , are used to define a diffusion coefficient, D u u / γ ̇ , which is found to increase linearly with the distance from the wall. A solution of the transport equation accounting for this inhomogeneous stirring field describes quantitatively the concentration profiles measured experimentally. It exhibits a super-diffusive character, a consequence of the increase of the stirring strength with distance from the wall. Movies are available with the online version of the paper.

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See supplementary material at http://dx.doi.org/10.1063/1.4918613 for three movies: Movie 1) Dispersion of a rhodamine layer in a sheared suspension of particles, Movie 2) PIV in the flowing suspension, and Movie 3) Rolling-coating mechanism.

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