

The dispersion of dye in Newtonian and shear-thinning fluids has been studied optically in transparent rough model fractures. The evolution of the displacement front is investigated as a function of time and mean velocity $U$. For a wall roughness given by a random distribution of single-sized obstacles on a flat plane, the front spreading is diffusive ($\Delta x \propto  t^{0.5}$) and characterized by a dispersivity $l_d$. At low velocities, $l_d \simeq$  cst.: dispersion is driven by spatial velocity fluctuations which are increased for shear-thinning fluids; at high velocities, Taylor dispersion is dominant and $l_d$ increases with $U$ but slower for shear thinning fluids. For a fracture with complementary self-affine rough walls with a relative lateral displacement perpendicular to $U$, macroscopic channels parallel to $U$ appear and extend all along the length of the model: the global front width increases linearly with time, more rapidly for the shear-thinning fluids (advective spreading). In contrast, the local front spreading remains diffusive and governed by Taylor dispersion.
