Abstract In subsurface environments, incomplete mixing at the pore scale limits reaction rates, rendering their prediction by Darcy‐scale models challenging. Such pore scale concentration gradients are enhanced by the deformation of solute fronts and decay under the action of molecular diffusion and solute filament merging. It is currently unclear how these processes govern concentration gradient dynamics under different flow rates. We measure experimentally pore scale concentrations in solute fronts transported in a two‐dimensional porous micromodel over an extensive range of flow rates. We demonstrate that pore‐scale shear flow increases concentration gradients up to a time predicted by the lamellar mixing theory in shear flow. However, the flow rate‐dependency of the mean concentration gradient at this so‐called mixing time is weaker than predicted theoretically, a discrepancy which we explain quantitatively by accounting for lamellae aggregation. These findings shed new light on the pore‐scale mechanisms driving mixing dynamics in porous media.
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