Reactive Mixing Fronts in Porous Media Under Transverse Velocity Gradients

Abstract Mixing‐limited reactions in porous media depend on concentration gradients at interfaces, yet their behavior in nonuniform co‐flow remains poorly understood. Here we develop a closed‐form theory for irreversible reactions in a co‐flow where a transverse velocity gradient makes transverse hydrodynamic dispersion vary linearly across the mixing zone. The solution shows that the velocity gradient mainly displaces the front toward slower flow rather than increasing the leading‐order mixing width. This migration reduces local transverse dispersion and compensates for the spatial variation in dispersion, so the mixing‐limited reaction intensity follows the uniform‐flow scaling. At larger distances, hydrodynamic dispersion weakens and molecular diffusion controls the remaining gradients, accelerating reaction‐intensity decay. These results show that velocity‐dependent dispersion redistributes reactive fronts without sustaining stronger mixing, constraining the spatial persistence of reactive mixing in nonuniform porous media.