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Mixing fronts, where fluids of different chemical compositions mix with eachother, are typically subjected to velocity gradients, ranging from the porescale to the catchment scale due to permeability variations and flow linegeometries. A common trait of these processes is that the mixing interface isstrained by shear. Depending on the P\'eclet number $Pe$, which represents theratio of the characteristic diffusion time to the characteristic advectiontime, and the Damk\"ohler number $Da$, which represents the ratio of thecharacteristic diffusion time to the characteristic reaction time, the localreaction rates can be strongly impacted by the dynamics of the mixinginterface. This impact has been characterized mostly either in kinetics-limitedor in mixing-limited conditions, that is, for either very low or very high$Da$. Here the coupling of shear flow and chemical reactivity is investigatedfor arbitrary Damk\"ohler numbers, for a bimolecular reaction and an initialinterface with separated reactants. Approximate analytical expressions for theglobal production rate and reactive mixing scale are derived based on areactive lamella approach that allows for a general coupling between stretchingenhanced mixing and chemical reactions. While for $Pe<Da$, reaction kineticsand stretching effects are decoupled, a scenario which we name "weakstretching", for $Pe>Da$, we uncover a "strong stretching" scenario where newscaling laws emerge from the interplay between reaction kinetics, diffusion,and stretching. The analytical results are validated against numericalsimulations. These findings shed light on the effect of flow heterogeneity onthe enhancement of chemical reaction and the creation of spatially localizedhotspots of reactivity for a broad range of systems ranging from kineticlimited to mixing limited situations.

Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damk\"ohler numbers