The lamellar structure of reactive mixtures in porous media: Pore scale experimental imaging and upscaling

International audienceEffective reaction rates in porous media are controlled by the spatial organization of chemical species concentrations at the pore scale. From high resolution millifluidic pore scale imaging of reactive tracers we report experimental evidence of the formation of well-developed lamellar structures in reactive mixtures transported through porous media (de Anna et al., Environ. Sci. Technol., 2014). The latter are highlighted by a chemioluminescent reaction producing photons that localize along spatially coherent lines, representing hotspots of mixing and reaction at pore scale. These elongated spatial structures are naturally created by the stretching action of the pore scale velocity field, which induces a dynamic deformation of the material elements carrying solutes (Le Borgne et al., Phys. Rev. Lett., 2013). This particular spatial organization is shown to have a major impact on global reactivity by increasing the surface available for reactive mixing and by enhancing local chemical gradients (de Anna et al., Geophys. Res. Lett. 2014). We quantify this phenomenon for different flow topologies using a reactive lamella representation, which links fluid deformation, diffusion and reaction at the elementary scale. The upscaled reaction rates, estimated by integrating the distribution of local deformation rates, are shown to follow different temporal behavior depending on the distribution of local velocity gradients. This approach allows for the systematic evaluation of the temporal evolution of upscaled reaction rates, and establishes a direct link between the global reaction efficiency and the spatial characteristics of the underlying pore scale flow field.References:[1] P. de Anna, J. Jimenez-Martinez, H. Tabuteau, R. Turuban, T. Le Borgne, M. Derrien,and Yves Méheust, Mixing and reaction kinetics in porous media : an experimental pore scale quantification, Environ. Sci. Technol.48, 508-516, 2014.[2] de Anna, P., Dentz, M., Tartakovsky A. and Le Borgne, T., The filamentary structure of mixing fronts and its control on reaction kinetics in porous media flows, Geophys. Res. Lett., 41, 4586–4593 (2014)[3] Le Borgne, T., M. Dentz, E. Villermaux, Stretching, coalescence and mixing in porous media, Phys. Rev. Lett., 110, 204501 (2013