Experimental imaging of pore scale stretching and coalescence as drivers for solute mixing in porous media

The combined action of stretching and diffusion within solute plumes controls mixing in flows through soils and fractured rocks, ultimately affecting the rates of subsurface reactions. Stretching enhances mixing by increasing the area available for diffusion to act and steepening concentration gradients. Ultimately, the resulting solute filaments coalesce which drives the transition of concentration profiles toward uniformity. While the role of stretching is well described by current models, the effect of coalescence on mixing has been more challenging to understand, partly because the spatial extent and distribution of coalesced regions depends on the geometric structure of the medium. Here we present a new set of experiments designed to isolate the role of coalescence on mixing in porous media. Using stereolithography 3D printing, we have fabricated transparent porous models with different geometric structures. By imaging pulses of fluorescent dye as they mix in flows through these models, we have resolved the dependence of mixing rates on both Peclet number and the medium geometry. We observe that converging streamlines downstream of stagnation points establish local zones in the flow where coalescence is enhanced. From these observations, we describe the statistics of coalescence and its impact on mixing and reaction rates. These findings support the ongoing effort to improve our predictions of mixing and reactive transport in the subsurface.