Mixing driven reactions in porous media
Mixing processes in porous media are controlled by the combination of heterogeneous advection and diffusion. The former acts to spread the reactants in the local direction of the flow. Diffusion, on the other hand tends to homogenize the spatial contrasts in chemicals concentration. Key mechanisms affecting the reaction front geometry include stretching by the flow field, splitting by random obstacles and trapping in low velocity areas, in particular close to no slip boundaries at the solid-fluid interfaces.
The video below (watch it in high resolution!) represents an example of two-dimensional heterogeneous poly-disperse porous medium where an injected solution A (top) react with a resident solution B to form a conservative chemical C (bottom).
Details can be found in the article: "The filamentary structure of mixing fronts and its control on reaction kinetics in porous media flows" (pdf), Geophysical Research Letters
Below a video (watch it in high resolution!) of an experimental set up to characterize mixing and reactive transport in porous media with a high spatial resolution at the pore scale.
The analogous porous medium consists of a Hele-Shaw cell containing a single layer of cylindrical solid grains built by soft lithography. Considering a fast bimolecular reaction A + B → C occurring as A displaces B, we quantify the rate of product formation from the spatially resolved measurement of the pore scale reaction rate, using a chemiluminescent reaction.
Details can be found on the article: "Mixing and Reaction Kinetics in Porous Media: An Experimental Pore Scale Quantification" (pdf), Environmental Science & Technology