Scalar mixtures in porous media

Using a technique allowing for in situ measurements of concentrations fields, the evolution of scalar mixtures flowing within a porous medium made of a three-dimensional random stack of solid spheres, is addressed. Two distinct fluorescent dyes are injected from separate sources. Their evolution as they disperse and mix through the medium is directly observed and quantified, which is made possible by matching the refractive indices of the spheres and the flowing interstitial liquid. We decipher the nature of the interaction rule between the scalar sources, explaining the phenomenon that alters the concentration distribution of the overall mixture as it decays toward uniformity. Any residual correlation of the initially merged sources is progressively hidden, leading to an effective fully random interaction rule of the two distinct subfields.