File:Mixing in Porous Medium.jpg

English: Two fluorescent dyes are injected into stationary upward water flow inside three-dimensional porous medium made of a random stack of solid transparent beads of diameter 16 mm (the planar cross-sections of the spherical beads are seen as black circles on the image). Based on this single photograph, the evolution of the dye concentrations and their interaction rules in the porous medium have been deciphered and explained (Kree and Villermaux, Physical Review Fluids, 2, 104502). Technical/scientific details (see the paper for further details): 1) The porous medium is made transparent by matching the refraction indices of water and the beads; 2) The fluorescent dyes are made visible only at the central plane of the 3D medium using a planar laser sheet, elsewhere the dyes remain transparent; 3) The stationary (time-independent) flow leads to efficient mixing due to high velocity gradients resulting from no-slip boundary conditions at the surfaces of the beads; 4) Any residual correlation of the initially merged distinct dyes is progressively hidden, leading to an effective fully random interaction rule of the two distinct subfields, manifested by a Beta distribution of the mixing ratio.