Invasion front structure in a 3-D model porous medium under a hydrostatic pressure gradient

An experimental analysis of the structure of a Wood metal invasion front into a crushed glass sample is described. The correlation function of the invading fluid repartition is determined in horizontal plane cuts over the whole vertical extent of the invasion front. A fractal-like geometry is observed at length scales between the individual grain size R and a correlation length of up to 20 R. At large invading fluid saturations, the fractal dimension D f is close to the value 2.5 predicted for 3D infinite percolation clusters; at lower saturations, it decreases to D f approximately=2. At high and medium saturations, the mean Wood metal saturation S MW in a horizontal plane decreases linearly with height, due to finite size effects and at low saturations, S MW decreases slowly corresponding to an 'invasion tail'. The interpretation of these experimental results as a percolation process with a spatial percolation parameter gradient is then discussed.