Imbibition in layered systems of packed beads

It is well known that during imbibition of uniform porous media, the penetration distance increases as the square root of time. However, this ``diffusive'' result is not observed when the spatial arrangement of the beads is not uniform. We investigate two specific cases of inhomogeneity: (i) a system made with two layers each of a uniform bead size, (ii) a gradient of permeability. In both cases, deviations from the ``diffusive'' dynamics are observed and explained theoretically.