Imbibition in layered systems of packed beads

We study imbibition of a wetting fluid into layers of packed beads and examine how variations of permeability in the flow direction affect the dynamics of imbibition. In the case of systems made of two distinct layers, we show that the imbibition dynamics exhibit three regimes including diffusive-like imbibition as well as an unusual constant speed imbibition, which can be fine-tuned by adjusting the bead diameters and the length of the initial layer. We also perform experiments with gradients of permeability by ordering layers of beads of increasing (or decreasing) sizes in a tube. A continuum description is used to explain the results from the discrete experimental systems. In all cases, analytical models based on Darcy's equation show excellent agreement with the experiments.