Spreading and sorption of droplets on layered porous substrates.

In this paper spreading and sorption of a droplet on an anisotropic, layered porous substrate are investigated numerically. Flow in the saturated part of the porous material is governed by Darcy's law, assuming a sharp wetting front separating the saturated regions from the dry regions. Numerical results are presented for spreading and sorption of droplets in their dependence on the material and process parameters for axisymmetric configurations. Limiting cases of sorption into infinitely thick and very thin porous layers are considered. For an analytical sorption model for thin substrates fed by an infinite reservoir a correction term taking into account the flow resistance in the inlet region is derived and the consistence of the modified model with numerical and experimental results is shown. For two-layer substrates, numerical results on the influence of the layer permeabilities on the sorption kinetics are presented.