Spreading on soft porous and poroelastic substrates

When wetting drops are deposited on porous or poroelastic materials, such as gels, paper or cotton fabrics, their spreading is accompanied with absorption of the liquid into the material. Here, we consider the coupled effects of wetting and liquid absorption using two experimental model systems.

On a flat poroelastic substrate, spreading is limited by the absorption within the material, leading to a maximal wetting radius that is independent of the liquid’s viscosity but depends on the material properties. On a fibrous mesh, the drop adopts a shape that strongly depends on the fiber arrangements, i.e. on the porosity of the grid; the spreading radius increases as the porosity decreases. For favorable solvents that can penetrate and swell the fibers, swelling can further induce a spontaneous collapse of adjacent fibers, as well as a global deformation of the mesh, which strongly affect the spreading dynamics. We rationalize these observations with models coupling wetting, poroelasticity and elasto-capillarity.