Spreading on textiles: Dynamics of drops on model poroelastic fibrous materials

When wetting drops are deposited on textiles, such as paper or cotton fabrics, their spreading is generally accompanied with absorption of the liquid into the fibrous material and often expansion (i.e., swelling), which may cause large deformations. Using experimental model systems, we consider the coupled effects of perfect wetting, capillary forces, and liquid absorption and swelling. On a flat poroelastic substrate, a wetting drop readily spreads; spreading is only limited by the absorption within the material, leading to a maximal wetted radius and a localized deformation that slows down the absorption. On an assembly of fibers, a wetting drop adopts a compact shape that is governed by the fiber's geometry. On a single fiber, this finite spreading leads to a large absorption time as the fiber becomes quickly saturated below the drop, leaving only slow diffusion towards the unsaturated regions to absorb the liquid. This time may be strongly reduced if the drop is allowed to spread, which is possible between two parallel fibers, provided the distance between them is small enough. As a drop applies a capillary force on the fibers, it may deform them, reducing the distance between them and inducing a spontaneous spreading. For favorable solvents that can penetrate and swell the fibers, swelling can further induce a spontaneous collapse of adjacent fibers, and thus a rapid absorption. On a fibrous grid, where spreading decreases with increasing porosity, localized swelling induces a large out-of-plane deformation of the mesh, forming a bulge whose shape depends on the grid pattern. All these effects affect the spreading dynamics and absorption times of drops on fibrous assemblies, and we rationalize these observations with models coupling wetting, poroelasticity, and elastocapillarity.