Droplet Bridging into Porous Media

If the top of a sessile droplet is brought into contact with an opposing solid surface, the droplet can transfer depending on the relative speeds and wettabilities of the surfaces. What if the surface receiving the liquid was porous? We use side-view high-speed imaging to capture the transfer of liquid from a solid substrate to an opposing porous surface. The parameters that were varied include donor wettability, the porosity and pore size of the receiving surface, and the droplet’s volume, surface tension, and viscosity. Generally, the transfer process is split into two major regimes, wetting and wicking. The wetting regime is split into two more regimes, the donor-independent and donor-dependent regimes. The donor-independent regime follows the dynamics of droplet coalescence, starting in the inertially-limited viscous regime and diverging into capillary-inertial or viscocapillary regimes depending on the liquid. The donor-dependent regime is limited by the flow in the receding contact line’s viscous wedge. The wicking regime is governed by Darcy’s Law, completing the transfer of the droplet.