Capillary compression of a soft sponge

The capillary entry pressure of a porous medium is the applied pressure at which a non-wetting fluid will first invade the pore space by displacing the wetting fluid from the largest pore throats. For a rigid porous medium, the entry pressure is a characteristic of the two fluids, the solid material, and the pore structure. For a soft porous medium, however, the applied pressure will also compress the medium, thereby changing the pore structure and thus the entry pressure itself. This capillary compression complicates the basic concept of entry pressure as a material property. Here, we use experiments and modelling to study the capillary entry pressure of a soft polyurethane sponge. We show that the measurement of capillary pressure provides a sensitive probe of the complex mechanics of these materials. We highlight the strong, non-monotonic relationship between water content and volumetric strain.