Capillary entry pressure of a hydrogel packing

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 coupling 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 model soft porous medium: a packing of hydrogel beads (diameter ~1 mm), in which the individual grains are themselves soft porous media on a much smaller scale (pore size ~10 nm). We show that the measurement of entry pressure provides a sensitive probe of the complex mechanics of these materials. We highlight in particular the strong interactions between entry pressure, viscoelastic creep, and stress-strain hysteresis.