Under pressure: Mechanics of swelling hydrogels under confinement

Hydrogels are polymer networks that can absorb considerable amounts of water. They are thus promising additives to soil in arid conditions, increasing water retention and decreasing the need for plant irrigation. However, field measurements indicate that confinement in soil alters both the ability of hydrogels to hold water, as well as the properties of the soil itself—and the underlying physical reasons remain unknown. We have developed the ability to directly visualize hydrogel swelling within a three-dimensional porous medium that mimics soil. Using this platform, we quantify how the presence of the solid grains around a hydrogel hinders its ability to swell. By testing different applied loads and sizes of porous media, we show that the deformations of the hydrogel and the medium can be described by a balance between the osmotic swelling pressure of the hydrogel, the local elastic strain energy needed for the hydrogel to swell into the pores, and the frictional interactions holding grains together. Our results thereby provide a general framework by which hydrogel swelling can be understood, potentially improving their ability to help plants survive drought, and informing applications in new settings like oil fields and lab-on-a-chip devices.