(494f) Under Pressure: Hydrogel Swelling in a Granular Medium

Hydrogels are polymer networks that can absorb considerable amounts of water. They thus hold promise in agriculture as reservoirs of water in dry soil, potentially decreasing the burden of plant irrigation. However, recent tests indicate that confinement in soil can drastically reduce the ability of hydrogels to absorb water and swell---in some cases, by as much as ~90 vol%. Unfortunately, the underlying reason remains unknown. We shed light on this issue by directly visualizing the swelling of a hydrogel confined in a three-dimensional (3D) granular medium, similar to unconsolidated soil. When confined, hydrogels deform strongly and swell less; the total extent of swelling depends sensitively on the magnitude of the confining stress. We demonstrate that this behavior is determined by the competition between the osmotic swelling pressure of the hydrogel and the confining stress transmitted by the surrounding grains. Our experiments also reveal that the medium can itself be restructured by hydrogel swelling. We demonstrate that this behavior is in turn determined by the balance between osmotic swelling, confining stress, and inter-grain friction. Together, our results provide quantitative principles to predict how hydrogels behave in confinement, potentially improving their use in agriculture as well as informing other applications such as oil recovery, construction, mechanobiology, and filtration.