  Hydrogels hold promise in agriculture as reservoirs of water in dry soil,
potentially alleviating the burden of irrigation. However, confinement in soil
can markedly reduce the ability of hydrogels to absorb water and swell,
limiting their widespread adoption. Unfortunately, the underlying reason
remains unknown. By directly visualizing the swelling of hydrogels confined in
three-dimensional granular media, we demonstrate that the extent of hydrogel
swelling is determined by the competition between the force exerted by the
hydrogel due to osmotic swelling and the confining force transmitted by the
surrounding grains. Furthermore, the medium can itself be restructured by
hydrogel swelling, as set by the balance between the osmotic swelling force,
the confining force, and intergrain 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.
