Under pressure: Hydrogel swelling in a granular medium

TL;DR

This study reveals how hydrogel swelling in granular media is governed by a balance of osmotic forces and confining stresses, providing insights for agricultural and industrial applications.

Contribution

It introduces a direct visualization method to quantify hydrogel swelling under confinement and establishes predictive principles for their behavior in granular media.

Findings

Swelling extent is determined by osmotic and confining force balance.

Hydrogel swelling can restructure the granular medium.

Quantitative principles predict hydrogel behavior in confinement.

Abstract

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.