Measuring the elastic properties of granular hydrogels: effects of capillary interaction and ionic conditions

The elastic properties of granular hydrogels are commonly characterised under wet conditions, yet the influence of capillary interactions remains unclear. In practical applications, hydrogels operate in aqueous environments containing dissolved ionic species, where swelling and elastic behaviour depend sensitively on ionic conditions. In this study, an experimental setup is developed to measure the elastic responses of granular hydrogels under wet conditions. This setup directly observes liquid-bridge formation and its evolution during compression. Our results show that neglecting capillary contributions leads to a systematic underestimation of Young's modulus of hydrogels. This underestimation due to the capillary interaction increases as the sample size or intrinsic stiffness decreases. In addition to the swelling ratio, the tested samples were also prepared under controlled salinity conditions. The experimentally observed dependence of stiffness on swelling and salinity conditions is well captured by a modified constitutive model. This study offers a robust testing protocol for measuring the elastic properties of hydrogels under various environmental conditions.