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

TL;DR

This study develops an experimental method to measure the elastic properties of granular hydrogels, revealing the significant influence of capillary interactions and ionic conditions on their stiffness, which are often overlooked.

Contribution

The paper introduces a new testing protocol that accounts for capillary effects and ionic conditions, improving accuracy in measuring hydrogel elasticity.

Findings

Capillary interactions significantly affect measured Young's modulus.

Neglecting capillary effects underestimates hydrogel stiffness.

Elastic response depends on swelling and salinity levels.

Abstract

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 elastic responses of granular hydrogels under wet conditions. This setup directly observes liquid bridges formation and its evolution during compression. Our results show that neglecting capillary contributions leads to a systematic underestimation of the Young's modulus of hydrogels. Such an underestimation due to the capillary interaction increases as the sample size or its intrinsic stiffness decreases. In addition to the swelling ratio, the tested samples were also prepared under controlled salinity levels. The experimentally observed dependence of stiffness on swelling and salinity conditions is well captured by a modified constitutive model. The development of this study offers a robust testing protocol for measuring elastic properties of hydrogels under various environmental conditions.