Hyperelastic rheology of hydrogels during compression and sliding

Hydrogels are biphasic, swollen polymer networks where elastic deformation is coupled to nanoscale fluid flow. Examples include cartilage, gelatin, agarose, and many cosmetic products. They are ubiquitous in applications such as tissue engineering and soft robotics due to their hydrophilic nature. Here we show how sliding and shear forces produce dilation in hydrogels. First, we investigated the relaxation of the normal force in centimeter-scale polyacrylic acid hydrogel spheres subjected to strain-controlled compression. A collapse of the data revealed a multi-day relaxation timescale, and the effective poroelastic diffusion coefficient was measured to be of order of 2 x 10^-9 m²/s, which is consistent with the self-diffusivity of water. Additionally, using a custom-built pin-on-disk tribometer and rheometer, we observed an increase in the normal force (or swelling) upon sliding the spherical hydrogel. This hyperelastic coupling between shear strain and dilation (Poynting effect) induces fluid imbibition in the hydrogel, as evidenced by slow relaxation upon cessation of sliding.