Capillary-induced adhesive contact dynamics determines dissipation and flow structure in wetted hydrogel packings

The bulk response of a granular material is strongly influenced by particle and contact properties, such as friction coefficients, particle softness, lubrication on the contact scale and adhesion between particles. This study explores the bulk flow of wetted hydrogel particles, which are soft but also weakly adhesive due to capillary bridges. This simplified granular material with minimal contact friction reveals key insights in the role of capillary stresses on the macroscopic flow. At the micro-scale, we demonstrate a direct correlation between relative humidity (RH) and liquid bridge size between two wetted hydrogel spheres, with an average rupture distance increasing with humidity. On the macro scale, the wetted hydrogel sphere packings show remarkable flow dissipation and flow behavior in the split-bottom shear cell. We retrieve flow fields of the hydrogel packing with magnetic resonance imaging and measure flow resistance with a rheometric technique. The shear bands for the adhesive hydrogels are much narrower than for dry grain flows. The change in flow resistance due to a change in filling height can be interpreted with a minimization argument, indicating that the flow dissipation is set entirely by the capillary bridge stress: the capillary stress at all filling heights dominates the gravitational stress. We confirm this view by exposing the flowing packing to an external pressure. Beyond a confining stress of 250 Pa, the shear bands become significantly thinner, approaching some plateau at 360 Pa. This underscores the importance of understanding micro-scale interactions in controlling macroscopic hydrogel particle packing behavior.