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

Area of Science:

• Soft Matter Physics
• Granular Materials Science
• Rheology

Background:

• Bulk response of granular materials depends on particle properties like friction, softness, and adhesion.
• Wetted hydrogel particles are soft and weakly adhesive due to capillary bridges, offering a simplified system to study granular flow.

Purpose of the Study:

• To investigate the role of capillary stresses in the macroscopic flow of wetted hydrogel particles.
• To explore the relationship between micro-scale interactions and macro-scale flow behavior.

Main Methods:

• Micro-scale experiments correlating relative humidity (RH) with liquid bridge size and rupture distance.
• Macro-scale experiments using a split-bottom shear cell to observe bulk flow.
• Magnetic resonance imaging (MRI) to retrieve flow fields.
• Rheometric techniques to measure flow resistance.

Main Results:

• A direct correlation between RH and liquid bridge size was observed, with increased humidity leading to larger average rupture distances.
• Wetted hydrogel packings exhibited narrow shear bands, significantly narrower than dry granular flows.
• Flow resistance was dominated by capillary bridge stress, independent of filling height, and influenced by external pressure.

Conclusions:

• Capillary stresses are a critical factor in controlling the macroscopic flow behavior of wetted hydrogel particle packings.
• Micro-scale capillary interactions directly influence macro-scale flow dissipation and shear band formation.
• Understanding these micro-scale properties is essential for predicting and controlling granular material flow.