Moving under pressure Time dependent response of soft granular materials under stress

This thesis examines how granular materials made out of soft grains—such as hydrogels, foams, and food grains—respond to loading, highlighting the importance of particle deformability. Often ignored in studies of granular materials made of rigid grains, accounting for deformability is essential to predict how these materials pack, flow, and react to forces.
Chapter 2 investigates hydrogel particles as a model system. Using a custom tool, we measured contact forces during compression and sliding and developed a simple model with five key parameters to describe particle behavior and relaxation dynamics. We characterized the material and relaxation dynamics originating at the grain level.
In Chapter 3, hydrogel packings under strain revealed two characteristic timescales in force response. We conjectured that one timescale is associated with grain-level relaxation. These findings show how microscopic responses integrate into macroscopic behavior.
Chapter 4 presents the strain-dependent viscoelastic model (SDVES), which captures nonlinear behaviors such as force recovery and the absence of adhesion at the end of contact with a non-adhesive material. The model effectively explains behavior at moderate strains but requires refinements to account for faster or more extreme deformations.
Finally, Chapter 5 compares flowability metrics for powders with different properties. The Hausner ratio emerges as a potentially scalable alternative to traditional methods for cohesive powders, though additional data are needed to confirm its reliability.
Overall, this work advances the understanding of soft granular materials by linking particle-level dynamics to bulk behavior and developing models and metrics for better prediction and control.