Entangled Granular Chains: Geometry, Cohesion, and Emergent Mechanics

Abstract

Granular materials, composed of discrete macroscopic particles, are ubiquitous in daily life, industry, and natural processes. Unlike conventional solids or liquids, their behavior is complex, driven by dissipative grain-grain interactions rather than thermal energy. Understanding their mechanics and flow properties is crucial for diverse applications, from pharmaceutical manufacturing and mining to construction materials like railway ballast and road foundations, and even in predicting natural phenomena such as landslides and avalanches. The mechanical and flow properties of dry granular systems are significantly influenced by interparticle cohesion. While traditional cohesion, arise from attractive forces like Van der Waals or capillary bridges, “geometric cohesion” emerges solely from particle shape and contact friction, enabling strong interlocking or entanglement. This phenomenon is particularly pronounced in materials with aspherical or non-convex particles, such as U-shaped staples, Z-shaped particles, or flexible granular chains. These systems exhibit emergent properties, including enhanced mechanical rigidity, increased resistance to deformation, and distinctive flow characteristics. They can form rigid, freestanding structures, such as tall piles or columns, and display unusually high angles of repose, defying typical cohesion less granular behavior. Despite their widespread importance, a microscopic understanding of these complex systems, about how geometric cohesion influences their mechanical and flow properties, remains limited. Specifically, quantitative insights into how such systems transmit forces and evolve under deformation are scarce. Granular chains are well-suited for studying entanglement-driven phenomena due to their tunable entanglement by varying the chain length. The flexibility introduced by the links allows the chains to form loops and entangle, giving rise to an effective cohesion that depends strongly on chain length and geometry. This makes them ideal for systematically probing how internal constraints influence macroscopic behavior such as shear strength, flow stability, and structural rigidity. This thesis addresses these critical questions through three dedicated experimental investigations. We systematically examined the behavior of granular chains under repose, studying how chain length and boundary conditions influence pile formation and stability. Further, we analyzed their flow dynamics through a hopper to understand how geometric cohesion impacts flow stability and jamming. We finally explored their response to shear, investigating force transmission and deformation characteristics