Emergence of Dynamic Heterogeneity and Facilitation in 3D Sheared Granular Materials

Plastic deformation in granular materials involves complex, correlated microscale dynamics, yet the nature of this correlated motion remains poorly understood. While such behavior has been extensively studied in the context of dynamic heterogeneity (DH) and dynamic facilitation (DF) in 2D amorphous materials, similar analyses in 3D granular systems are limited. Here, we use 3D in-situ X-ray tomography during triaxial compression to track thousands of particles and quantify microscale dynamics across strain increments. We find that DH and DF emerge strongly in the transition regime between the elastic and the critical state regimes, but weaken inside the shear band at the critical state under large strain increments. This implies a qualitative shift in local dynamics upon strain localization. Under small strain increments, both DH and DF remain weak across all deformation regimes. These results reveal that correlated dynamics in 3D granular materials depends strongly on both strain increment and deformation regime, bridging the behavior of granular matter and glassy dynamics in amorphous solids.