Correlation between Structural Order and Diffusion Length in Granular Flow

We investigate how structural ordering, i.e., crystallization, affects the flow of bidisperse granular materials in a quasi-two-dimensional silo. By systematically varying the mass fraction of two particle sizes, we finely tune the degree of local order. Using high-speed imaging and kinematic modeling, we show that crystallization significantly enhances the diffusion length b, a key parameter controlling the velocity profiles within the flowing medium. We reveal a strong correlation between b, the hexatic order parameter ψ_{6}, and the cluster size ξ, highlighting the role of local structural organization in governing macroscopic flow. Furthermore, pressure gradients within the silo stabilize orientational order even without crystallization, thus intrinsically increasing b with height. These results highlight a direct link between microstructural order, pressure, and transport properties in granular silo flows and suggest that similar mechanisms may operate in other particulate systems, such as colloids, foams, or emulsions, where local structural ordering and confinement affect flow and transport.