Structural Order Drives Diffusion in a Granular Packing

TL;DR

This study shows that crystallization and local structural order significantly influence diffusion and flow profiles in granular silo flows, with pressure gradients also playing a stabilizing role.

Contribution

It demonstrates a direct link between microstructural order, pressure, and transport properties in granular flow, highlighting the role of crystallization and local order.

Findings

Crystallization enhances the diffusion length $b$ in granular flow.

A strong correlation exists between $b$ and the hexatic order parameter.

Pressure gradients promote orientational order, increasing $b$ with height.

Abstract

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 $b$ and the hexatic order parameter $\left<|\psi_6|\right>_t$, highlighting the role of local structural organization in governing macroscopic flow behavior. Furthermore, we demonstrate that pressure gradients within the silo promote the stabilization of orientational order even in the absence of crystallization, thus intrinsically increasing $b$ with height. These findings establish a direct link between microstructural order, pressure, and transport properties in granular silo flows.