Structural transitions in jammed asymmetric bidisperse granular packings

TL;DR

This study investigates how local structural order changes during jamming in asymmetric bidisperse granular packings, revealing a transition characterized by shifts in contact configurations and the presence of local crystalline structures.

Contribution

It introduces a contact orientational order parameter to quantify local structure and demonstrates structural transitions at jamming in bidisperse packings, highlighting local crystalline order.

Findings

Structural transition at jamming indicated by a jump in contact order.

Different contact configurations contribute uniquely to the structure.

Presence of local HCP and FCC structures in bidisperse packings.

Abstract

We study the local structural changes along the jamming transitions in asymmetric bidisperse granu\-lar packings. The local structure of the packing is assessed by the contact orientational order, $\tilde{Q}_{\ell}$, that quantifies the contribution of each contact configuration (Large-Large, Small-Small, Large-Small, Small-Large) in the jammed structure. The partial values of $\tilde{Q}_{\ell}$ are calculated with respect to known ordered lattices that are fixed by the size ratio, $\delta$, of the particles. We find that the packing undergoes a structural transition at $\phi_J$, manifested by a sudden jump in the partial $\tilde{Q}_{\ell}$. Each contact configuration contributes to the jammed structure in a different way, changing with $\delta$ and concentration of small particles, $X_{\mathrm{S}}$. The results show not only that the packing undergoes a structural change upon jamming, but also that bidisperse packings exhibit local HCP and FCC structures also found in monodisperse packings. This suggests that the jammed structure of bidisperse systems is inherently endowed with local structural order. These results are relevant in understanding how the arrangement of particles determines the strength of bidisperse granular packings.