Role of Friction on the Formation of Confined Granular Structures

TL;DR

This study investigates how friction influences the formation of glass- or crystal-like structures in confined granular systems, revealing that lower friction promotes crystalline order while higher friction leads to amorphous arrangements.

Contribution

It provides experimental evidence linking surface friction properties to the structural organization of confined granular materials, introducing a new diagram for particle states based on friction levels.

Findings

Higher friction increases velocity fluctuations.

Lower friction favors crystal-like structures.

Friction level determines amorphous or crystalline organization.

Abstract

Unstable systems of fluidized grains in a very-narrow vertical tube can auto-defluidize after some time, the settling particles forming either a glass- or crystal-like structure. We carried out experiments using different polymer spheres, of known friction and roughness, fluidized in water. A diagram was obtained for the \RefereeA{shell-settled} particles when the coefficient of friction is of the order of 0.1, and their structure is characterized through an analysis of the nearest neighbors' angles. We show that the level of velocity fluctuations is higher for the high friction material, and that, once defluidized, the decrease in the coefficient of sliding friction leads to more organized (crystal-like) structures, while those with higher friction coefficients are amorphous (glass-like structures). Our results bring new insights for understanding the formation of glass- and crystal-like structures based on the material surface properties.