Undulating compression and multi-stage relaxation in a granular column consisting of dust particles or glass beads

TL;DR

This study investigates the compression and relaxation behaviors of dust particles and glass beads in granular columns, revealing undulating force patterns and different stress decay mechanisms, with implications for understanding hierarchical granular structures.

Contribution

The paper provides experimental characterization of hierarchical granular matter, highlighting undulating compression forces and distinct relaxation behaviors for dust particles and glass beads, and proposes empirical models for these phenomena.

Findings

Periodic undulation observed in dust particles

Sudden force drops in glass beads

Stress relaxation follows exponential and logarithmic decay

Abstract

For fundamentally characterizing the effect of hierarchical structure in granular matter, a set of compression-relaxation tests for dust particles and glass beads confined in a cylindrical cell was performed. Typical diameter of both grains is approximately 1~mm. However, dust particles are produced by binding tiny ($\sim 5$~{\textmu}m) glass beads. The granular columns were compressed with a piston until reaching a maximum load force of 20~N with a constant compression rate $v$ ($0.17 \leq v \leq 2000$~{\textmu}m~s$^{-1}$). After that, the piston was stopped and the relaxation process was quantified. From the experimental results, we found that the compression force $F$ nonlinearly increases with the increase of compression stroke $z$ depending on particles. Besides, periodic undulation and sudden force drops were observed on $F(z)$ in dust particles and glass beads, respectively. The relaxation process was characterized by an exponential decay of stress followed by a logarithmic dependence one in both kinds of particles. These experimental findings are the main point in this study. To understand the underlying physics governing the compression mechanics, we assumed empirical forms of $F(z)$; $F\propto z^{\alpha}$ for dust particles and $F \propto \exp(z/z_G)$ for glass beads ($\alpha=2.4$ and $z_G=70$~{\textmu}m). Then, we found that the growing manners of periodic undulation and force drops were identical to those of mean compression forces, i.e., power law in dust particles and exponential in glass beads. In addition, the undulation amplitude and wavelength decreased as $v$ increased in dust-particles compression. On the basis of experimental results and the difference between dust particles and glass beads, we also discuss the origin of undulation and the physical meaning of granular-compression models used in engineering fields.