Sound propagation and force chains in granular materials

TL;DR

This study investigates how sound propagates through granular materials by combining experiments and models, revealing the influence of force chains and particle contacts on acoustic amplitude and speed.

Contribution

It provides particle-scale measurements of sound propagation in granular media and links force chains to acoustic responses, advancing understanding of heterogeneous material behavior.

Findings

Wave amplitude correlates with force chains and contact forces.

Speed of wave front matches 1D chain predictions.

Transient force chains affect wave propagation.

Abstract

Granular materials are inherently heterogeneous, leading to challenges in formulating accurate models of sound propagation. In order to quantify acoustic responses in space and time, we perform experiments in a photoelastic granular material in which the internal stress pattern (in the form of force chains) is visible. We utilize two complementary methods, high-speed imaging and piezoelectric transduction, to provide particle-scale measurements of both the amplitude and speed of an acoustic wave in the near-field regime. We observe that the wave amplitude is on average largest within particles experiencing the largest forces, particularly in those chains radiating away from the source, with the force-dependence of this amplitude in qualitative agreement with a simple Hertzian-like model of particle contact area. In addition, we are able to directly observe rare transient force chains formed by the opening and closing of contacts during propagation. The speed of the leading edge of the pulse is in quantitative agreement with predictions for one-dimensional chains, while the slower speed of the peak response suggests that it contains waves which have travelled over multiple paths even within just this near-field region. These effects highlight the importance of particle-scale behaviors in determining the acoustical properties of granular materials.