Grains unchained: local fluidization of a granular packing by focused ultrasound

TL;DR

This study demonstrates that focused ultrasound can remotely induce local and global fluidization in granular materials, revealing an intermittent and hysteretic unjamming transition with no clear structural precursors.

Contribution

It provides experimental and numerical evidence of ultrasound-induced unjamming in granular packs, introducing a two-state model to explain the observed intermittency and hysteresis.

Findings

Ultrasound causes local rearrangements and global motion in granular packs.

Fluidization is intermittent and hysteretic with respect to acoustic pressure.

Unjamming occurs without detectable structural precursors.

Abstract

We report experimental results on the dynamics of a granular packing submitted to high-intensity focused ultrasound. Acoustic radiation pressure is shown to remotely induce local rearrangements within a pile as well as global motion around the focal spot in an initially jammed system. We demonstrate that this fluidization process is intermittent for a range of acoustic pressures and hysteretic when the pressure is cycled. Such a first-order-like unjamming transition is reproduced in numerical simulations in which the acoustic pressure field is modeled by a localized external force. Further analysis of the simulated packings suggests that in the intermittent regime unjamming is not associated with any noticeable prior structural signature. A simple two-state model based on effective temperatures is proposed to account for these findings.