Drastic slowdown of shear waves in unjammed granular suspensions

TL;DR

This study investigates how shear elastic waves slow down dramatically in unjammed granular suspensions, revealing nonlinear behavior, shear modulus weakening, and grain rearrangements using ultrafast ultrasound.

Contribution

It introduces an experimental approach to observe nonlinear shear wave behavior and proposes a friction model linking elasticity and plasticity in granular suspensions.

Findings

Shear wave velocity softens with increased amplitude

Two regimes identified: shear modulus weakening and grain rearrangement

Modulus decreases up to 88% during grain rearrangement

Abstract

We present an experimental investigation of shear elastic wave propagation along the surface of a dense granular suspension. Using an ultrafast ultrasound scanner, we monitor the softening of the shear wave velocity inside the optically opaque medium as the driving amplitude increases. For such nonlinear behavior two regimes are found: in the first regime, we observe a significant shear modulus weakening, but without visible grain rearrangements. In the second regime, there is a clear grain rearrangement accompanied by a modulus decrease up to 88%. A friction model is proposed to describe the interplay between nonlinear elasticity and plasticity, which highlights the crucial effect of contact slipping before contact breaking. Investigation of these nonlinear shear waves may bridge the gap between two disjoint approaches for describing the dynamics near unjamming: linear elastic soft modes and nonlinear collisional shock.