Ultrasonic tracking of a sinking ball in a vibrated dense granular suspension

TL;DR

This study uses ultrasonic probing to analyze how a steel ball sinks in a vibrated dense granular suspension, revealing that vibration reduces friction and yield stress through acoustic lubrication, differing from traditional liquefaction mechanisms.

Contribution

It demonstrates the application of ultrasonic techniques to study granular suspension dynamics and introduces the concept of acoustic lubrication reducing friction without grain rearrangements.

Findings

Vibration decreases static friction coefficient and effective viscosity.

Sinking depth increases with vibration intensity due to reduced yield stress.

Acoustic lubrication reduces inter-particle friction without visible grain rearrangements.

Abstract

Observing and understanding the movement of an intruder through opaque dense suspensions such as quicksand remains a practical and conceptual challenge. Here we use an ultrasonic probe to investigate the dynamics of a steel ball sinking in a 3D dense glass bead packing saturated by water. We show that the frictional model developed for dry granular media can be used to describe the ball motion induced by horizontal vibration. From this rheology we infer the static friction coefficient and effective viscosity that decrease when increasing the vibration intensity. Our main finding is that the vibration-induced reduction of the yield stress and increase of the sinking depth are presumably due to induced slipping at the grain contacts but without visible plastic rearrangements of grains, in contrast to dry granular packings. To explain these results, we propose a mechanism of acoustic lubrication that reduces the inter-particle friction and leads to a decrease of the yield stress. This scenario is different from the mechanism of liquefaction usually invoked in loosely packed quicksands where the vibration-induced compaction increases the pore pressure and decreases the confining pressure on the solid skeleton, thus reducing the granular resistance to external load.