High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming

TL;DR

This study uses high-speed ultrasound imaging to reveal that impact-induced solidification in dense suspensions occurs via shear jamming without densification, providing new insights into the internal dynamics of such transitions.

Contribution

The paper introduces a novel application of ultrasound imaging to directly observe shear jamming in dense suspensions, challenging previous densification-based models.

Findings

Solidification occurs without increased packing fraction.

Shear intensity peaks at the jamming front.

Proposed model explains anisotropic front propagation.

Abstract

A remarkable property of dense suspensions is that they can transform from liquid-like at rest to solid-like under sudden impact. Previous work showed that this impact-induced solidification involves rapidly moving jamming fronts; however, details of this process have remained unresolved. Here we use high-speed ultrasound imaging to probe non-invasively how the interior of a dense suspension responds to impact. Measuring the speed of sound we demonstrate that the solidification proceeds without a detectable increase in packing fraction, and imaging the evolving flow field we find that the shear intensity is maximized right at the jamming front. Taken together, this provides direct experimental evidence for jamming by shear, rather than densification, as driving the transformation to solid-like behavior. Based on these findings we propose a new model to explain the anisotropy in the propagation speed of the fronts and delineate the onset conditions for dynamic shear jamming in suspensions.