Boundary compliance selects heterogeneous dynamics in shear-thickening suspensions

TL;DR

This study demonstrates how boundary compliance influences the formation and stability of heterogeneous structures in shear-thickening suspensions, revealing distinct flow regimes and the role of boundary mechanics in shear jamming phenomena.

Contribution

It introduces a method to tune boundary compliance using a viscous oil layer, revealing its impact on flow heterogeneities and jamming in shear-thickening suspensions.

Findings

Compliant boundaries lead to long-lived density waves and clusters.

Resistant boundaries cause transient jamming and secondary stress waves.

Onset stress for jamming is independent of boundary viscosity.

Abstract

The mechanical properties of confining boundaries can fundamentally alter the flow behaviour of shear-thickening suspensions. We study a dense cornstarch suspension sheared beneath a viscous silicone-oil layer, using the oil viscosity to tune boundary compliance. Flow visualisation and rheometry reveal two distinct regimes. With compliant boundaries, long-lived heterogeneities emerge via density waves or persistent clusters, maintained by a balance between interface deformation and particle rearrangement. With more resistant confinement, we observe transient jamming events, marked by abrupt spanning of load-bearing structures across the suspension thickness and the emergence of secondary stress waves. The onset stress of these events remains constant at the DST threshold, independent of bounding viscosity. Our results reveal that boundary compliance selects the lifetime and morphology of heterogeneous structures, offering a means to amplify otherwise short-lived microscopic processes and providing new insight into the interplay between shear thickening, shear jamming, and confinement mechanics.