Macroscopic Discontinuous Shear Thickening vs Local Shear Jamming in Cornstarch

TL;DR

This study combines rheometry and MRI to investigate shear thickening in cornstarch, revealing that macroscopic DST arises from flow separation into distinct density regions rather than a uniform rheological change.

Contribution

It provides new evidence linking macroscopic shear thickening to local shear jamming and flow heterogeneity, challenging previous interpretations of DST as a uniform rheological transition.

Findings

DST occurs only with flow separation into different density regions

Local rheology in the flowing region is shear-thinning, not shear-thickening

Shear jamming occurs at volume fractions below random close packing

Abstract

We study the emergence of discontinuous shear-thickening (DST) in cornstarch, by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI) measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region, is not DST but, strikingly, is often shear-thinning. Our data thus show that the stress jump measured during DST, in cornstach, does not capture a secondary, high-viscosity branch of the local steady rheology, but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.