Abstract
Recent theories predict that discontinuous shear-thickening (DST) involves an instability, the nature of which remains elusive. Here, we explore unsteady dynamics in a dense cornstarch suspension by coupling long rheological measurements under constant shear stresses to ultrasound imaging. We demonstrate that unsteadiness in DST results from localized bands that travel along the vorticity direction with a specific signature on the global shear rate response. These propagating events coexist with quiescent phases for stresses slightly above DST onset, resulting in intermittent, turbulent-like dynamics. Deeper into DST, events proliferate, leading to simpler, Gaussian dynamics. We interpret our results in terms of unstable vorticity bands as inferred from recent model and numerical simulations.
Highlights
Instabilities are commonly observed in simple, Newtonian fluids when forced to flow under increasingly large Reynolds numbers
Our results demonstrate that the unsteady dynamics observed in the discontinuous shear thickening (DST) of a dense cornstarch suspension originates from localized, intermittent propagating events that proliferate as the shear stress is increased
By carefully investigating a dense cornstarch suspension with a combination of rheometry and ultrasound imaging over long timescales, we have pinned the origin of unsteady dynamics in DST to the existence of transient localized bands that travel along the vorticity direction
Summary
Instabilities are commonly observed in simple, Newtonian fluids when forced to flow under increasingly large Reynolds numbers. Such hydrodynamic instabilities lead to fully developed turbulence [1,2], yet following multiple pathways in which vortices [3,4] or turbulent puffs and spots [5,6] may arise and mediate unsteady or chaotic large-scale flow dynamics. While inertia is at the heart of flow instabilities in simple fluids, non-Newtonian fluids, such as polymer or self-assembled surfactant solutions, may display instabilities at vanishingly small Reynolds numbers due to elasticity [7,8] or due to a strong coupling between the flow and the fluid microstructure [9].
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