Uncovering instabilities in the spatiotemporal dynamics of a shear-thickening cornstarch suspension

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.

• The paper uses combined rheology and ultrasound imaging to uncover dynamic instabilities driven by propagating vorticity bands in shear-thickening cornstarch.
• Two dynamic regimes were identified based on stress, revealing intermittent, turbulent-like statistics at low stress and Gaussian statistics at higher stress.
• While propagation speed is constant, band frequency increases with stress, suggesting DST models should include spatiotemporal factors.

Insights into the Dynamics of Shear-Thickening in Cornstarch Suspensions

The study titled "Uncovering instabilities in the spatiotemporal dynamics of a shear-thickening cornstarch suspension" offers a significant contribution to the understanding of discontinuous shear-thickening (DST) in non-Newtonian fluids. This work explores DST through an innovative combination of rheological measurements and ultrasonic imaging to capture the complex dynamics of cornstarch suspensions under shear stress.

Study Overview

Shear-thickening behavior in particulate suspensions denotes a transition from a low to a high viscosity state under increasing shear. Traditionally, this phenomenon has been attributed to mechanisms such as hydroclustering, particularly in colloidal suspensions, while recent models suggest a transition mediated by frictional interactions among particles, as exemplified by the Wyart and Cates model. This study, however, explores uncharted aspects of DST by focusing on temporal and spatial dynamics, particularly with respect to instabilities and band formations.

Methodology

The authors employed a well-matched density mixture of water and cesium chloride to suspend cornstarch, which was then subjected to controlled shear stresses in a Taylor-Couette geometry. This setup allowed for the measurement of shear rates and the use of ultrasound imaging to obtain velocity maps across the suspension. Such an approach addresses limitations in prior studies that often overlooked spatial variability within the suspensions.

Key Findings

1. Propagation of Vorticity Bands: The study identifies that the unsteady dynamics observed in DST are attributed to the formation and propagation of localized vorticity bands along the vorticity direction. These bands propagate through the suspension with a consistent speed, leading to fluctuations in the shear response.
2. Statistical Analysis of Dynamics: Two dynamic regimes were observed. For shear stresses just above the DST onset, the shear rate exhibited intermittent, turbulent-like statistics, while higher stresses revealed Gaussian statistics indicative of more regular dynamics. The researchers detailed a statistical characterization using probability density functions and spectral analyses to support these findings.
3. Impact of Parameters: The propagation speed of the vorticity bands remained relatively constant but their frequency increased notably with higher applied stresses, aligning with theoretical predictions of a transition from lubricated to frictional particle contacts.

Theoretical Implications

The presence of propagating vorticity bands raises considerable questions about our understanding of DST. Traditionally, vertical regions on flow curves have been linked to steady-state shear banding. However, this study suggests these regions may instead signify dynamic processes at play, such as stress-bearing structures traversing the material. This could imply that existing models need to incorporate more complex spatiotemporal factors to accurately represent discontinuous shear-thickening.

Practical Implications and Future Work

This research enhances our comprehension of the fundamental mechanics behind DST, potentially influencing industrial processes where shear-thickening materials are prevalent. The findings of this study call for further refinement in models to predict and control flow behavior in such systems better. Future investigations should focus on the link between wall slip phenomena and bulk dynamics while also incorporating advanced imaging techniques to confirm the hypothetical link between mechanical response and light scattering intensity as a function of local concentration.

In conclusion, the investigation presents a comprehensive view of DST dynamics by capturing the nuances of shear-induced instabilities. This work paves the way for refined modeling and control strategies in applications leveraging the unique properties of shear-thickening systems. The innovative integration of rheology and ultrasound imaging sets a new standard in exploring complex flow behaviors in non-Newtonian fluids.