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, turbulentlike 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.

Density waves in shear-thickening suspensions

“…Our results on cornstarch differ from the ones obtained by Saint-Michel et al (22). In their recent work, the authors have studied cornstarch suspensions with local velocity measurements.…”

“…At this stage, there exists no direct experimental evidence of these bands and of their dynamics: signs of their possible existence are fluctuating inhomogeneities of stress (21) and of velocity (22). This mainly comes from the difficulty to measure the temporal and the spatial evolution of local variables such as solid-fraction profiles with high resolution.…”

Competing Timescales Lead to Oscillations in Shear-Thickening Suspensions

“…With only two dynamical variables, lacking spatial variation, our model cannot capture such inhomogeneous flow. It nevertheless well captures the development of relaxation oscillations en route to aperiodic unsteady flows, which are widely seen [14,15,17,39].…”

Interparticle attraction controls flow heterogeneity in calcite gels

“…More generally, we emphasize that strong fluctuations seem inherent to the shear-induced dynamical transitions of soft materials. Indeed, dense suspensions of frictional particles that undergo discontinuous shear thickening also display large temporal variations of global and local quantities, both in experiments 6,[65][66][67][68][69] and in simulations. 70,71 Thus, fluctuations are not restricted to shear-banding systems, whether yield stress fluids or shear-thinning fluids, but appear as much more widespread.…”