Acoustic modulation of shear thickening transition in dense adhesive suspensions

TL;DR

This study reveals that ultrasound shifts the shear-thickening transition in dense adhesive suspensions to higher shear rates by fluidizing force networks, rather than directly reducing viscosity, enabling better control of flow instabilities.

Contribution

It uncovers the mechanism by which ultrasound influences shear thickening, emphasizing the role of fluctuating hydrodynamic forces and force network stability in dense suspensions.

Findings

Ultrasound shifts shear-thickening transition to higher shear rates.

Stress distributions collapse onto master curves indicating continuous transition.

Ultrasound modifies force network stability via fluctuating hydrodynamic forces.

Abstract

Discontinuous shear thickening (DST) in dense suspensions leads to flow instabilities that limit processing in many systems. While high-power ultrasound has been reported to reduce the apparent viscosity of such materials, the origin of this effect remains unclear. Here, we investigate dense adhesive cornstarch suspensions, where shear thickening arises from fragile, load-bearing force networks embedded in heterogeneous density-wave structures. Using a rheo-ultrasound setup, we show that ultrasound does not directly reduce viscosity but instead shifts the shear-thickening transition toward higher shear rates. This is evidenced by the collapse of stress probability distributions onto master curves, revealing a continuous evolution toward more fluid-like states without a sharp threshold. We interpret these results through a separation of time scales, in which the suspension behaves as an effectively immobile porous medium subjected to high-frequency interstitial flows. Fluidization then arises from a combination of boundary slip, bulk destabilization of force networks by drag-force fluctuations, and localized acoustic streaming. Beyond these mechanisms, we propose that ultrasound modifies the stability of force networks by introducing fluctuating hydrodynamic forces at the pore scale. As a result, larger stresses or shear rates are required to sustain jammed states, leading to a continuous renormalization of the DST transition. These findings provide a consistent physical picture of acoustic fluidization in adhesive suspensions and establish ultrasound as a powerful tool to control discontinuous shear thickening in confined flows.