Dynamics and memory of boundary stresses in discontinuous shear thickening suspensions during oscillatory shear

TL;DR

This study uses boundary stress microscopy to reveal how boundary stresses and transient solid-like phases form and dissipate in dense suspensions during oscillatory shear, linking microscopic behavior to macroscopic rheology.

Contribution

It provides the first direct spatially resolved measurements of boundary stresses in shear thickening suspensions during LAOS, showing the formation and dynamics of solid-like phases.

Findings

Boundary stresses correlate with viscosity increase.

Solid-like phases form transiently at high strains.

Persistence of structures varies with frequency.

Abstract

We report direct measurements of spatially resolved surface stresses of a dense suspension during large amplitude oscillatory shear (LAOS) in the discontinuous shear thickening regime using boundary stress microscopy. Consistent with previous studies, bulk rheology shows a dramatic increase in the complex viscosity above a frequency-dependent critical strain. We find that the viscosity increase is coincident with that appearance of large heterogeneous boundary stresses, indicative of the formation of transient solid-like phases (SLPs) on spatial scales large compared to the particle size. The critical strain for the appearance of SLPs is largely determined by the peak oscillatory stress, which depends on the peak shear rate and the frequency-dependent suspension viscosity. The SLPs dissipate and reform on each cycle, with a spatial pattern that is highly variable at low frequencies but remarkably persistent at the highest frequency measured ($\omega = 10$ rad/sec).