Structure of propagating high stress fronts in a shear thickening suspension

TL;DR

This study reveals persistent high-stress fronts propagating in shear thickening suspensions, showing their structure, particle-fluid flow dynamics, and role in viscosity increase, advancing understanding of shear thickening mechanisms.

Contribution

It provides direct measurements of stress, particle velocity, and fluid migration associated with propagating fronts in shear thickening suspensions, highlighting their structural and dynamic properties.

Findings

High stress fronts propagate at boundary speed.

Fluid migration is localized and reverses across fronts.

Propagating fronts account for viscosity increase.

Abstract

We report direct measurements of spatially resolved stress at the boundary of a shear thickening cornstarch suspension revealing persistent regions of high local stress propagating in the flow direction at the speed of the top boundary. The persistence of these propagating fronts enables precise measurements of their structure, including the profile of boundary stress measured by Boundary Stress Microscopy (BSM) and the non-affine velocity of particles at the bottom boundary of the suspension measured by particle image velocimetry (PIV). In addition, we directly measure the relative flow between the particle phase and the suspending fluid (fluid migration) and find the migration is highly localized to the fronts and changes direction across the front, indicating that the fronts are composed of a localized region of high dilatant pressure and low particle concentration. The magnitude of the flow indicates that the pore pressure difference driving the fluid migration is comparable to the critical shear stress for the onset of shear thickening. The propagating fronts fully account for the increase in viscosity with applied stress reported by the rheometer and are consistent with the existence of a stable jammed region in contact with one boundary of the system that generates a propagating network of percolated frictional contacts spanning the gap between the rheometer plates and producing strong localized dilatant pressure.