Lasting effects of discontinuous shear thickening in cornstarch suspensions upon flow cessation

TL;DR

This study investigates how cornstarch suspensions retain a memory of their shear-thickening state after flow stops, revealing that residual stresses and relaxation times depend on the prior viscosity, indicating persistent frictional contacts.

Contribution

It provides new insights into the relaxation dynamics and memory effects in shear-thickening suspensions, highlighting the role of frictional contacts and boundary effects.

Findings

Residual stress correlates with prior steady-state viscosity.

Two-step stress relaxation exhibits near linear dependence on viscosity.

Frictional force chains persist, hindering complete relaxation.

Abstract

Dense suspensions that exhibit discontinuous shear thickening (DST) undergo complex stress relaxation when the flow abruptly stops. Using rotational rheometry, we study the two-step relaxation of aqueous cornstarch suspensions out of the DST state upon flow cessation and show that the DST fluid retains the memory of its shear-thickening state until the shear stress reaches a constant value at late times. We find that this residual stress at the end of the relaxation increases with the steady-state viscosity before the cessation. Furthermore, the timescales that characterize the two-step exponential decay of the shear stress exhibit near linear dependence on the steady-state viscosity. Within the current framework that ascribes DST to the breakdown of hydrodynamic lubrication layers leading to interparticle frictional contacts, the lasting effects of the steady-state viscosity suggest that the memory of frictional contacts persists until the end of the relaxation, despite the presence of repulsive forces between the particles. These results indicate that complete, spontaneous relaxation of the system out of DST is stalled by the partial retention of the frictional force chains, which may be caused by the stationary boundaries and the adhesion between cornstarch particles.