Rheology of dense suspensions under shear rotation

TL;DR

This study investigates how dense suspensions respond to shear rotations, revealing transient viscosity drops and complex stress behaviors dependent on solid volume fraction, supported by experiments, simulations, and a constitutive model.

Contribution

It introduces an experimental setup to systematically study shear rotations in dense suspensions and links the stress responses to particle interactions and volume fraction effects.

Findings

Transient decrease in shear resistance during rotations.

Orthogonal shear stress varies with rotation angle and volume fraction.

Particle simulations and models confirm experimental observations.

Abstract

Dense non-Brownian suspensions exhibit a spectacular and abrupt drop in viscosity under change of shear direction, as revealed by shear inversions (reversals) or orthogonal superposition. Here, we introduce an experimental setup to systematically explore their response to shear rotations, where one suddenly rotates the principal axes of shear by an angle $\theta$, and measure the shear stresses with a bi-axial force sensor. Our measurements confirm the genericness of the transient decrease of the resistance to shear under unsteady conditions. Moreover, the orthogonal shear stress, which vanishes in steady state, takes non-negligible values with a rich $\theta$-dependence, changing qualitatively with solid volume fraction $\phi$, and resulting in a force that tends to reduce or enhance the direction of flow for small or large $\phi$. These experimental findings are confirmed and rationalized by particle-based numerical simulations and a recently proposed constitutive model. We show that the rotation angle dependence of the orthogonal stress results from a $\phi$-dependent interplay between hydrodynamic and contact stresses.