Sheared active fluids: thickening, thinning and vanishing viscosity

TL;DR

This paper investigates the complex rheological behaviors of active polar particle suspensions under shear, revealing phenomena like viscosity changes, non-monotonic stress responses, and yield stress due to active stresses and flow alignment.

Contribution

It provides a comprehensive analysis of how active stresses and flow alignment influence the rheology of active fluids under shear, highlighting novel behaviors such as thickening, thinning, and vanishing viscosity.

Findings

Active stresses can cause viscosity to increase or decrease.

Non-monotonic stress/strain-rate relations observed.

Large activities can induce yield stress phenomena.

Abstract

We analyze the behavior of a suspension of active polar particles under shear. In the absence of external forces, orientationally ordered active particles are known to exhibit a transition to a state of non-uniform polarization and spontaneous flow. Such a transition results from the interplay between elastic stresses, due to the liquid crystallinity of the suspension, and internal active stresses. In the presence of an external shear we find an extremely rich variety of phenomena, including an effective reduction (increase) in the apparent viscosity depending on the nature of the active stresses and the flow-alignment property of the particles, as well as more exotic behaviors such as a non-monotonic stress/strain-rate relation and yield stress for large activities.