# Active suspensions have non-monotonic flow curves and multiple mechanical equilibria

## TL;DR

This paper demonstrates that confined active fluids like bacterial suspensions can exhibit negative viscosity and non-monotonic flow behavior, revealing complex mechanical properties and multiple equilibria in active matter systems.

## Contribution

It introduces a minimal model predicting negative viscosity in bacterial suspensions and explains its origin through non-monotonic flow curves, aligning with experimental data.

## Key findings

- Bacterial suspensions can behave as negative viscosity fluids.
- Negative viscosity arises from non-monotonic velocity profiles.
- Active fluids exhibit non-equivalent stress and strain rate ensembles.

## Abstract

We point out unconventional mechanical properties of confined active fluids, such as bacterial suspensions, under shear. Using a minimal model of an active liquid crystal with no free parameters, we predict the existence of a window of bacteria concentration for which a suspension of \textit{E.~Coli} effectively behaves, at steady-state, as a negative viscosity fluid and reach quantitative agreement with experimental measurements. Our theoretical analysis further shows that a negative apparent viscosity is due to a non-monotonic local velocity profile, and is associated with a non-monotonic stress vs. strain rate flow curve. This implies that fixed stress and fixed strain rate ensembles are not equivalent for active fluids.