Rheology of active emulsions with negative effective viscosity

TL;DR

This paper investigates the complex rheological behavior of active emulsions through lattice Boltzmann simulations, revealing regimes of negative effective viscosity, shear-thickening, and multistability influenced by activity and external forcing.

Contribution

It introduces a detailed numerical analysis of active emulsions, highlighting the emergence of negative viscosity and multistability, which are novel insights into active matter rheology.

Findings

Identification of shear-thickening at high activity

Discovery of stable negative effective viscosity regimes

Observation of multistability influenced by activity and forcing

Abstract

We numerically study by lattice Boltzmann simulations the rheological properties of an active emulsion made of a suspension of an active polar gel embedded in an isotropic passive background. We find that the hexatic equilibrium configuration of polar droplets is highly sensitive to both active injection and external forcing and may either lead to asymmetric unidirectional states which break top-bottom symmetry or symmetric ones. In this latter case, for large enough activity, the system develops a shear-thickening regime at low shear rates. Importantly, for larger external forcing a regime with stable negative effective viscosity is found. Moreover, at intermediate activity a region of multistability is encountered and we show that a maximum entropy production principle holds in selecting the most favorable state.