Activity-suppressed phase separation

TL;DR

This paper investigates how activity influences phase separation in a mixture of active nematic and passive fluid, revealing that bulk activity can suppress phase separation and lead to a dynamic emulsion state.

Contribution

The study distinguishes the roles of interfacial and bulk active stresses, showing bulk activity's significant effect on arresting phase separation through sustained fluid stirring.

Findings

Bulk active stresses suppress phase separation.

Phase separated state becomes a dynamic emulsion.

Droplet size depends on activity levels.

Abstract

We use a continuum model to examine the effect of activity on a phase separating mixture of an extensile active nematic and a passive fluid. We highlight the distinct role of previously considered interfacial active stresses and bulk active stresses that couple to liquid crystalline degrees of freedom. Interfacial active stresses can arrest phase separation, as previously demonstrated. Bulk extensile active stresses can additionally strongly suppress phase separation by sustained self-stirring of the fluid, substantially reducing the size of the coexistence region in the temperature/concentration plane relative to that of the passive system. The phase separated state is a dynamical emulsion-like steady state of continuously splitting and merging droplets, as suggested by recent experiments. Using scaling analysis and simulations, we identify various regimes for the dependence of droplet size on activity. These results can provide a criterion for identifying the mechanisms responsible for arresting phase separation in experiments.