Capillary interfacial tension in active phase separation

TL;DR

This paper investigates how activity alters interfacial tension in phase separation, revealing two different tensions govern interface fluctuations and coarsening, with activity potentially leading to negative tension and novel self-organized states.

Contribution

It introduces the concept of two distinct tensions in active phase separation and demonstrates how activity can induce negative interfacial tension, leading to new self-organized states.

Findings

Active systems have two different interfacial tensions.

Strong activity can cause negative capillary tension.

Negative tension leads to microphase separation or active foam states.

Abstract

In passive fluid-fluid phase separation, a single interfacial tension sets both the capillary fluctuations of the interface and the rate of Ostwald ripening. We show that these phenomena are governed by two different tensions in active systems, and compute the capillary tension $\sigma_{cw}$ which sets the relaxation rate of interfacial fluctuations in accordance with capillary wave theory. We discover that strong enough activity can cause negative $\sigma_{cw}$. In this regime, depending on the global composition, the system self-organizes, either into a microphase-separated state in which coalescence is highly inhibited, or into an `active foam' state. Our results are obtained for Active Model B+, a minimal continuum model which, although generic, admits significant analytical progress.