Negative interfacial tension in phase-separated active suspensions

TL;DR

This study numerically investigates phase separation in active suspensions, revealing a negative interfacial tension as a genuine non-equilibrium effect, challenging traditional thermodynamic models.

Contribution

It demonstrates that active suspensions can exhibit negative interfacial tension, a phenomenon not explained by passive thermodynamic theories, through numerical simulations and analysis.

Findings

Negative interfacial tension observed in active suspensions.

Stable phase separation occurs despite negative tension.

Results challenge effective thermodynamic descriptions of active matter.

Abstract

We study numerically a model for active suspensions of self-propelled repulsive particles, for which a stable phase separation into a dilute and a dense phase is observed. We exploit that for non-square boxes a stable "slab" configuration is reached, in which interfaces align with the shorter box edge. Evaluating a recent proposal for an intensive active swimming pressure, we demonstrate that the excess stress within the interface separating both phases is negative. The occurrence of a negative tension together with stable phase separation is a genuine non-equilibrium effect that is rationalized in terms of a positive stiffness, the estimate of which agrees excellently with the numerical data. Our results challenge effective thermodynamic descriptions and mappings of active suspensions onto passive pair potentials with attractions.