Residual Force Determines Surface Tension in Active Systems

TL;DR

This paper introduces a new method to accurately measure surface tension in active particle systems by analyzing microscopic force distributions, revealing positive surface tension contrary to previous negative or zero estimates.

Contribution

It presents a total-force framework that accounts for both active and interaction forces, providing a more consistent measure of surface tension in nonequilibrium active matter systems.

Findings

Interaction forces oppose active propulsion, reducing net force.

The total-force approach yields positive, physically consistent surface tension.

Method bridges microscopic forces and macroscopic interfacial properties.

Abstract

The mechanical tension at the interface of motility-induced phase separating active Brownian particles (ABPs) remains an open question. Here, we determine the surface tension by analyzing the spatial distribution of forces at the molecular level in a slab-confined system of ABPs exhibiting high and low density regions separated by a one-dimensional active interface. Unlike previous approaches that evaluate active and interaction stresses independently - often producing near-zero or negative surface tension - we show that on average, interaction forces act antagonistically to active propulsion, reducing the net force experienced by particles. By evaluating the work required to bring a particle to the interface using this total-force framework, we find a positive and physically consistent surface tension. These results reframe the mechanical interpretation of local stresses and provide a generalizable method for connecting microscopic force distributions to emergent interfacial properties in nonequilibrium systems.