Curvature-dependent tension and tangential flows at the interface of motility-induced phases

TL;DR

This study investigates the interface mechanics of motility-induced phase separation in active particles, revealing curvature-dependent tension, tangential flows, and a stabilizing Marangoni-like effect that maintains interface stability.

Contribution

It introduces a local frame analysis of active particle interfaces, uncovering curvature-dependent tension and tangential flows that explain interface stability in MIPS.

Findings

Negative tension correlates with local curvature.

Presence of sustained tangential particle flows.

Interface stability maintained by out-of-equilibrium Marangoni effect.

Abstract

Purely repulsive active particles spontaneously undergo motility-induced phase separation (MIPS) into condensed and dilute phases. Remarkably, the mechanical tension measured along the interface between these phases is negative. In equilibrium this would imply an unstable interface that wants to expand, but these out-of-equilibrium systems display long-time stability and have intrinsically stiff boundaries. Here, we study this phenomenon in detail using active Brownian particle simulations and a novel frame of reference. By shifting from the global (or laboratory) frame to a local frame that follows the dynamics of the phase boundary, we observe correlations between the local curvature of the interface and the measured value of the tension. Importantly, our analysis reveals the presence of sustained local tangential motion of particles within a surface layer in both the gas and the dense regions. The combined tangential current in the gas and self-shearing of the surface of the dense phase suggest a stiffening interface that redirects particles along itself to heal local fluctuations. These currents restore the otherwise wildly fluctuating interface through an out-of-equilibrium Marangoni effect. We discuss the implications of our observations on phenomenological models of interfacial dynamics.