Bulk condensation by an active interface

TL;DR

This paper demonstrates that a small number of active, motile grains can induce bulk vapor-liquid condensation in a hard-bead fluid, supported by experiments, simulations, and a novel continuum theory.

Contribution

It introduces a continuum model with a non-reciprocal Cahn-Hilliard structure to explain active interface-induced condensation phenomena.

Findings

Active grains form a polarized monolayer that induces condensation.

Oppositely aligned active layers immobilize the non-motile component.

The continuum theory's predictions match experimental observations.

Abstract

We present experiments, supported by mechanically detailed simulations, establishing bulk vapor-liquid condensation of a hard-bead fluid by a tiny population of orientable motile grains that self-assembles into a moving polarized monolayer. In a quasi-1D geometry two such layers, oppositely aligned, immobilize the condensed non-motile component. We account for our observations through a continuum theory with a naturally non-reciprocal Cahn-Hilliard structure, whose predicted trends as a function of packing fraction are consistent with our observations.