Activity-induced phase separation and self-assembly in mixtures of active and passive particles

TL;DR

This study explores how mixtures of active and passive particles undergo phase separation and self-assembly, revealing that even a small fraction of active particles can induce complex structures and crystallization.

Contribution

It demonstrates that active-passive mixtures can self-organize into structured phases and that a small proportion of active particles can trigger phase separation and crystallization.

Findings

Phase separation occurs with as little as 15% active particles.

Active-passive segregation leads to passive rafts in an active sea.

A fully segregated initial state can self-assemble into an active corona that induces crystallization.

Abstract

We investigate the phase behavior and kinetics of a monodisperse mixture of active (\textit{i.e.}, self-propelled) and passive isometric Brownian particles through Brownian dynamics simulations and theory. As in a purely active system, motility of the active component triggers phase separation into a dense and a dilute phase; in the dense phase we further find active-passive segregation, with "rafts" of passive particles in a "sea" of active particles. We find that phase separation from an initially disordered mixture can occur with as little as 15 percent of the particles being active. Finally, we show that a system prepared in a suitable fully segregated initial state reproducibly self-assembles an active "corona" which triggers crystallization of the passive core by initiating a compression wave. Our findings are relevant to the experimental pursuit of directed self-assembly using active particles.