Phase separation of passive particles in active liquids

TL;DR

This paper investigates how passive colloidal particles behave in active liquids, revealing dynamic clustering and phase separation driven by active noise, effective potential, and size ratios through experiments and simulations.

Contribution

It provides the first combined experimental and numerical analysis of phase separation of passive colloids in active media, highlighting the role of size ratio and interactions.

Findings

Passive colloids form clusters due to active noise and effective attraction.

Cluster size increases with colloid-to-bacteria size ratio.

Macroscopic phase separation occurs at large size ratios.

Abstract

The transport properties of colloidal particles in active liquids have been studied extensively. It has led to a deeper understanding of the interactions between passive and active particles. However, the phase behavior of colloidal particles in active media has received little attention. Here, we present a combined experimental and numerical investigation of passive colloids dispersed in suspensions of active particles. Our study reveals dynamic clustering of colloids in active media due to an interplay of active noise and an attractive effective potential between the colloids. The size-ratio of colloidal particles to the bacteria sets the strength of the interaction. As the relative size of the colloids increases, the effective potential becomes stronger and the average size of the clusters grows. The simulations reveal a macroscopic phase separation of passive colloids at sufficiently large size-ratios. We will present the role of density fluctuations and hydrodynamic interactions in the emergence of effective interactions.