Active fluids form system-spanning filamentary networks

TL;DR

This paper uses continuum theory to analyze how activity influences phase separation in active liquid crystals, revealing suppression of phase boundaries and the emergence of a system-spanning filamentary active network.

Contribution

It provides an analytical derivation of activity-induced phase boundary suppression and uncovers a new filamentous active network morphology in phase-separated active fluids.

Findings

Activity shifts the critical point of phase separation.

A new filamentous active network morphology emerges.

Passive droplets are trapped within the active network.

Abstract

Recent experimental realizations of liquid-liquid phase separation of active liquid crystals have offered an insight into the interaction between phase separation, ubiquitous in soft matter and biology, and chaotic active flows. In this Letter, we use continuum theory to examine phase separation of an active liquid crystal and a passive fluid and report two new results. First, we provide an analytical derivation of the activity-induced suppression of the phase boundary of the coexistence region - a result first reported in simulations and experiments. We show that the shift in the critical point is a result of the balance between self-stirring active flows and phase-separating diffusive fluxes. Second, we show that this same balance is responsible for dramatically changing the morphology of the phase separated state, resulting in the emergence of a new mixed active phase consisting of a dynamical filamentous active network that invades the entire system area, trapping droplets of passive material. This structure exists even for very low volume fractions of active material. Our work provides an important step towards the goal of understanding how to use activity as a new handle for sculpting interfaces.