Synchronisation and liquid crystalline order in soft active fluids

TL;DR

This paper develops a phenomenological theory for soft active fluids capable of spontaneous synchronization and liquid crystalline order, revealing how these phenomena can coexist and influence each other in systems like micro-swimmer suspensions.

Contribution

It introduces a new theoretical framework describing the coupled emergence of orientational order and synchronization in soft active fluids, supported by explicit microscopic examples.

Findings

Synchronization promotes orientational order formation.

Liquid crystalline order enhances synchronization.

The theory applies to micro-swimmer suspensions with cyclic strokes.

Abstract

We introduce a phenomenological theory for a new class of soft active fluids, with the ability to synchronise. Our theoretical framework describes the macroscopic behaviour of a collection of interacting anisotropic elements with cyclic internal dynamics and a periodic phase variable. This system (i) can spontaneously undergo a transition to a state with macroscopic orientational order, with the elements aligned: a liquid crystal, (ii) attain another broken symmetry state characterised by synchronisation of their phase variables or (iii) a combination of both types of order. We derive the equations describing a spatially homogeneous system and also study the hydrodynamic fluctuations of the soft modes in some of the ordered states. We find that synchronisation can promote the transition to a state with orientational order; and vice-versa. Finally, we provide an explicit microscopic realisation: a suspension of micro-swimmers driven by cyclic strokes.