Spontaneous symmetry breaking in active droplets provides a generic route to motility

TL;DR

This paper demonstrates that active droplets can spontaneously break symmetry to achieve motility without treadmilling, providing a universal mechanism relevant to cells and bacterial aggregates.

Contribution

It introduces a simple model showing how spontaneous symmetry breaking can induce motility in active matter, independent of treadmilling processes.

Findings

Motility can arise from spontaneous symmetry breaking of polarity.

Reduced wall friction enhances SSB-mediated motility.

SSB leads to both rotational and translational motion in bacterial aggregates.

Abstract

We explore a generic mechanism whereby a droplet of active matter acquires motility by the spontaneous breakdown of a discrete symmetry. The model we study offers a simple representation of a "cell extract" comprising, e.g., a droplet of actomyosin solution. (Such extracts are used experimentally to model the cytoskeleton.) Actomyosin is an active gel whose polarity describes the mean sense of alignment of actin fibres. In the absence of polymerization and depolymerization processes ('treadmilling'), the gel's dynamics arises solely from the contractile motion of myosin motors; this should be unchanged when polarity is inverted. Our results suggest that motility can arise in the absence of treadmilling, by spontaneous symmetry breaking (SSB) of polarity inversion symmetry. Adapting our model to wall-bound cells in two dimensions, we find that as wall friction is reduced, treadmilling-induced motility falls but SSB-mediated motility rises. The latter might therefore be crucial in three dimensions where frictional forces are likely to be modest. At a supra-cellular level, the same generic mechanism can impart motility to aggregates of non-motile but active bacteria; we show that SSB in this (extensile) case leads generically to rotational as well as translational motion.