Spontaneously rotating clusters of active droplets

TL;DR

This study demonstrates that active droplets can spontaneously form rotating clusters through collective dynamics, with stable rotation emerging from cooperative interactions among droplets, independent of geometric asymmetries.

Contribution

It reveals a new mechanism for spontaneous rotation in active emulsions driven by collective droplet interactions, not geometric asymmetries.

Findings

Droplets form stable, rotating clusters in active emulsions.

Individual droplets exhibit helical swimming in specific activity ranges.

Cluster rotation is due to cooperative coupling of droplet rotational modes.

Abstract

We report on the emergence of spontaneously rotating clusters in active emulsions. Ensembles of self-propelling droplets sediment and then self-organise into planar, hexagonally ordered clusters which hover over the container bottom while spinning around the plane normal. This effect exists for symmetric and asymmetric arrangements of isotropic droplets and is therefore not caused by torques due to geometric asymmetries. We found, however, that individual droplets exhibit a helical swimming mode in a small window of intermediate activity in a force-free bulk medium. We show that by forming an ordered cluster, the droplets cooperatively suppress their chaotic dynamics and turn the transient instability into a steady rotational state. We analyse the collective rotational dynamics as a function of droplet activity and cluster size and further propose that the stable collective rotation in the cluster is caused by a cooperative coupling between the rotational modes of individual droplets in the cluster.