Anchoring-driven spontaneous rotations in active gel droplets

TL;DR

This study explores how active gel droplets with specific surface anchoring conditions spontaneously rotate due to internal activity, leading to diverse dynamic behaviors and shape deformations.

Contribution

It reveals the conditions under which active gel droplets spontaneously rotate and how anchoring and activity interplay to produce various morphologies and dynamics.

Findings

Droplets rotate spontaneously when activity and anchoring conditions are sufficiently strong.

Contractile droplets rotate only with planar anchoring; extensile droplets only with normal anchoring.

Various morphologies including steady rotation, oscillations, and irregular trajectories are observed.

Abstract

We study the dynamics of an active gel droplet with imposed orientational anchoring (normal or planar) at its surface. We find that if the activity is large enough droplets subject to strong anchoring spontaneously start to rotate, with the sense of rotation randomly selected by fluctuations. Contractile droplets rotate only for planar anchoring and extensile ones only for normal anchoring. This is because such a combination leads to a pair of stable elastic deformations which creates an active torque to power the rotation. Interestingly, under these conditions there is a conflict between the anchoring promoted thermodynamically and that favoured by activity. By tuning activity and anchoring strength, we find a wealth of qualitatively different droplet morphologies and spatiotemporal patterns, encompassing steady rotations, oscillations, and more irregular trajectories. The spontaneous rotations we observe are fundamentally different from previously reported instances of rotating defects in active fluids as they require the presence of strong enough anchoring and entail significant droplet shape deformations.