Interfacial instability of confined 3D active droplets

TL;DR

This study combines experiments and theory to analyze how activity influences the stability and shape evolution of 3D active nematic droplets confined between plates, revealing the role of active flows and physical parameters.

Contribution

It introduces a minimal nematohydrodynamic model to predict interfacial instability growth rates in active droplets, aligning theoretical results with experimental observations.

Findings

Active flows induce interface undulations and shape changes.

Growth rates depend on droplet size, gap height, and active timescale.

Experimental and theoretical results show good agreement on instability modes.

Abstract

Instabilities of fluid-fluid interfaces are ubiquitous in passive soft matter. Adding activity to the interface or either fluid can dramatically change the stability of the interface. Using experiment and theory, we investigate the interfacial instability of a deformable 3D active nematic liquid crystal droplet in the isotropic phase surrounded by a passive fluid and confined between two parallel plates. Spontaneous active flows drive the growth of undulations along the active/passive interface, with the mode number of the fastest-growing mode increasing with droplet radius and decreasing with gap height. We apply the lubrication approximation to a minimal nematohydrodynamic model to determine the growth rates of all interfacial modes. The magnitude of the growth rate is determined by the active timescale and the relaxation timescales associated with liquid crystalline order, as well as capillary and viscous stresses. We find multiple points of agreement between experiment and theory, including the shape evolution of individual droplets, the growth rates of unstable modes averaged across many droplets, and the extensional shear flows observed within droplets.