Active extensile stress promotes 3D director orientations and flows

TL;DR

This study uses simulations and analysis to show how extensile active stresses promote out-of-plane director orientations and flows in nematic layers, influencing layer formation and defect structures.

Contribution

It reveals the distinct effects of extensile versus contractile stresses on director orientation and flow, highlighting extensile stress as a driver for 3D layer formation in active nematics.

Findings

Extensile stress promotes out-of-plane director orientations.

Contractile stress suppresses out-of-plane flows and favors in-plane directors.

Extensile activity leads to 3D growth from planar drops.

Abstract

We use numerical simulations and linear stability analysis to study an active nematic layer where the director is allowed to point out of the plane. Our results highlight the difference between extensile and contractile systems. Contractile stress suppresses the flows perpendicular to the layer and favours in-plane orientations of the director. By contrast, extensile stress promotes instabilities that can turn the director out of the plane, leaving behind a population of distinct, in-plane regions that continually elongate and divide. This supports extensile forces as a mechanism for the initial stages of layer formation in living systems, and we show that a planar drop with extensile (contractile) activity grows into three dimensions (remains in two dimensions). The results also explain the propensity of disclination lines in three dimensional active nematics to be of twist-type in extensile or wedge-type in contractile materials.