Activity gradients in two- and three-dimensional active nematics

TL;DR

This study uses numerical simulations to explore how spatial activity gradients influence defect orientation and flow patterns in 2D and 3D active nematic systems, revealing defect alignment behaviors and transitions to turbulence.

Contribution

It provides new insights into defect alignment and flow responses in active nematics under activity gradients in both two and three dimensions.

Findings

In 2D, +1/2 defects align parallel to activity gradients with defect heads pointing towards contractile regions.

In 3D, disclination lines tend to lie perpendicular to activity gradients, with defect type influencing orientation.

High activity induces defect creation and transition to active turbulence.

Abstract

We numerically investigate how spatial variations of extensile or contractile active stress affect bulk active nematic systems in two and three dimensions. In the absence of defects, activity gradients drive flows which re-orient the nematic director field and thus act as an effective anchoring force. At high activity, defects are created and the system transitions into active turbulence, a chaotic flow state characterized by strong vorticity. We find that in two-dimensional (2D) systems active torques robustly align $+1/2$ defects parallel to activity gradients, with defect heads pointing towards contractile regions. In three-dimensional (3D) active nematics disclination lines preferentially lie in the plane perpendicular to activity gradients due to active torques acting on line segments. The average orientation of the defect structures in the plane perpendicular to the line tangent depends on the defect type, where wedge-like $+1/2$ defects align parallel to activity gradients, while twist defects are aligned anti-parallel. Understanding the response of active nematic fluids to activity gradients is an important step towards applying physical theories to biology, where spatial variations of active stress impact morphogenetic processes in developing embryos and affect flows and deformations in growing cell aggregates, such as tumours.