Orienting-Field Effects on Instability and Mode Selection in Active Nematics

TL;DR

This paper investigates how orienting fields influence instabilities and mode selection in confined active nematics, revealing how field direction and strength can tune thresholds and control modes.

Contribution

It provides a theoretical framework showing how orienting fields can lower activity thresholds and select instability modes in active nematic systems.

Findings

Orienting fields can lower activity thresholds for instability.

Field direction determines whether the system is stabilized or destabilized.

Linear analysis accurately predicts long-time state symmetries.

Abstract

We examine the instabilities of a confined active nematic subjected to an orienting field using a low Reynolds number Ericksen-Leslie framework with active stresses and field-induced torques. Linear analysis reveals two distinct modes, with odd and even director symmetry, the instabilities of which depend on the interplay between activity and field strength. We derive exact and approximate analytic forms of the stability boundaries and show that an orienting field that aligns the director perpendicular to the substrate anchoring direction cooperatively lowers activity thresholds and enables a field-driven even symmetry mode instability, while an orienting field that aligns the director parallel to the substrate anchoring tends to stabilise the system. Numerical solutions of the full nonlinear equations show that the linear stability analysis correctly identifies the symmetries of long-time states. These results demonstrate how orienting fields can promote an instability below the classical critical activity and can be used to both tune the instability onset and control the mode selection in confined active nematics.