Memory effects, arches and polar defect ordering at the cross-over from wet to dry active nematics

TL;DR

This study investigates how increased friction influences active nematics, revealing that it enhances memory effects, defect trail formation, and arch-like patterns, leading to polar defect ordering and organized director fields.

Contribution

It demonstrates how friction induces persistent defect trails and arch patterns, providing new insights into defect dynamics and ordering in active nematic systems.

Findings

Increased friction enhances defect trail persistence.

Trails act as nucleation sites for defect formation.

Arch-like director patterns emerge as steady states.

Abstract

We use analytic arguments and numerical solutions of the continuum, active nematohydrodynamic equations to study how friction alters the behaviour of active nematics. Concentrating on the case where there is nematic ordering in the passive limit, we show that, as the friction is increased, memory effects become more prominent and $+1/2$ topological defects leave increasingly persistent trails in the director field as they pass. The trails are preferential sites for defect formation and they tend to impose polar order on any new $+1/2$ defects. In the absence of noise and for high friction, it becomes very difficult to create defects, but trails formed by any defects present at the beginning of the simulations persist and organise into parallel arch-like patterns in the director field. We show aligned arches of equal width are approximate steady state solutions of the equations of motion which co-exist with the nematic state. We compare our results to other models in the literature, in particular dry systems with no hydrodynamics, where trails, arches and polar defect ordering have also been observed.