Defect self-propulsion in active nematic films with spatially-varying activity

TL;DR

This paper investigates how spatial variations in activity influence the motion and interactions of topological defects in active nematic films, revealing mechanisms for defect trapping, alignment, and accumulation at activity interfaces.

Contribution

It introduces a detailed analysis of defect dynamics under spatially-varying activity, highlighting new defect behaviors near activity gradients and sharp interfaces.

Findings

+1/2 defects align with activity gradients due to induced vorticity.

Sharp activity jumps trap defects and influence their motion and orientation.

Negative defects are attracted to activity interfaces, leading to accumulation.

Abstract

We study the dynamics of topological defects in active nematic films with spatially-varying activity and consider two setups: i) a constant activity gradient, and ii) a sharp jump in activity. A constant gradient of extensile (contractile) activity endows the comet-like $+1/2$ defect with a finite vorticity that drives the defect to align its nose in the direction of decreasing (increasing) gradient. A constant gradient does not, however, affect the known self-propulsion of the $+1/2$ defect and has no effect on the $-1/2$ that remains a non-motile particle. A sharp jump in activity acts like a wall that traps the defects, affecting the translational and rotational motion of both charges. The $+1/2$ defect slows down as it approaches the interface and the net vorticity tends to reorient the defect polarization so that it becomes perpendicular to the interface. The $-1/2$ defect acquires a self-propulsion towards the activity interface, while the vorticity-induced active torque tends to align the defect to a preferred orientation. This effective attraction of the negative defects to the wall is consistent with the observation of an accumulation of negative topological charge at both active/passive interfaces and physical boundaries.