# The effect of self-induced Marangoni flow on polar-nematic waves in active-matter systems

## TL;DR

This paper explores how Marangoni flow affects density waves in active matter systems with polar-nematic symmetry.

## Contribution

The study reveals how Marangoni flow influences the propagation and stability of polar-nematic density waves in active matter.

## Key findings

- Density waves broaden and their speed changes with increasing Marangoni parameter.
- Density waves can disappear via saddle-node or Hopf bifurcations.
- Wave behavior depends on wavelength and mean density.

## Abstract

We study the formation of propagating large-scale density waves of mixed polar-nematic symmetry in a colony of self-propelled agents that are bound to move along the planar surface of a thin viscous film. The agents act as an insoluble surfactant, i.e. the surface tension of the liquid depends on their density. Therefore, density gradients generate a Marangoni flow. We demonstrate that for active matter in the form of self-propelled surfactants with local (nematic) aligning interactions such a Marangoni flow nontrivially influences the propagation of the density waves. Upon gradually increasing the Marangoni parameter, which characterises the relative strength of the Marangoni flow as compared to the self-propulsion speed, the density waves broaden while their speed may either increase or decrease depending on wavelength and overall mean density. A further increase in the Marangoni parameter eventually results in the disappearance of the density waves. This may occur either discontinuously at finite wave amplitude via a saddle-node bifurcation or continuously with vanishing wave amplitude at a wave bifurcation, i.e. a finite-wavelength Hopf bifurcation.