Spontaneous membrane formation and self-encapsulation of active rods in an inhomogeneous motility field

TL;DR

This paper investigates how active rods self-organize into membranes at interfaces of different motility, trapping other rods and enhancing local trapping through self-encapsulation, demonstrated via computer simulations.

Contribution

It introduces the concept of active membrane formation and self-encapsulation in inhomogeneous motility fields, supported by simulation results.

Findings

Active membranes spontaneously form at motility interfaces.

Self-encapsulation enhances trapping efficiency.

Phenomenon observable in various geometries.

Abstract

We study the collective dynamics of self-propelled rods in an inhomogeneous motility field. At the interface between two regions of constant but different motility, a smectic rod layer is spontaneously created through aligning interactions between the active rods, reminiscent of an artificial, semi-permeable membrane. This "active membrane" engulfes rods which are locally trapped in low-motility regions and thereby further enhances the trapping efficiency by self-organization, an effect which we call "self-encapsulation". Our results are gained by computer simulations of self-propelled rod models confined on a two-dimensional planar or spherical surface with a stepwise constant motility field, but the phenomenon should be observable in any geometry with sufficiently large spatial inhomogeneity. We also discuss possibilities to verify our predictions of active-membrane formation in experiments of self-propelled colloidal rods and vibrated granular matter.