Dynamically Generated Patterns in Dense Suspensions of Active Filaments

TL;DR

This study uses simulations to explore how active filaments in dense layers exhibit various non-equilibrium phases, including collective flow and spiral formations, driven by activity levels and filament flexibility.

Contribution

It provides a detailed phase diagram of active filament systems, revealing novel activity-driven phase transitions and shape-changing mechanisms not seen in simpler active particle models.

Findings

Identification of multiple non-equilibrium phases including flowing melt and spiral states.

Discovery of an activity-driven transition from collective flow to individual filament spirals.

Observation of strong density fluctuations associated with active phases.

Abstract

We use Langevin dynamics simulations to study dynamical behaviour of a dense planar layer of active semi-flexible filaments. Using the strength of active force and the thermal persistence length as parameters, we map a detailed phase diagram and identify several non-equilibrium phases in this system. In addition to a slowly flowing melt phase, we observe that for sufficiently high activity, collective flow accompanied by signatures of local polar and nematic order appears in the system. This state is also characterised by strong density fluctuations. Furthermore, we identify an activity-driven cross-over from this state of coherently flowing bundles of filaments to a phase with no global flow, formed by individual filaments coiled into rotating spirals. This suggests a mechanism where the system responds to activity by changing the shape of active agents, an effect with no analogue in systems of active particles without internal degrees of freedom.