Swarm behavior of self-propelled rods and swimming flagella

TL;DR

This study compares swarm behaviors of self-propelled rods and flagella, revealing similar overall patterns but distinct differences in cluster formation, shape, and dynamics due to their interaction mechanisms.

Contribution

It provides a comparative analysis of two different self-propelled particle systems using simulations, highlighting their similarities and differences in swarm behavior.

Findings

Clusters of rods are wedge-shaped due to blocked forward motion.

Flagella form elongated clusters due to hydrodynamic synchronization.

Flagella clusters have shorter lifetimes compared to rod clusters.

Abstract

Systems of self-propelled particles are known for their tendency to aggregate and to display swarm behavior. We investigate two model systems, self-propelled rods interacting via volume exclusion, and sinusoidally-beating flagella embedded in a fluid with hydrodynamic interactions. In the flagella system, beating frequencies are Gaussian distributed with a non-zero average. These systems are studied by Brownian-dynamics simulations and by mesoscale hydrodynamics simulations, respectively. The clustering behavior is analyzed as the particle density and the environmental or internal noise are varied. By distinguishing three types of cluster-size probability density functions, we obtain a phase diagram of different swarm behaviors. The properties of clusters, such as their configuration, lifetime and average size are analyzed. We find that the swarm behavior of the two systems, characterized by several effective power laws, is very similar. However, a more careful analysis reveals several differences. Clusters of self-propelled rods form due to partially blocked forward motion, and are therefore typically wedge-shaped. At higher rod density and low noise, a giant mobile cluster appears, in which most rods are mostly oriented towards the center. In contrast, flagella become hydrodynamically synchronized and attract each other; their clusters are therefore more elongated. Furthermore, the lifetime of flagella clusters decays more quickly with cluster size than of rod clusters.