Aggregation of self-propelled particles with sensitivity to local order

TL;DR

This paper investigates a model of self-propelled particles where particles switch between aligning and nonaligning states based on local order, revealing complex clustering behaviors and phase transitions influenced by density and thresholds.

Contribution

It introduces a novel switching mechanism based on local order, leading to new mixed phases and insights into clustering and percolation in self-propelled particle systems.

Findings

Clusters of nonaligners emerge with increasing threshold.

Giant nonaligner clusters form at high density, indicating a percolation transition.

Speed differences are crucial for segregation of particle types.

Abstract

We study a system of self-propelled particles (SPPs) in which individual particles are allowed to switch between a fast aligning and a slow nonaligning state depending upon the degree of the alignment in the neighborhood. The switching is modeled using a threshold for the local order parameter. This additional attribute gives rise to a mixed phase, in contrast to the ordered phases found in clean SPP systems. As the threshold is increased from zero, we find the sudden appearance of clusters of nonaligners. Clusters of nonaligners coexist with moving clusters of aligners with continual coalescence and fragmentation. The behavior of the system with respect to the clustering of nonaligners appears to be very different for values of low and high global densities. In the low density regime, for an optimal value of the threshold, the largest cluster of nonaligners grows in size up to a maximum that varies logarithmically with the total number of particles. However, on further increasing the threshold the size decreases. In contrast, for the high density regime, an initial abrupt rise is followed by the appearance of a giant cluster of nonaligners. The latter growth can be characterized as a continuous percolation transition. In addition, we find that the speed differences between aligners and nonaligners is necessary for the segregation of aligners and nonaligners.