Microphase separation in active filament systems maintained by cyclic dynamics of cluster size and order

The onset of polar flocking in active matter is discontinuous, akin to gas-liquid phase transitions, except that the steady state exhibits microphase separation into polar clusters. While these features have been observed in theoretical models and experiments, little is known about the underlying mesoscopic processes at the cluster level. Here we show that emergence and maintenance of polar order are governed by the interplay between the assembly and disassembly dynamics of clusters with varying size and degree of polar order. Using agent-based simulations of propelled filaments in a parameter regime relevant for actomyosin motility assays, we monitor the temporal evolution of cluster statistics and the transport processes of filaments between clusters. We find that, over a broad parameter range, the emergence of order is determined by nucleation and growth of polar clusters, where the nucleation threshold depends not only on the cluster size but also on its polar moment. Growth involves cluster self-replication, and polar order is established by cluster growth and fragmentation. Maintenance of the microphase-separated, polar-ordered state results from a cyclic dynamics in cluster size and order, driven by an interplay between cluster nucleation, coagulation, fragmentation and evaporation of single filaments. These findings are corroborated by a kinetic model for the cluster dynamics that includes these elementary cluster-level processes. It consistently reproduces the cluster statistics as well as the cyclic turnover from disordered to ordered clusters and back. Such cyclic kinetic processes could represent a general mechanism for the maintenance of order in active matter systems.