Jeffrey M. Moore, Tyler N. Thompson, Matthew A. Glaser, and Meredith D. Betterton (2020). Soft Matter 16, 9436-9442. DOI: 10.1039/D0SM01036G. arXiv DOI:1909.11805. Download
In active matter systems, self-propelled particles can self-organize to undergo collective motion, leading to persistent dynamical behavior out of equilibrium. In cells, cytoskeletal ﬁlaments and motor proteins exhibit activity and self-organization into complex structures important for cell mechanics, motility, and division. Collective dynamics of cytoskeletal systems can be reconstituted using ﬁlament gliding experiments, in which cytoskeletal ﬁlaments are propelled by surface-bound motor proteins. These experiments have observed diverse behavior, including ﬂocks, polar streams, swirling vortices, and single ﬁlament spirals. Recent experiments with microtubules and kinesin motor proteins found that the eﬀective repulsive interaction between ﬁlaments can be tuned by crowding agents in solution, altering the collective behavior. Adding a crowder reduced ﬁlament crossing, promoted alignment, and led to a transition from active, isotropically oriented ﬁlaments to locally aligned polar streams. These results suggest that tunable soft repulsion can control active phase behavior, but how altering steric interactions and ﬁlament stiﬀness alter collective motion is not fully understood. Here we use simulations of driven ﬁlaments with tunable soft repulsion and rigidity in order to better understand how the interplay between ﬁlament ﬂexibility and steric eﬀects can lead to diﬀerent active steady states. We identify swirling ﬂocks, polar streams, buckling bands, and spirals, and describe the physics that govern transitions between these states. In addition to repulsion, tuning ﬁlament stiﬀness can promote collective behavior, and controls the transition between active isotropic ﬁlaments, locally aligned ﬂocks, and polar streams.